F05.303.Lecture notes
Author's note:
I have placed all the lecture notes into one file for easy downloading. Since the PowerPoint files are significantly larger, each lecture has a separate file for the slides. Additionally, some of the leture notes, particularly at the end of the day's notes, might not have been discussed. I have left them in for completeness, but I won't test you in this class on materials that I don't talk about in class.
F05.303.L1
Reading: Earth: An introduction to Physical Geology, 8th Edition
Dan Karner
Office: 2019 Schultz
Phone: 664-2854
karner@sonoma.edu
Office hours: Tuesday, Thursday 3-4, Wednesday 1-2
Office hour meetings that require a confidential setting should be made in
advance.
Homework: I will assign homework bi-weekly. You will have approximately one
week to return the assignment (I will notify you of the due date). Late assignments
are docked by 10%. Homework can be completed by hand, but it should be neat.
If the assignment requires you to make calculations, I expect to see those
calculations with your returned assignment.
Exam structure: Exams will consist of fill-ins, diagrams, and short essays.
How you write, including clarity of thought, spelling, grammar, etc., will
count toward your score; effective communication is one of the most important
skills that we can teach you.
Grading structure: This is a four-unit course, with one unit coming from the
lab and three units from lecture. Thus, lab constitutes 25% of your final grade
for the class, and lecture constitutes 75%. The distribution of points for
the class is as follows:
Lab = 25%
Homework = 10%
Midterm 1 = 20%
Midterm 2 = 20%
Midterm 3 (Final) = 25% (this will be semi-cumulative- I will select questions
from the first two midterms to have you answer again in addition to questions
on the new material.
Your final letter grade for the class will be determined by calculating your
overall score for your work in the class, compared with the scores obtained
by your peers.
Lecture Lecture
Date Reading Topics
1 8-25 Introduction and logistics Nothing of substance
2 8-30 Ch. 1: Introduction to Geology Big Bang, expanding universe, supernovae,
Nebular hypothesis
3 9-01 Ch. 3: Matter and Minerals elements, nucleosynthesis (S and R processes),
E=mc^2, radioactivity; periodic table: isotopes, magic numbers
4 9-06 Ch. 3: Matter and Minerals bonding, Bowens reaction series
5 9-8 Ch. 4 Igneous Rocks Density layering of Earth; Origin and components
of magma, composition of magma
6 9-13 Ch. 4 Igneous Rocks plutonic vs. volcanic rocks, Phase diagrams, predicting
eruptions (Video: Volcano’s Deadly Warning
7 9-15 Ch. 9: Geologic Time Relative Dating, Numerical dating, Geologic Time
Scale: Precambrian events
8 9-20 Ch. 9: Geologic Time Phanerozoic Eon
9 9-22 Ch. 12: Earth’s interior internal structure, seismic waves, reflection
and refraction, “sound” channels
10 9-27 Ch. 11: Earthquakes Seismology, intensity scales, tsunamis, Video:
The Next Wave: Science of Tsunamis
11 9-29 Ch. 11: Earthquakes San Andreas fault system
10-04 Midterm 1
12 10-06 Ch. 2: Plate tectonics global scale, mantle convection, triple junctions,
plate boundaries, paleomagnetism
13 10-11 Ch. 5: Volcanoes MORB, subduction volcanism, hot spots, rift volcanism,
calderas
14 10-13 Ch. 8: Metamorphism & metamorphic rocks Agents of, regional, hydrothermal,
and contact metamorphism
15 10-18 Ch. 10: Crustal deformation Structural geology, folding and faulting,
plate tectonic context
16 10-20 Ch. 6: Weathering and soil Forming sediment, chemical and physical weathering,
soils
17 10-25 Ch. 7: Sedimentary rocks detrital and chemical sedimentary rocks
18 10-27 Ch. 7 (continued) Stratigraphy, Facies, Walther’s Law, Turbidites,
bedding
19 11-01 no chapter isotopes for environmental reconstruction
11-03 Ch. 20: Shorelines waves, erosion, deltas, Coriolis Forcing, longshore
transport
20 11-08 Ch. 20 Shorelines coastal landforms, human intervention
11-10 Midterm 2
21 11-15 Ch. 19: Deserts and winds Hadley cells, geographic location, dunes
22 11-17 Ch. 16: Running water Erosion and sediment transport, river systems,
meanders, baselevel
23 11-22 Ch. 17: Groundwater US aquifers, Sonoma County usage and issues
11-24 Thanksgiving break
24 11-29 Ch. 18: Glaciers and glaciation Historical context of Ice Ages
25 12-01 Ch. 15: Mass wasting Erosion at grander scale, friction and water, creep,
Northern California issues
26 12-06 No Chapter Economic geology: Oil, Middle East, California coasts
27 12-8 Final lecture Karner’s research: Volcanic Hazards for Rome
12-15 Final Exam 2:00-3:50
F05.303.L2
Topics (write on board):
Big Bang
Expanding Universe
Supernovae
Nebular Hypothesis
Pass out: Age of Earth (Science Perspective by Jacobson, 2003). Have students
read it by Thursday
Check Chemistry background of students
Introduction chapter
1) Physical Geology
2) Historical Geology
1 must come before 2, but often it doesn’t.
Much of our program at SSU is physical geology, and this provides you with
the background that you will need to collect data for the basis of interpretation.
But Geology 303 is somewhat loosely defined in terms of the materials that
we expect to cover in this class, and so I will use a part of this class to
provide context for how the different classes that you will take might relate
to each other.
Earth’s spheres
Lithosphere (or geosphere)
Biosphere
Hydrosphere
Atmosphere
We live at the boundary between these four spheres, and so to work as a geologist
requires some knowledge of how they all interact. Failure to recognize the
contributions by each of these spheres can lead to very erroneous interpretations
of the rock record.
Internal versus external heat engine
I view geologic processes to be battle between two competing heat engines.
Internal heat engine = radioactive decay, gravitational settling, latent heat
of fusion
…these create igneous processes, and also the metamorphic processes.
The geosphere is controlled by the internal heat engine
External heat engine = Sun = radiation in all of its forms which drive weathering.
Physical and chemical weathering break down minerals and form new ones.
The hydrosphere, atmosphere, and biosphere are controlled mostly by the external
heat engine.
Of these two, the internal processes had to occur first in Earth history, and
tend to be more straightforward to understand. Surface processes tend to be affected
by several or all of the external processes, and so can be more complicated.
So I will spend the first part of this course talking about the internal heat
engine.But before we start in with Earth, let’s take a step further back
in time to see where we came from.
Early evolution of the Universe
Old idea: Big Bang, Big crunch- makes sense if you are trying to explain how
matter got here- explain it with another unanswerable puzzle!
Where did matter come from if Einstein’s famous equation is correct?
Primordial ooze = I dunno answer to the question of where matter comes from.
There is no consensus to these questions yet, but astronomy is making progress
toward answering some of them
Subatomic particles formed during the Big Bang
For most of geology, we can use the simplified view of atoms, with:
p+
n
e-
in a planetary model.
[[Write following section on board!]]
However, for sake of thoroughness, let’s talk about the smaller particles
that first existed.
Early Big Bang too hot for subatomic particles to form yet. All particles were:
baryons = photons, neutrinos, electrons, quarks
as cooling progressed down to 300 billion degrees K,
hadrons formed from the quarks = protons and neutrons
but these could not yet combine with other subatomic particles to form atoms
the other particles at that time were called leptons = electrons, photons, and
neutrinos.
So, the protons and neutrons combined to form the nuclei of simple atoms:
p+
p+ + n = deuteron (D)
p+ + 2n = tritium (T)
D + p+ = 3He nucleus
T + p+ = 4He (alpha particle)
Note: isotope = same number of protons, but different number of neutrons.
The Universe had expanded to the point where heavier nuclei would decay faster
than they formed, and so nothing would get done:
Example:
3H = tritium
t1/2 = 12.33 years
n -> b- decay -> p+ + e-
So 3H -> 3He
By the end of the Big Bang, there were 10 H for every He atom.
Where heavier elements formed
Center of stars: Nuclear fusion : He to Fe wll form in stars, but heavier elements
form from additional source of neutrons: Slow and Rapid processes of nucleosynthesis-
we’ll do this Thursday.
Expanding Universe, Age of Universe
Edwin Hubble- determined that a galaxy’s velocity was related to its distance…If
twice as far away it was moving twice as fast. This means that everything in
the universe is moving away from everything else- it is like the surface of a
balloon that is being blown up.
Note: This would have to be true if the Big Bang were true.
Age of the Universe
If everything in the Universe formed at the same place and at the same time,
then we can calculate how long ago that was by the present position of the stars
and the speed at which they are moving away from us.
Time = Distance /velocity (distance/time) (this is a simplification of Hubble’s
Law, which included the Hubble constant instead of velocity). Hubble found that
many galaxies yielded the same answer. His answer for the age of the universe
turned out to be wrong, at about 2 Ga.
Distance = brightness of an object
Velocity = redshift of objects
Hubble was using whole galaxies for his measurements, and since galaxies rotate
around a central object (black hole?) they do not have uniform velocities relative
to us. So Hubble wasn’t very accurate.
Supernovae: Modern astronomy and improving on the Hubble estimate.
Needed a standard candle for brightness = type 1a supernova
Details about a type 1a supernova
Two stars about the size of our sun undergo fusion of H-> He. One runs out
of fuel and so stops fusing. This causes the outer parts of the star to expand
and the core to contract. The star then is a Red Giant. The outer layers are
lost, and the core contracts to form a White Dwarf. Then the same thing happens
to the other star. However, when its outer layers expand, they get sucked into
the White Dwarf, causing that star to re-ignite. When the White dwarf reaches
a mass of 1.4 solar masses, it collapses gravitationally and explodes as a supernova.
Since this happens with the same mass (=same amount of fuel for the explosion)
they all have the same luminosity when the blow up (and this is billions of times
the light given off by the Sun, so it can be seen across the Universe).
Since Supernovae are singular objects, they do have a uniform velocity away from
Earth. Distant supernovae have light spectra that are shifted to the red more
than closer supernovae, and so this tells us that the farther objects have moved
away from us faster than closer ones.
Most distance supernova = 1997ff = 11.3 Ga
Accelerating expansion of Universe
Expansion of the Universe appears to have accelerated starting 5 Ga, and this
is counter to what is expected from a Universe that is dominated by gravity (should
contract through time).
This requires the mass of the Universe to be in other forms primarily
Dark energy
energy in space that has negative pressure- it acts opposite to gravity (been
called repulsive gravity), and is used to explain the fact that the Universe’s
expansion is now accelerating- something that would not be possible is gravitational
attraction was the only force acting on mass.
Dark energy is supposed to represent 65% of all mass in the Universe
Two proposed forms of dark energy:
Cosmological constant (Einstein): Dark energy is homogeneously distributed throughout
the universe, and opposes the attractive force of gravity; hence our Universe
is not contracting. The observation that the Universe’s expansion is accelerating
could be explained by a slightly positive cosmological constant.
Quintessence: A dynamic energy field that can change strength and concentration
through space and time. This could explain the acceleration of the universe’s
expansion in the past 5 Gyr.
Dark Matter
Dark energy should not be mistaken for dark matter. The latter represents about
30% of the Universe’s mass, and is caused by materials that do not give
off light, as do the stars. Consequently, we cannot see the dark matter, but
we can note that it is present by the gravitational effect it has on visible
objects.
What is dark matter? It is probably a combination of these:
1) MACHOs (massive compact halo objects). These are dwarf stars, rogue planets,
black holes.
2) Neutrinos Billions of these pass through your body every second. They can
pass through the whole earth. They are subatomic particles that are emitted from
all sorts of nuclear reactions, and they have a small mass.
3) WIMPs (weak interacting massive particles)- particles that have mass but that
don’t interact with ordinary matter.
Nebular Hypothesis
Solar system forms from debris from a supernova (radioactive elements that formed
during supernova event are found in all materials in our solar system, and they
can be dated to determine the age of different bodies, such as Earth, Moon, Mars,
asteroids).
Age solar system 4,567 Ma (4.567 Ga)
Age Earth: 4.557 Ga
Age Moon: 4.537 Ga
So the Solar System formed very quickly following the supernova event! Only about
30 Myr from the supernova to the formation of the Moon!
Read paper on How Old is Planet Earth?
Moon-forming event
Moon now believed to have formed from a large asteroid (the size of Mars) colliding
with early Earth. Moon is chemically very similar to Earth’s Mantle. But
the Moon has no core! This means that the Earth had to be separated into a core
and mantle before the asteroid hit us, so that the asteroid only excavated the
outer part of the Earth. This material was ejected into orbit around Earth, where
is then coalesced into the Moon.
Note: Moon too close to be a captured asteroid; Moon has been moving farther
away from Earth through time, due to the gravitational drag of Earth’s
water on the moon. This tidal drag causes the Moon to slow its orbit, and this
has caused it to recede.
Earth’s heat
Sources:
1) impacts
2) Radioactive decay
3) Gravitational settling
4) Latent heat of fusion (outer-> inner core)
F05.303.L3
Nebular Hypothesis
Solar system forms from debris from a supernova (radioactive elements that
formed during supernova event are found in all materials in our solar system,
and they can be dated to determine the age of different bodies, such as Earth,
Moon, Mars, asteroids).
Age solar system 4,567 Ma (4.567 Ga)
Age Earth: 4.557 Ga
Age Moon: 4.537 Ga
So the Solar System formed very quickly following the supernova event! Only
about 30 Myr from the supernova to the formation of the Moon!
Read paper on How Old is Planet Earth?
Moon-forming event
Moon now believed to have formed from a large asteroid (the size of Mars) colliding
with early Earth. Moon is chemically very similar to Earth’s Mantle.
But the Moon has no core! This means that the Earth had to be separated into
a core and mantle before the asteroid hit us, so that the asteroid only excavated
the outer part of the Earth. This material was ejected into orbit around Earth,
where is then coalesced into the Moon.
Note: Moon too close to be a captured asteroid; Moon has been moving farther
away from Earth through time, due to the gravitational drag of Earth’s
water on the moon. This tidal drag causes the Moon to slow its orbit, and this
has caused it to recede.
Moon’s retreat caused by tidal action with earth
Present Moon = 380,000 km
Measurements of Moon indicate that it is receding from Earth about 4 cm/year.
Cause: Tidal pull of Moon on Earth causes Earth’s rotation to slow. This
in turn causes Moon’s orbit to recede from Earth. At 900 Ma: Earth days
faster: 18 Hours. 1 yr = 481 days! At 4 cm/yr the Moon was 4x109 cm closer
to Earth 1 Ga. That is 4x107 m = 40,000 km retreat/Gyr. At age of Moon formation,
Distance to Moon would have been 200,000 km instead of 380,000 km. That is
too close to Earth for Moon to have been a captured satellite!
Earth’s heat
Sources:
1) impacts
2) Radioactive decay
3) Gravitational settling
4) Latent heat of fusion (outer-> inner core)
(Geo)Chemistry
While there are more complex models to explain how atoms are constructed, it
suffices for this class to use the planetary model of electron shell configuration
(star w/orbiting planets)
[[Figure of the periodic table]]
Notes about the periodic table
Write the following sequentially on board while talking students through them.
Leave periodic table up for students while discussing this.
1) Shows us arrangement of p and e-
2) Table wraps around
3) Rows = number of electron shells
4) Columns= similar bonding behavior
5) Elements want to mimic noble gasses- REALLY want their outer shell of electrons
to be filled. Do this by bonding
6) Distance from noble gas affects how they do it
What is immediately necessary for you to understand about the periodic table
is how to do the basic accounting of charges so that you will be able to construct
mineral formulas from memoryChemical Bonding (4 types)
ionic bonding = transfer of electrons- weak bonds
e.g. Na+Cl (evaporite minerals most ionic bonds)
covalent bonding= sharing electrons = strong! Si and O bond covalently, and
so very difficult to break quartz bonds.
looking ahead. As we learn about silicate minerals, I want you to be aware
that many minerals have a combination of covalent and ionic bonds. As a consequence
of the strength of the covalent bonds, it is the element that is ionically
bonded that will determine cleavage locations.
Metallic: valence electrons are not bound to a nucleus; they migrate throughout
a solid. This is why metallic bonds are so good at conducting electricity.
Van der Waals: atoms that are electrically neutral (due to bonding with another
atom) can have uneven distribution of charge. These can bond to other atoms
with uneven charge.
F05.303.L4
Topics (write on board):
Chapters 2, 3
Periodic table
isotopes
magic numbers
Radioactivity
Nucleosynthesis
S and R process
Age of the Earth (Jacobsen paper)
Mineralogy
Electronegativities (Handout showing ionic versus covalent bonding)
Bowens Reaction SeriesIsotopes: Neutrons are dealt with separately. Neutrons
control the stability
of the atom (radioactivity) and are not represented in the periodic table.
[[Table of isotopes]]
“Magic numbers”
There are nuclear “shells” just like there are electron shells.
Nuclear shells filled with 2, 8, 20, 28, 50, 82, 126 protons or neutrons are
particularly abundant.
Nuclei with magic numbers of both protons and neutrons are particularly stable
(He, O, Ca, Fe)
When magic numbers are obtained, the nuclei get smaller, which means that they
are less likely to interact with other particles.Radioactivity
Controlled by number of neutrons. These push protons away from each other;
generally, the more neutrons, the less stable the nucleus.
Any nucleus that can be broken into two pieces that, when combined have less
mass than the whole, is unstable.
i.e. if breaking it releases more energy than it took to break it, then it
is unstable.
Nuclear decay
Beta decay
Last time I fudged up on beta decay.
B- = neutron -> proton + e- + neutrino + Q
changes atomic number by +1
B+ decay (positron decay) = proton -> neutron + e+ (positron) + neutrino
+ Q
changes atomic number by –1
electron capture = nucleus captures an electron from the innermost electron
shell
P + e- -> neutron + neutrino
changes atomic number by –1
alpha emission = emission of a 4He nucleus from atomic nucleus
changes mass number by 4 and atomic number by 2
The decay of 238U, 235U to 207Pb and 206Pb releases 8 and 7 alpha particles.
These are our sources of 4He on Earth (very little primordial He left- it all
escaped).
Fission- the breaking of nuclei into nuclei larger than alpha particles, plus
some of the lighter particles mentioned above.
Nucleosynthesis in stars produces elements up to Fe. After that, nucleosynthesis
stops releasing energy.
Nuclei still can capture neutrons, provided that a source of neutrons is nearby.
S and R process of nucleosynthesis
[[see chart that has the nuclides]]
S = Slow accumulation of neutrons ~100,000 years.
Nuclei grow until magic number is reached, then neutron
capture cross section gets small; unlikely to be hit by another
neutron. Source: Asymptotic Giant Branch (AGB) stars.
208Pb, 209Bi heaviest nuclei produced by the S process.
R = Rapid accumulation of neutrons. Rate of accumulation
must faster than beta decay. These accumulate neutrons
until they reach the magic number bottlenecks. But they
reach the bottleneck with lots of neutrons- unstable
Source: Neutron star.Age of the Earth
Some radioisotope chronometers work very well for determining how long it took
for the Earth to form following the supernova that formed the solar nebula.
182Hf -> 182W (B- decay to 182Ta half life = 9 Myr, then 115 day half life
B- decay to 182W)
Hf atomic number (Z) = 72
W = 74
Hf = lithophile = goes with silicate (to mantle)
W = siderophile = goes with Fe (to core)
If unusual isotopic ratio of 182W to 183W occurs in Earth relative to meteorites,
then you can say that the Earth had already gone through a core forming process
with the 182Hf isotope was still alive. If the ratio was the same as that of
meteorites, then core formation will have been after the chronometer was dead
(>50 Myr)Mineralogy
Determining ionic versus covalent bonding
[[Electronegativity scale]]: Pauling scale. Range : 4.0 = F to 0.7.
Note that the periodic table has negativities that are largest at the upper
right and lowest at the lower left.
Electronegativity is a measure of how strongly an ion attracts valence electrons.
If big difference between two ions attracting electrons, the high one wins,
and the electron spends most time around that ion = ionic bonding.
If low difference then electron spends time around both ions= covalent bonding,
For silicates: O2- is the negatively charged ion that bonds with everything
else.
with Si = electronegitivity = 1.7 = covalent bond
For all other atoms in silicates, electronegativity > 1.7 = ionic bonding.
generally speaking, covalent bonding stronger than ionic bonding. As a consequence,
the atoms that ionically bond are the ones that break. This leads to cleavage
forming that is controlled by the silicate structure.
Arrangement of silica tetrahedra:
single tetrahedron = no cleavage
single chain = 2 cleavage directions at 90 degrees
double chain = 2 cleavage directions at 60,120 degrees
sheet = one direction of cleavage
framework = depends on how and where atoms substitute in the crystal lattice.
quartz = no cleavage = no weaknesses
feldspar = 2-3 directions of cleavage
Bowens Reaction Series: the order in which minerals crystallize from a basaltic
melt.
density of minerals related to the temperature of crystallization
Hi temp = single tetrahedral, so requires 4 charges from other elements = Fe2+
Mg2+
Ratio O:Si
single tetrahedral = 4
single chain = 3
double chain = 2.75
Sheet = 2.5
framework = 2
F04.303.L5
Topics (write on board):
Layered Earth
Magma components
Igneous rock cycle
phase diagrams
Cleavage
Silicates: Silicate cleavage: Si-O covalent bonds, all other bonds =ionic =
weaker.
This means that planes of ionic bonds in silicate minerals will be
planes of weakness.
Single tetrahedral = no cleavage b/c all bonds same strength (ionic)
Chain silicates: 2 planes of weakness parallel to chains
Sheets= 1 plane parallel to sheet
3-D frameworks = 2 parallel to something, or nothing for quartz = no cleavage
because all bonds same strength.
Bowens Reaction Series: the order in which minerals crystallize from a basaltic
melt.
density of minerals related to the temperature of crystallization
Hi temp = single tetrahedral, so requires 4 charges from other elements = Fe2+
Mg2+
Ratio O:Si
single tetrahedral = 4
single chain = 3
double chain = 2.75
Sheet = 2.5
framework = 2
Details about crystallization
Note that inherent to this process is that the mineral phase that crystallizes
is in equilibrium with the magma. But as minerals crystallize, the chemistry
of the magma changes. If this occurs slowly, and crystals remain in contact
with the magma, they will react to form a new mineral phase that is in equilibrium
with the magma again.
If cooling is fast, or if crystals settle to the bottom of the magma chamber,
they may be sealed off from interacting with the magma, and so the magma can
become very different in composition. This is why igneous rocks can look so
different form one place on earth to the next.
Petrography predictions: First minerals crystallize in melt, and so produce
euhedral crystals. Later ones crystallize in confined space and have irregular
shapes.
Igneous petrologists run this backward to find out the P-T conditions of a
rock at the time of crystallization.
Igneous Rocks: Chapter 4
By definition igneous rocks had to be the first rocks to form on Earth. We’ll
start our discussion of Earth with them.
Layered Earth
Early Earth- hot
Draw cross section of Earth
Earth is subdivided compositionally and on its physical properties (basically
the presence of melt, which results from pressure changes with depth).
Core: 2900-6378 km
Outer core = liquid (1216-3486 km)
Inner core = solid (0-1216 km)
Composition: Fe-Ni (~90% Fe)
density ~ 11 g/cc
The inner core is growing at the expense of the outer core.
liquid Fe and Ni sink, taking Fe-loving metals (siderophiles) with them. This
includes Pt, Ir, Os, Co, W)…form core
Evidence of core
Fe-Ni meteorites
seismic waves
Average density of Earth = 5.5 g/cm3
Aside about the core:
Appears to be some material excluded from the inner core- silicates? These are
less dense than the outer core, and so float up to the Core-Mantle Boundary.
Form
D’’ layer.
While not proven, it has been suggested that mantle plumes arise from the CMB.
If so, the build-up of D’’ could be related to that.
Mantle plumes- unknown trigger, but produce million km^3 of lava in short time
span (~1 Million years). These coincide with extinction events. Could be causal,
or symptomatic of impact event?
F05.303.L6
Topics (write on board):
Layered Earth
Magma components
Igneous rock cycle
phase diagrams
Mantle- from about 3478 km from center. For outer surface:
Upper mantle = 670 km discontinuity= liquid on top, solid below
Lower mantle = 670-2900 km density change probably a change in the mineralogy
and in the liquid content (none below 670).
density = 3.3 - 6 g/cm3- density largely an effect of pressure
Evidence of mantle = xenoliths
(show xenolith from Hawaii)
Rocky materials (mainly Si and O) float on top of this…form mantle = peridotite
Minerals = olivine (Mg,Fe)2SiO4
orthopyroxene= (Mg,Fe)SiO3
clinopyroxene=(Mg,Fe)2Si2O6
pyrope garnet= Mg3Al2Si3O12 (important for the formation of feldspars)
Crust 5-70 km
Through time, Crust forms from mantle material as it cools
Upper mantle and crust also divided into lithosphere and asthenosphere, based
on physical properties:
Lithosphere: rigid (about 100 km thick-includes crust and upper mantle)
Mohorovicic discontinuity- a seismic velocity jump across this boundary- change
from crust to mantle- so there is a major compositional change within the lithosphere,
but this material behaves in unison. Nonetheless seismic waves that had to pass
through the Moho arrive faster than those traveling near the surface.
asthenosphere: softer- a zone of melting at its top (~100-670 km). This melting
mesosphere = lower mantle
Core = core
Magma Components
Magma- partly molten material
3 components
liquid
solid
volatile (H2O, CO2, SO2)
How melts are generated
1) change pressure – pressure increases the boiling temperature of material.
This is why the outer core and upper mantle are the liquid parts.
2) Change temperature (increase heat melts material = increase depth of burial.
3) Add volatiles = volatiles increase the mobility of ions, and so their presence
leads to lower melting temps (also note that as magmas crystallize, the volatiles
stay in the melt, so the last bits of melt are very volatile rich.
Where are melts generated
To address this, I will introduce the:
Igneous Rock cycle
[[Draw rock cycle]]
First generation igneous rock cycle:
Note: MORB and hotspot volcanoes = first generation melt of mantle
Formation of ocean crust (MORB) at divergent plate boundaries. Here, upwelling
magma circulated heat away from Earth’s interior. Rising material experiences
less pressure, and so by the time it reaches the surface it undergoes a small
amount of DECOMPRESSION MELTING. The small amount of melt involves only some
of the mantle material.
Hotspots include a mantle plume that comes from some great depth, Increases
temperatures by about 100 degrees CCrystallization begins following Bowens
Reaction series. Formation of ophiolite sequence. Heaviest minerals settle
to bottom of magma chamber, become isolate from rest of magma. Magma composition
evolves to be richer in SiO2.
Bowens Reaction Series: the order in which silicate minerals crystallize from
a basaltic melt.
density of minerals related to the temperature of crystallization
Hi temp = single tetrahedral, so requires 4 charges from other elements = Fe2+
Mg2+
Ratio O:Si
single tetrahedral = 4
single chain = 3
double chain = 2.75
Sheet = 2.5
framework = 2
Named after N. L. Bowen
Igneous petrologists spend much of their time in the labs using furnaces to
simulate conditions in the interior. One thing that they do is melt rocks to
certain temperatures and pressures, and then watch what happens as the melt
cools and crystallizes.
Most of this work is experimental (meaning that results we’re predicted,
they were observed).
Bowen found that minerals form in a particular pattern. This is observable
in rocks too, by finding that SiO2 fills spaces between other minerals. That
means it forms last.
This also can be used as a clue to how hot a magma chamber was prior to eruption.
Composition of Igneous rocks
mostly composed of silicate minerals. Fortunately there are only a few.
Mafic minerals = top of Bowens Reaction Series. Rich in Mg and Fe
Sialic minerals = bottom of Bowens Reaction serires. Rich in Si and Al
[[Figure 4.7- mineralogy of common igneous rocks and magmas form which they
form]]
recognize that while there are some differences in the compositions of magmas,
most of the differences come about because of the physical processing of the
magma:
Amount of crystallization
depth of magma chamber (determines when melt is buoyant enough to rise to surface)
presence of volatiles
F04.303.L7
Topics (write on board):
Igneous rock cycle (second generation, third generation)
mantle sources
generating melt
Compositions of igneous rocks
Igneous Rock cycle
[[Draw rock cycle]]
Mantle sources:
Hotspots versus mantle plumes. Plumes are the first burst of magma to the surface, which produce large flood basalts. These plumes create conduits that allow magma to continue to rise to the surface. The slower, continued eruptions that proceed for 10s of Myr from these sources are know as hot spots. include a mantle plume that comes from some great depth, Increases temperatures by about 100 degrees C
Plume and Hotspot mantle source: Primitive mantle (mantle that has not yet seen the surface, nor has it had an ocean crust component removed from it. Probable from the lower mantle, or even the CMB.
By comparison: Mid Ocean ridge basalt taps a shallower source of mantle, one that has been depleted in some geochemical components such as helium. This is known as depleted mantle.
Generating Melt
Add heat: Geothermal gradient = 20-30 C/km
Decrease pressure = confining pressure keeps ions from mobilizing
Volatiles: addition of 0.1% by weight of volatiles decreases melting temp. of basalt by 100C.
mostly composed of silicate minerals. Fortunately there are only a few.
F05.303.L8
Chapter 9: Geologic Time
Topics:
Absolute ages (radioactivity)
Catastrophism – -Bishop James Ussher (1581-1656). Sudden, violent short-lived events sculpted the Earth into its present configuration. Calculated from Bible that earth formed on Sunday October 23, 4004 B.C.
Idea of how Earth evolved- recognized that sedimentary layers found above sea level
Neptunism- Abraham Werner (1749-1817) the notion that the rocks all precipitated out of a primordial ocean attributed to the biblical flood.
Formation of rocks rich in SiO2 were said to be "acidic” because they were formed from silicic acid (H4SiO4). So terms “basic” and “acidic” igneous rocks are artifacts of neptunism.
Large caves under ground occasionally collapsed causing sea level to drop, and earthquakes to occur
Nicholas Steno (1638-1686)- Accredited with being the world’s first stratigrapher. Anatomist- compared living fossils with ancient fossils and realized that they had to be the same.
Recognized that hard pieces (e.g. fossils) found within other hard pieces (e.g. sedimentary rocks) meant that the rocks had to solidify at some date after the deposition of the fossils. This concept applied not only to fossils, but to mineral grains and layers or rocks (strata).
1669: Preliminary discourse to a dissertation on a solid body naturally contained within a solid
Steno recognized that strata were formed from sediments falling out of suspension in water, and so the layers were original deposited horizontally. Thus, any strata that were not horizontal were deformed by subsequent processes.
The presence of fossils in a rock led him to recognize that the fossil had to be deposited first and then the sediment around it had to be deposited- the fossil could not have grown in place, otherwise it would have been distorted by the preexisting materials.
This applied also to sedimentary beds, where the stratigraphically lower beds are seen to shape the layers above them. Steno recognized that this meant there was a definite order to the deposition of sedimentary layers, and led to his second law:
2) Law of Superposition- layers of rock are formed in a time sequence with the older rocks on the bottom and the younger ones on top.
This law is so fundamentally important to geologists that is does not only apply to sedimentary rocks; it also applies to metamorphic and igneous rocks, and to geologic structures. It is the basis for our being able to work out the geologic history of an area.
3) Law of Lateral Continuity. strata are not only laid down in order, but they are laterally extensive as well (a uniform physical process).
-----------------------------------
Uniformitarianism- James Hutton (1726-1797).
“The present is the key to the past”
The physical processes that govern our world today can also explain the geologic history. However, what was needed was a LOT of time.
Plutonism – James Hutton (1726-1797)- rocks all formed from molten interior.
-----------------------------------
(William Smith- 1769-1839)
Principle of Faunal Succession
William Smith- first geologic map
“Map that changed the world”
-----------------------------------
Charles Lyell (1797-1875).
First Geologic Time Scale
Principle of Inclusions
Unconformity- period of time when geologic record removed by erosion or nondeposition- you cannot have deposition in same place for all time!
1) Angular unconformity- angular discordance above and below an unconformity- involves uplift, erosion, and tilting of strata
2) Disconformity- evidence of erosion, but no change in bedding orientations above and below
3) Nonconformity- special case for igneous and metamorphic rocks, which might not have layers
F05.303.L9
Chapter 9: Geologic Time
Topics:
Geologic Time Scale
[[show PPT figure of decay systems in use by Geologists]
14C produced in atmosphere from cosmic rays (protons) spallating neutrons from other atmospheric nuclei:
14N(n,p)14C
14C -> CO2 -> plants and animals.
When die, 14C starts to diminish
14C -> 14N ß- decay
t1/2 = 5730 ± 40 yr
Problem with 14C dating is in the assumption that production is constant. We now know that this is not true.
[[Figure showing 14C production rate through time]]
comparison of 14C dates with tree ring dates shows a systematic change in age through time. A significant correction needs to be applied to Libby Years in order to make them consistent with calendar years.
The most versatile dating method is K-Ar dating. It can be used to date any rock on Earth, except those less than a few 1,000 years old.
I just dated some rocks from Italy that were 30 ka ± 0.3 ka.
Paul Renne dated the Mt. Vesuvius eruption
Potassium is the 8th most abundant element in Earth
39K = 93.3% , 40K = 0.01%, 41K = 6.7%
40K -> 40Ca (B- emission) 89.3%
40K -> 40Ar* (e- capture) 10.7%
40Ar*=le/l40K(elt-1) (production of 40Ar)
t1/2= 1.28x109 yr. ~ 4 half lives since Earth formed. 16X as much radioactivity associated with K when the Earth formed.
Decay constant=l= 5.543(±0.010)x10-10
le= portion that decays to 40Ar by electron capture = 0.581 x10-10
lb=portion that decays to 40Ca by beta decay = 4.962 x10-10
General Form:
Daughter=D
Parent=P
1) D=Pe-lt (production daughter related to amt. parent)
l is related to t1/2 as follows
t1/2 = 0.693 (=ln(2)) / l
Proof:
at t1/2 D=1/2P
substituting this into eqn. 1
2) 1/2P=Pe-lt1/2
simplifying…
= -ln(1/2)=lt1/2
note that –ln(x) = ln(1/x)
ln(2)/l=t1/2
0.693/l=t1/2
40K/40Ar Method
1) 40Ar=40K le/l (elt-1) Note that e-lt ==elt-1
Rearranging 1) to solve for t:
2) t= 1/l ln(40Ar*/40K l/le +1)
-need to measure total concentrations of K, Ar
Analyze:
K through wet chemical techniques
Ar using
a resistance furnace and noble gas mass spectrometer
Problems: need lots of material (1013 atoms 40Ar*)
400,000 year sample with 15% K2O requires about 10 g
of crystal
Contamination possible by older grains
Insufficient heat to completely de-gas sanidine
One shot gas release- if there has been some leakage, you can’t determine that from K-Ar dating. Also, if there are inclusions that have mantle Ar, you can’t figure those out either.
40Ar/39Ar Method
A neat trick invented by Brent Merrihue and Grenville Turner at Berkeley. Measuring Ar isotopes in meteorites, they realized that :
39K(n,p)39Ar
for this they determined that neutron irradation could be used as an improvement on K-Ar dating.
39Ar = 39K ƒ¢T ∫j(e)s(e)de
- all of these terms relate to the research reactor, and are hard to monitor. So, the trick is to co-irradiate a sample (standard) of known age, and from this you can cast the age of an unknown in light of the age of the standard
Measure ratio of 40Ar*/39Ar
Analysis can be done on single sand-sized grains
(109 atoms 40Ar*)
Contamination by older material can be eliminated
t= 1/l ln (40Ar/39Ar x J +1)
[show photos of BGC]
[Show step-heating diagram]
Geologic time scale
Show Geologic Time Scale
(Bring several copies of earlier handout)
In many cases the boundaries between geologic times appear arbitrary, and many have been modified by study at different locations where more strata from those time periods existed.
two important boundaries exist:
Paleozoic-Mesozoic Boundary 248 Ma (flood basalt, asteroid?)
Mesozoic-Cenozoic Boundary 65 Ma (asteroid impact, flood basalt?)
[[show Fig. 11.3: Precambrian portions of continents]]
Precambrian shields- nearly flat. Younger rocks accrete to edges of Precambrian shields due to collisional processes (subduction, obduction)
Oldest Rocks on Earth = Acasta Gneiss, Northwest Territories. Age = 4.03 Ga
Gneiss is metamorphosed sedimentary rock, so must have been older rocks
atmosphere starts out “volcanic” = volcanic outgassing of CO2, H2O, SO2, N2
compared with modern atmosphere = 78%N2, 21% O2, and 1% Ar ± 1-5% H2O
This early atmosphere was a reducing environment:
Fe2+= reduced forms magnetite Fe3O4 ==Ferrous Fe = 2+
Fe3+=oxidized forms hematite = Fe2O3 ==Ferric Fe = 3+
~3.5 Ga = evidence of prokaryote cells (bacteria)
6 H2O + 6 CO2 = C6H12O6 +6 O2 (glucose + oxygen)
Oxygen produced by photosynthesis led to the oxygenation of the oceans and the precipitation of iron oxides:
Banded Iron Formations ~2.5-1.7 Ga (Note, Michigan State University has a very detailed website with the history of iron mining in the Great Lakes Region
About 2 billions years ago: layers of iron-rich sediment were deposited in the Proterozoic sea.
Up to 1 km thick.
Oxidized Fe = not soluble = ferric
~50% Fe minerals
Iron has been the chief industry for Michigan, Minnesota and Wisconsin for 150 years.
Auto industry there for a reason- Iron!
It is not until after the Fe was removed from the oceans that oxygen could start to build up in the atmosphere.
[[Figure of ice age that ended 12 ka]]
- we are not talking about modern ice ages…Snowball Earth would have been much worse
[[show some images of the glacial deposits and the cap carbonates. Get picture of snowball earth.]]
This is still a controversial story, but it is one that is gaining a lot of support.
Earth frozen ~ -50 C. All oceans frozen over, bottoms kept warm by Earth’s internal heat. Frozen water increases albedo. Volcanoes build up CO2 in the atmosphere. Eventually enough CO2 to start a runaway greenhouse warming. Ice melts, CO2 goes into the oceans, precipitates out as CaCO3.
Shortly after this life on Earth gets much more diverse and we start to see lots of animals with hard shells.
Perhaps it was the environmental trauma caused by the Snowball earth that caused the animals to evolve?
F05.303.L10
Chapter 9: Geologic Time
Topics:
Geologic Time Scale
Geologic time scale
Students should know the following from the Geologic Time Scale:
Eons
Eras
Periods
Epochs (Tertiary and Quaternary only)
Show Geologic Time Scale
(Bring several copies of earlier handout)
In many cases the boundaries between geologic times appear arbitrary, and many have been modified by study at different locations where more strata from those time periods existed.
two important boundaries exist:
Paleozoic-Mesozoic Boundary 248 Ma (flood basalt, asteroid?)
Mesozoic-Cenozoic Boundary 65 Ma (asteroid impact, flood basalt?)
[[show Fig. 11.3: Precambrian portions of continents]]
Precambrian shields- nearly flat. Younger rocks accrete to edges of Precambrian shields due to collisional processes (subduction, obduction)
Oldest Rocks on Earth = Acasta Gneiss, Northwest Territories. Age = 4.03 Ga
Gneiss is metamorphosed sedimentary rock, so must have been older rocks
atmosphere starts out “volcanic” = volcanic outgassing of CO2, H2O, SO2, N2
compared with modern atmosphere = 78%N2, 21% O2, and 1% Ar ± 1-5% H2O
This early atmosphere was a reducing environment:
Fe2+= reduced forms magnetite Fe3O4 ==Ferrous Fe = 2+
Fe3+=oxidized forms hematite = Fe2O3 ==Ferric Fe = 3+
~3.5 Ga = evidence of prokaryote cells (bacteria)
6 H2O + 6 CO2 = C6H12O6 +6 O2 (glucose + oxygen)
Oxygen produced by photosynthesis led to the oxygenation of the oceans and the precipitation of iron oxides:
Banded Iron Formations ~2.5-1.7 Ga (Note, Michigan State University has a very detailed website with the history of iron mining in the Great Lakes Region
About 2 billions years ago: layers of iron-rich sediment were deposited in the Proterozoic sea.
Up to 1 km thick.
Oxidized Fe = not soluble = ferric
~50% Fe minerals
Iron has been the chief industry for Michigan, Minnesota and Wisconsin for 150 years.
Auto industry there for a reason- Iron!
It is not until after the Fe was removed from the oceans that oxygen could start to build up in the atmosphere.
[[Figure of ice age that ended 12 ka]]
- we are not talking about modern ice ages…Snowball Earth would have been much worse
[[show some images of the glacial deposits and the cap carbonates. Get picture of snowball earth.]]
This is still a controversial story, but it is one that is gaining a lot of support.
Earth frozen ~ -50 C. All oceans frozen over, bottoms kept warm by Earth’s internal heat. Frozen water increases albedo. Volcanoes build up CO2 in the atmosphere. Eventually enough CO2 to start a runaway greenhouse warming. Ice melts, CO2 goes into the oceans, precipitates out as CaCO3.
Shortly after this life on Earth gets much more diverse and we start to see lots of animals with hard shells.
Perhaps it was the environmental trauma caused by the Snowball earth that caused the animals to evolve?
PhanerozoicEON(visible life)
[[SHOW QUICKTIME MOVIE]]
Watching the continents migrate around Earth might be helpful for you to appreciate why evolution has occurred. It is not all due to asteroid impacts. Land masses traveled from the poles to the equator, and sometimes were near the oceans and at other times were far from the oceans. If animals could not migrate, they became extinct.
Can play with this movie for a while-
Show North America
Western U.S. grows in the Tertiary
Show India collision with Asia to form Himalaya
Show closure of the Mediterranean between Africa and Europe
Point out how ocean circulation must have changed through time- affected heat transfer to poles.
At the start of the Phanerozoic, the continents were in completely different places.
[[Fig. 11.7 : Gondwanaland]] Five continents together here.
Because fossils were abundant throughout the Phanerozoic and because animals evolve through time, it was possible to subdivide the Phanerozoic:
ERA:
Paleozoic (ancient life) C,O=invertebrates, S-D =fishes, M,P,P=amphibians
North America~300 Ma Appalachian Orogeny: Caused by the collision of Africa with North America. The part of North America East of the Appalachians was African crust prior to the collision.
PERIOD
Cambrian= Cambria = name for England during the Roman Empire
Age of trilobites
Ordovician= Ordovices- ancient Welsh tribe
Age of brachiopods
armor-plated fishes begin to occur
Silurian= Silures- ancient Welsh tribe
Land plants begin to grow
Devonian=Devonshire, England
Trees >10 meters tall were growing.
Age of the fishes- the earlier fish evolved to have scales.
Late Devonian- lung fish + lobe-finned fish adapted to land- amphibians.
Carboniferous = major source of coal made at this time.
(Mississippian, Pennsylvanian = US Carboniferous equivalents)
Large tropical swamps at this time.
Permian= Perm, western Russia
During the Permian the Urals were formed by the collision of Siberia and Europe with Western Asia
BY the end of the Permian Period, all the World’s land masses had combined to form PANGAEA
[[Show Fig. 11.9: Pangaea]]
Pangaea led to a drier climate with vast deserts over much of North America
Perhaps 90% of all marine organisms died at the end of the Permian Period.
Perhaps Climate, perhaps Siberian Traps volcanism
F05.303.L11
Topics (write on board):
Chapter 9: Geologic Time
Chapters 12, 11: Earth’s interior
Topics:
Phanerozoic Eon
Seismic waves
Earth’s interior
PhanerozoicEON(visible life)
At the start of the Phanerozoic, the continents were in completely different places.
[[Fig. 11.7 : Gondwanaland]] Five continents together here.
Because fossils were abundant throughout the Phanerozoic and because animals evolve through time, it was possible to subdivide the Phanerozoic:
ERA:
Paleozoic (ancient life) C,O=invertebrates, S-D =fishes, M,P,P=amphibians
North America~300 Ma Appalachian Orogeny: Caused by the collision of Africa with North America. The part of North America East of the Appalachians was African crust prior to the collision.
PERIOD
Cambrian= Cambria = name for England during the Roman Empire
Age of trilobites
Ordovician= Ordovices- ancient Welsh tribe
Age of brachiopods
armor-plated fishes begin to occur
Silurian= Silures- ancient Welsh tribe
Land plants begin to grow
Devonian=Devonshire, England
Trees >10 meters tall were growing.
Age of the fishes- the earlier fish evolved to have scales.
Late Devonian- lung fish + lobe-finned fish adapted to land- amphibians.
Carboniferous = major source of coal made at this time.
(Mississippian, Pennsylvanian = US Carboniferous equivalents)
Large tropical swamps at this time.
Permian= Perm, western Russia
During the Permian the Urals were formed by the collision of Siberia and Europe with Western Asia
BY the end of the Permian Period, all the World’s land masses had combined to form PANGAEA
[[Show Fig. 11.9: Pangaea]]
Pangaea led to a drier climate with vast deserts over much of North America
Perhaps 90% of all marine organisms died at the end of the Permian Period.
Perhaps Climate, perhaps Siberian Traps volcanism
MesozoicEra (middle life) Age of reptiles
Evolutionary Note: Reptiles had hard-shelled eggs. This was important for the survival in drier climate of the Mesozoic. Fluid inside reptile eggs very similar to sea water.
This enabled dinosaurs to flourish throughout the Mesozoic.
Dinos very similar to mammals in their diversity- some flew, some returned to the oceans.
Evolutionary Note: Plants: Seed-bearing plants
Triassic: Tri-fold division of the rocks of this age in Germany
Most land exposed in North America- perhaps the break up of Pangaea caused high heat flow beneath the continents so they were high.
Jurassic: Jura Mountains in Switzerland
Oceans began to invade North America- some areas supported Dinosaurs (Montana) whereas others were desert (Southwest-Colorado Plateau).
Large sand dunes over North America- NAVAJO Sandstone up to 300 meter thick. It was DRY!- Sand seas similar to the Sahara Desert.
Break-up of PANGAEA caused formation of the Atlantic Ocean and also caused North America to override the Pacific Ocean plate.
Led to the formation of the California Coast Range and
the subduction-related volcanism of the Sierra Nevada.
~ 70 Ma – Larimide Orogeny- Formation of the Rocky Mountains and uplift of the Colorado Plateau- related to angle of subduction.
Cretaceous: Creta=chalk
Warm and humid time for North America. Large swamps led to the formation of coal swamps= coal for several western states.
By the Cretaceous, the land masses had broken up into their present configuration
[[Figure 11.12c: 100 Ma plates]]
[[End Cretaeous Extinction]]
Survivors through the K-T extinction: turtles, crocodiles, snakes, lizards
…Also age of flowering plants
Cenozoic mammals: Increased in size, increased brain capacity, specialized teeth, specialized limbs
Eastern U.S. very different from western U.S. Passive versus active continental margins
Tertiary = Early on older rocks were called Primary and Secondary, now obsolete terms
Note Tertiary cooling event.
[[From Wally’s book- bottom water cooling during the Tertiary in benthic d18O]]
[[Isolation of Antarctica creates circum-Antarctic current]]
[[Hadley circulation]]
Coriolis forcing
same angular velocity at all latitude, but different surface velocity.
wind and water bound to earth’s surface by friction, but this lessens with altitude
Air driven from equator toward poles deflected to right (N Hem)
Air driven from pole to equator deflected to left (N Hem)
Reverse pattern in southern hemisphere.
Consequence- easterlies at equator (trade winds)
At poles- Polar easterlies (at both poles)
This causes ocean circulation to circle Antarctica from East to west
Formation of Antarctic Bottom Water – cooling and formation of sea ice creates cold dense water which sinks. Fills deep ocean with cold water throughout the Tertiary
Leads to formation of Antarctic bottom water
cooling of oceans by about 10 degrees C throughout the Tertiary
Primary: Crystalline rocks
Secondary: Folded sedimentary strata
Quaternary: loose sediment.
Originally subdivided by Charles Lyell (incidentally Lyell was born the year that Hutton died) into the following percentages of species still living:
Eocene= 3.5%
Miocene= 18%
Pliocene= 33-50%
But this subdivision was doomed since the assessment of the geologic record was incomplete at that time, and so percentages would change for the periods of time.
EPOCH
(cene=recent)
Paleocene= (old recent)
Eocene= dawn of the recent
Grasses begin to spread, and animals begin to adapt to a diet of grass
Oligocene= slightly recent
Miocene= minor recent
Many large herbivores die, thought to coincide with development of grasses with more silica (fiber)- tooth destruction.
Pliocene= more recent
Closure of Isthmus of Panama- change in ocean circulation causes Ice Age?
Quaternary= Ice Age time we live in. We’ll spend a whole day talking about the Ice Age
Pleistocene= most recent
Holocene = completely recent
Chapter 12: Earth’s interior
[[Figure 16.3: Focus and Epicenter]]
Topics:
seismic waves
Earthquake is a vibration caused by the rapid release of energy. Usually caused by slippage along a fault.
Focus: The point in the ground where the earthquake occurs.
Epicenter: The point at the surface directly above the focus.
[[Figure 16.5- elastic rebound]]
Elastic Rebound Theory: Rocks can store elastic energy like the bending of a stick. Some of the strain in the stick is given back once you let go of it; that is recoverable strain. But sometimes too much elastic energy is put into the stick, and it breaks. Some of that energy goes into breaking the stick, but some of it is recovered strain- the two stick halves are not still bent- they are straight., Once the elastic energy overcomes the frictional resistance (=”glue”) that holds the rock together, it will rupture. This releases the elastic strain that was held in the rocks.
[[Photographs of 3 meter offset fence in Bolinas, toppled train at Pt. Reyes Station, Sonoma County Courthouse]]
It was following the 1906 earthquake that the elastic rebound theory was developed.
[[snap a stick demonstration]]
[[Focus and epicenter figure again- point out travels of seismic waves]]
Seismic energy travels out in all directions- can be portrayed as wave fronts or rays. (this works pretty well- as the wave front distorts, the rays bend towards the slower material.)
It turns out that there are several different kinds of seismic waves generated by an earthquake.
Thunder and lightning anology: People often count the distance to lightning storms by the amount of time between the lightning bolt and the sound of thunder. The lightning flash is instantaneous, but the thunder travels at the speed of sound, which is about 300 m/sec. So it takes about 5 seconds for the sound of thunder to travel about one mile.
In the same way, there are two pieces of information about earthquakes that can be used to tell you the distance to the quake. The P and S waves.
[[Figure 17.2 transmission of P and S waves]]
P waves= primary waves
compressional waves. They change the VOLUME of material that they pass through.
waves vibrate in the direction of propagation.
transmit through solids and liquids- both return to their initial volume after the stress is removed.
S waves- secondary waves.
Shear waves. They change the SHAPE of the material that they pass through.
waves vibrate perpendicular to the direction of propagation,
transmit through solids but not liquids; liquids have no shear strength- you change their shape and they don’t go back…they have no memory.
Surface waves:
concentrated at the Earth’s surface.
Surface layers are porous- they have air and water mixed with rock.
This mixture slows down the speed that waves are transmitted.
But as waves slow down, they have to increase in amplitude to conserve energy.
These produce the most damage, yet they are the most poorly explained process in any textbook that I have seen on seismic waves.
P waves and S waves combine to form the surface waves; the surface waves are NOT a separate kind of wave.
Love Wave: It's the fastest surface wave and moves the ground from side-to-side. My guess, but unverified: This is the horizontal S wave component that moves side-to-side.
Rayleigh Wave:A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves. My guess: This is the vertical component of the S wave and slows because it involves a boundary between the land and air.
F05.303.L12
Topics (write on board):
Chapters 12, 11: Earth’s interior
Chapter 12: Earth’s interior
[[Figure 16.3: Focus and Epicenter]]
Topics:
seismic waves
Seismic energy travels out in all directions- can be portrayed as wave fronts or rays. (this works pretty well- as the wave front distorts, the rays bend towards the slower material.)
It turns out that there are several different kinds of seismic waves generated by an earthquake.
Thunder and lightning anology: People often count the distance to lightning storms by the amount of time between the lightning bolt and the sound of thunder. The lightning flash is instantaneous, but the thunder travels at the speed of sound, which is about 300 m/sec. So it takes about 5 seconds for the sound of thunder to travel about one mile.
In the same way, there are two pieces of information about earthquakes that can be used to tell you the distance to the quake. The P and S waves.
[[Figure 17.2 transmission of P and S waves]]
P waves= primary waves
compressional waves. They change the VOLUME of material that they pass through.
waves vibrate in the direction of propagation.
transmit through solids and liquids- both return to their initial volume after the stress is removed.
S waves- secondary waves.
Shear waves. They change the SHAPE of the material that they pass through.
waves vibrate perpendicular to the direction of propagation,
transmit through solids but not liquids; liquids have no shear strength- you change their shape and they don’t go back…they have no memory.
Surface waves:
concentrated at the Earth’s surface.
Surface layers are porous- they have air and water mixed with rock.
This mixture slows down the speed that waves are transmitted.
But as waves slow down, they have to increase in amplitude to conserve energy.
These produce the most damage, yet they are the most poorly explained process in any textbook that I have seen on seismic waves.
P waves and S waves combine to form the surface waves; the surface waves are NOT a separate kind of wave.
Love Wave: It's the fastest surface wave and moves the ground from side-to-side. My guess, but unverified: This is the horizontal S wave component that moves side-to-side.
Rayleigh Wave:A Rayleigh wave rolls along the ground just like a wave rolls across a lake or an ocean. Because it rolls, it moves the ground up and down, and side-to-side in the same direction that the wave is moving. Most of the shaking felt from an earthquake is due to the Rayleigh wave, which can be much larger than the other waves. My guess: This is the vertical component of the S wave and slows because it involves a boundary between the land and air.
Waves Bent= refracted when they pass from one material into another.
Waves also reflected off discontinuities
Snell’s Law:
n1 sin theta 1 = n2 sin theta 2
where N1 and n2 are the indices of refraction through those materials:
space = 1
air = 1.00029
water = 1.33
quartz 1.46
glass = 1.52
diamond = 2.42
F05.303.L13
Topics (write on board):
Chapters 12, 11: Earth’s interior
Waves Bent= refracted when they pass from one material into another.
Waves also reflected off discontinuities
Note that at CMB, there is a marked drop in seismic velocity as seismic waves travel faster through solids than through liquids. What happens to seismic waves here is that as they pass from the lower mantle into the outer core, the waves refract AWAY from the mantle, and toward the direction of the outer core (note that this appears backward of the index of refraction, which would suggest that the seismic waves would be refracted LESS in the Outer core than in the mantle.
Thus refraction involves changes in the refractive index, and changes in velocity of material through different substances.
Deepest physical samples:
xenoliths
kimberlites
meteorites
deepest hole: 12.7 km Russia
Earth’s layers
Crust: thickness 3-70 km (Ocean vs. Continents)
Crust and mantle separated by the Mohorovicic discontinuity- little bit of melt here.
Mantle to 2900 km
Core: to center
Some phase transitions (Physical properties)
Lithosphere= crust + upper mantle ~100 km thick Up to 250 km below old crust. Only Few Km at mid-ocean ridges. So MOHO is within the lithosphere.
Asthenosphere= upper mantle below lithosphere. Relatively weak layer. Top of asthenosphere = partial melt.
Mesosphere== lower mantle
Outer Core
Inner Core
670 discontinuty- transition from olivine to perovskite- Si occupies a 6 oxygen site here (octahedral) rather than a tetrahedron.
D”= at the CMB
P wave Shadow zone- about 105 degrees from the earth quake focus About 35 degrees wide.
Caused by refraction of waves through the core.
Phase transitions:
410 km
670 km discontinuity: phase transition from olivine to perovskite. This transition moves Si from tetrahedron sites to octahedron sites (i.e. 4 oxygens to 6 oxygens; since each oxygen is shared by 2 Si atoms in perovskite, the Si: O ratio changes to 3
My thoughts about surface waves.
Rich’s page:
Maurice Ewing - SOFAR sound channel in the oceans.
(SOFAR stands for SOund Fixing And Ranging)
The SOFAR sound channel is a layer in the oceans, about 1 km deep, that is somewhat isolated from outside.
Velocity of sound varies in water as a function of depth.
5 meters per second per degree C.
If this effect dominated, the deeper it got, the slower sound would go.
At depth 1 km, the pressure effect overcomes the temperature effect, and there is a minimum in the sound speed.
Sound tends to bend towards this depth.
Same as waves bending towards the beach- they bend towards the slower direction
Sound produced at this depth at an upward or downward angle, is bent back to this depth.
It turns out that sound has no good way to enter the layer.
It is thus very quiet. There is very little sound in this layer except the music of whales.
emergency communications
small metal sphere dropped into the ocean, designed to crush at 1 km depth
Microphones could hear this ping
Triangulation
Sound Velocity in Air proportional to temperature.
Temp decreases with elevation
Until ozone layer. Then it increases.
So there is a sound channel in the atmosphere too!
detect the sound from a Russian nuclear explosion
From NY Times
1947 crash in the New Mexico desert
In 1946, Project Mogul was given a top-secret classification with the highest priority.
Soviets detonated their first nuclear bomb In August 1949. Mogul detected it
Project disbanded in 1950
The lithosphere also has a sound channel.
Seismic waves travel slowest at the Earth’s surface.
Waves that get there are trapped.
To conserve them, the amplitude has to grow (like water waves approaching the beach)
P waves and S waves combine to form the surface waves; the surface waves are NOT a separate kind of wave.
[[16.10. Typical Seismogram. (also 6.7)]]
[[6.12 Richter magnitude from the seismogram.]]
Point out that the distance to the earthquake affects the Richter Magnitude.
So without knowing how far away the epicenter is, you cannot gauge the magnitude
of the quake. You cannot say, “That felt like a M 4 quake to me!”
Modified Mercalli Intensity Scale = qualitative assessment of shaking where you are located- variable depending on composition of the rocks beneath you.
Moment magnitude: Involves measure of the length of fault rupture, amount of displacement, and rigidity of rocks. This better determines the amount of energy released from an earthquake event than does Richter.
Triangulation: Intersection of calculated distances from several seismograph locations determines the unique location of focus.
f05.303.L14
Ch. 2
Compelling story that launched modern thought:
Paleomagnetism: As magnetite cools below Curie Temperature (585 C), it locks in the magnetic field orientation of its location on Earth:
This works well for basalts, but magnetization of sediments also occurs- clay minerals often have weak magnetization and grains will rotate to align with magnetic field during deposition.
Polarity = Reverse or Normal
Inclination: Horizontal at equator and vertical at poles
Benefits of paleomag: globally synchronous, which means you can use reversals as a correlation tool, and also you can apply dates from one location to another
J. Tuzo Wilson recognized faults bound tectonic plates:
Plate Boundaries
Three types of plate boundaries exist;
1) Divergent Plate Boundaries= constructive plate margin.
This is the mid ocean ridges and the continental rifts
Ocean ridges constitute 20% of Earth’s area.
Changes in elevation of the spreading ridge could drastically affect sea level.
Average elevation of ridge is 2-3 km higher than nearby ocean crust.
NORMAL FAULTS- principal stress is gravity, minimum stress is confining pressure
Spreading rates average 5 cm/year, but vary from 2-17 cm/year.
Note that Atlantic is centered, but Pacific is NOT.
[[Figure 19.19]]- the development of a rift valley
[[Figure 19.20]- the East Africa Rift separates African and Arabian Plates
East African Rift
[[Picture of the Basin and Range?]]
Basin and Range
2) Convergent Plate Boundaries – destructive margins. Subduction when ocean-continent or ocean-ocean collision, but continent-continent collision doesn’t caused subduction (e.g., the India-Eurasia collision).
REVERSE FAULTS (thrust faults)= principal stress is horizontal and minimum stress is vertical
An aside- it appears that the lithosphere has delaminated under Italy, whereby the mantle has pealed (subducted) whereas the crust has not). This is related to a continent-continent collision, and you might expect that the recesses in the collision might have tremendous mantle flow as the mantle tries to get out of the way.
[[19.21A]
Accretionary wedge develops by scraping sediment off the down-going slab
-In California this is called the Franciscan Complex or Melange (mixture). Consists of sediment and pieces of oceanic crust that got scraped off
Up to 50% of the sediment can be scraped off the downgoing plate.
Ocean-Ocean collision
[[19.21B]]
[[19.17]]
Volcanic Island Arc
[[19.21C]]
[[19.23]] India Collision with Eurasian plate. Note that lithosphere MIGHT continue to go down by separating from crust.
3) Transform fault boundaries (conservative margins). Transform faults accommodate differences in plate velocities.
Strike-slip faults Principal stress is horizontal, minimum stress is also horizontal.
HOWEVER:
San Andreas Fault
[[19.25]]- shows that Juan de Fuca plate’s southern boundary is the Mendocino Fault. This is the northernmost extent of the san Andreas Fault.
San Andreas Fault joins the Juan De Fuca Ridge with the East Pacific Rise in the Gulf of Calofrnia.
F05.303.L15
Ch. 2
Topics
Triple Junctions
Mechanisms for plate tectonics
Volcanoes
Controls on violence of eruptions
Types of volcanoes: Conv. Div, Intraplate
Normal faults
reverse faults
strike-slip faults
Plate boundaries sometimes involve three plate boundaries. These are known as triple junctions, which involve different kinds of these faults:
D = divergent boundary = lines with shades on both sides to signify ridges
T = Tranforms = single line with or without arrows showing offset
C = Convergent = lines with teeth on upper place
Mendocino Triple Junction: San Andreas Transform, Mendocino Transform, Cascadia subduction zone
Note that some groups refer to these faults as Ridge (R), Trench (T) and Fault (F) triple junctions rather than C, D, T
Mechanisms for Plate tectonics:
Mantle Plumes and hotspots:
Evidence for a deep source of mantle plumes: a mid-ocean ridge moves right across a hotspot, as the Reunion hotspot in the Indian Ocean has done, the ridge doesn't excavate the hotspot; instead, volcanism jumps from one plate to the other.
Mantle plumes and hotspots
Idea: Source is core-mantle boundary.
Relationship between the two: First arises a mantle plume. which leads to a flood basalt. Then the tail creates a persistent conduit for slower release of magma for millions of years. The tail creates a weakness through the mantle.
Chemically distinct magma from MORB- plumes are more primitive- their materials have not seen Earth’s surface before. He, Nd, Hf, all have primordial values.
Upper mantle = depleted (loss of Volatiles due to repeated cycles of MORB formation.
Idea: Perhaps this material is D”, which would make it primordial if it is created from the solidification of the outer core.
Convection patterns:
1 layer convection = whole mantle convection
2-layer convection = upper and lower mantle convect separately
Lava lamp convection = irregular lower mantle layer that rises buoyantly as discrete blobs of material.
F05.303.L16
Ch. 2
Topics
Volcanoes
Controls on violence of eruptions
Types of volcanoes: Conv. Div, Intraplate
Ch. 5: Volcanic activity and other igneous processes
All igneous rocks start from magma
Magma components:
Magma sources: Mid-Ocean Ridge, Subduction zones, mantle plumes
In all cases the composition of the magma is very similar. The paths they take tot he surface lead to differences in what actually comes out.
How to melt rock:
Decompression – decrease pressure to lower temp of melt
Increase heat- increase heat = increase melt
add volatiles- more volatiles allows ions to move = melt
Magma resides in the crust where its density matches that of the surrounding crust.
There it begins crystallization
Once magma composition has changed to the point where the magma is less dense than surrounding material, it will rise buoyantly.
This will be different for ocean crust, continent crust –
Consider how different areas magmas form:
What determines eruptive behavior?
1) Composition
2) Temperature
3) Volatile content
These affect magma and lava viscosity
viscosity = stickiness
Factors affecting viscosity
[[Table 4.1]]
SiO2 content
Basalt= ~ 50%
Intermediate ~ 60%
Felsic ~ 70 %
Silica forms long chains in unstructured magma- increases viscosity.
Temperature:
Ranges from about 1100 C in basaltic eruptions to 900 C in felsic eruptions. Higher Temp= lower viscosity.
decrease viscosity, but more importantly, they fragment magma during ascent
can be 1-6% of the total weight!
Help to clear a conduit to the surface- erosion
water > Co2 > N2 > SO2 > Ar
Volatiles are dissolved until near the surface and then expand to fragment the magma
Volatiles increase in the upper part of the magma chamber-
[[Draw the igneous rock cycle and show primary and secondary magma sources]]
Primary magma: Recall that MORB are first melts from mantle- little interaction with other materials, so low contamination by H2O
Secondary magma- in subduction zones, involve water
magmatic differentiation
- increase in silica and volatiles as other materials crystallize out.
[[Figure 4.12- profiles of volcanoes]]
Note scale!
TYPES OF VOLCNOES
Shield Volcanoes
Composite Volcanoes
Cinder Cones
Shield Volcanoes
[[Figure volcano type- shield volcano- many thin flows
roof collapse common after large eruptions- form central caldera
Final stages- often more silicic and more pyroclastic in nature.
Basaltic composition, low volatiles, high eruption temperatures, HOTSPOTS
Cinder Cones-
lots of scoria- builds up higher angle of repose
short lived eruptions
Composite volcanoes
along subduction zones- steady supply of magma
Pyroclastic Eruptions
pyro=fire clast = fragment
high volatile content leads to the break up of lava
Despite these differences, all volcanoes share some common features
[[Figiure 4.9]]- discuss the parts of a volcano
Conduit- from magma chamber to surface
vent – at surface
crater- often forms at vent- due to removal of magma beneath the volcano
caldera- big crater
parasitic cone- often the main conduit gets solified by volcanic rock- new path found.
fumaroles = smoke holes- gasses often emitted at perimeter of volcano- Mammoth Mountain
Rhyolites – pyroclastic eruptions- gasses fragment lava into pieces, gas expansion causes violent release of rocks into the air.
pyroclastic eruptions
andesitic composition
Mount Fujiyama
Mount Shasta
Mount Saint Helens
Hotspots
Calderas
Intrusive (Plutonic) Igneous Bodies
Sonoma County Geology
Several students asked about Hawaii and Yellowstone- interesting type of volcanoes that are different than what we have discussed so far.
=Intraplate Volcanism
Different magma source than plate-tectonic-related volcanism
Mantle Plume- source of heat is at the CMB probably
[[Figure 7.something- Hawaiian Hotspot trace across the Pacific Ocean]]
Hawaii hotspot- source of heat has remained stationary while lithosphere has conveyed over it- for 80 Million Years!
Mantle ~ 150 C hotter at hotspots than surrounding mantle.
Hotspot volcanoes often have really large calderas associated with them.
[[Figure of Crater Lake caldera]]
[[Figure of how crater Lake caldera formed]]
[[Yelllowsotne Hotspot]]- probably easward migration of hotspot that formed the Columbia River Flood Basalt.- starts to melt the continental crust??
Long valley Caldera- California’s Caldera-
composed of many plutons
Sierra- Nevada Batholith – formed from old subduction volcanoes
F05.303.L17
Chapter 8: Metamorphic Rocks
Metamorphism- when rocks are subjected to temperature and pressures unlike those in which they formed.
Rocks change form to equilibrate with new environment.
Note that we are studying these rocks at Earth’s surface, and so metamorphic rocks are often unstable here. They can show signs of retrograde or prograde metamorphism
Prograde metamorphism: rocks are subjected to increased temps or pressures, and so multiple metamorphic mineral assemblages can develop along the way.
Retrograde metamorphism- the return to surface conditions passes the rocks through lower temp and pressure conditions, and so lower grades of metamorphism can be superimposed on higher degree metamorphism.
Big difference- prograde occurs with water-rich rocks, which speeds up the metamorphism. Retrograde occurs with dry rocks (water driven off during prograde processes) and so the changes are usually less dramatic.
A) Heat –
heat drives chemical reactions
1) Contact metamorphism – around igneous bodies- adjacent rocks get baked.
2) Regional metamorphism– heat caused by geothermal gradient- about 20-30C / km.
by temperatures of about 300C, clay minerals begin to metamorphose into chlorite and muscovite.
[[Figure 7.4- confining and differential pressures]]
Confining Pressure= pressure in all directions
Differential Pressure = directional pressure- causes minerals to form with a preferred orientation
C) Chemically reactive fluids- transports ions through the metamorphic environment.
Where grains are in contact= highest stress. Fluids dissolve materials here and move them to lower stress environment. This leads to minerals metamorphosing with a preferred orientation that is perpendicular to the principal stress.
Parent rock controls the minerals that will develop- the overall composition doesn’t change much. If shale- then metamorphic rocks will have lots of mica; the quartz doesn’t recrystallize yet, so it’s still there.
Foliation- the planar arrangement of mineral grains in a rock. Often this is the micas.
[[Figure 7.6- rotatioin of platy mineral grains]]
[[Figure 7.7- the dissolutin and reprecipitatin at lower stress portions of a clast elongates the rocks]]
Foliation can develop by rotation of crystals into alignment or recrystallization perpendicular to principal stress.
Slaty Cleavage- excellent splitting property into thin tabular slabs
Often this is not parallel to the bedding planes in rocks!
Schistocity- when the mineral grains grow large enough to be seen by eye.
Gneissic- minerals migrate into different bands
nonfoliated- partially due to the composition of the parent material- if CaCo3 or SiO2, then crystals remain equidimensional.
porphyroblasts- large mineral growth in a finer grained matrix. Garnet, staurolite and andalusite tend to grow few large crystals
Micas + quartz tend to grow a large number of small crystals.
[[Figure 7.24- metamorphic environments]]
Low-grade metamorphism to high-grade metamorphism
done in solid state- not melting
e.g.
shale
slate Low-grade
phyllite
schist
gneiss
migmatite High-grade
granite Igneous (melting)
1) Contact or thermal metamorphism
2) Hydrothermal metamorphism
3) Regional Metamorphism
4) Burial Metamorphism
5) Impact metamorphism
6) fault metamorphism
1) Contact Metamorphism- zone of alteration= aureole
These tend to grade outwards so that different zones of mineralization can occur.
[[Figure 7.21]]- the changes in mineralogy with temperature- these will occur at distance from the intrusive body
Because directed pressures minimal at contact metamorphism, rocks generally not foliated. ==Hornfels Facies
2) Hydrothermal Metamorphism- simultaneous with contact metamorphism
Often involves the ions that don’t go into usual mineral structures- Fe, Cu, Ni, Ag, Au can concentrate in the fluids and be removed from the igneous intrusion and the surrounding rock.
Fluids eventually cool at distance and precipitate minerals
-At Mid-Ocean ridges= black smokers
[[Show pictures of black smokers]]
3) Regional Metamorphism
Beneath Mountain Belts- isostasy. The amount of excess mass above ground has to be supported by an equal amount of displaced material below.
Boat floats because it displaces an equal mass of water from ocean. Crust does the same thing with mantle.
At convergent margins where subduction volcanism is occurring.
4) Burial Metamorphism- where deep sedimentary basins form (e.g. Gulf of Mexico). Here sediments can pile up so that pressures and temperatures at base of pile cause low grade metamorphism.
5) Fault Zones- RARE
Friction along fault causes mineralization and strong deformation of mineral grains- they elongate in fault zone- Mylonites
6) Impact (shock) metamorphism- Very rare!
High pressure impact wave move through crust at high velocity and cause minerals to deform. Some minor amounts of new minerals form (coesite and diamond).
Increasing metamorphic intensity leads to changes in texture and changes in the mineralogy of the rocks- typically noticed in the porphyroblasts
[[Figure 7.21]]= minerals stable at different
Review little bits of crustal deformation chapter- Anderson’s theory of faulting; faults and folds, sigma 1,2,3, ramp anticlines, balanced cross sections, anticlines in Middle East- reactivation of normal faults.
F05.303.L18
If layered sedimentary rocks no longer horizontal, then they’ve responded to some stress.
Stress = force/Area
compressional stress -> <- crustal shortening = Convergence
tensional stress <- -> crustal thinning = Divergence
shear stress == conservative = Transform
Strain= rock’s mode of response
Can be:
brittle (faulting) – stress exceeds rock’s ability to accommodate elastic strain
or
ductile (folding)
Rock properties that affect brittle versus ductile behavior:
1) Temperature – higher temp = greater ductility- this causes deeply-buried rocks to deform in a ductile manner.
2) Confining pressure- Increase confining pressure lads to greater ductility.
3) Composition – strength of rock or mineral’s chemical bonds
4) Time- stresses applied slowly can be accommodated by ductile deformation.
[[Strike and dip]]
Applies to all planar surfaces (bedding, fault planes, fold axes)
Bedding orientations
Strike: Dir. of horizontal line on planar surface
Dip: 90 degrees from strike= inclination of plane= noted directionally and by angle from horizontal.
Bedding can deform by faulting or folding
Faulting
Normal, reverse, thrust, strike-slip
Fault Angle ~30 degrees from principal stress
hanging wall and footwall (miner’s terms)
Why do these different types of faults develop?
[[Draw these out on the board!]]
Anderson’s theory of faulting: determined experimentally using squeezebox models.
Prediction: Faults occur at 30 degrees from directional vector of sigma 1
Stress directions, sigma 1, 2, 3 = perpendicular directions of greatest, intermediate and least stress.
Case 1: Sigma 1 vertical, Sigma 3 horizontal = normal faulting. Causes extension of crust. THESE FAULTS TEND TO FLATTEN WITH DEPTH
Large scale example- Basin and Range
Note that the reason they flatten with depth is that the confining pressures increase with depth, and so gravity is counter-balanced.
Case 2: Sigma 1 horizontal, sigma 3 vertical = thrust faulting. Crustal shortening
Case 3: Sigma 1 and 3 are horizontal, sigma 3 = vertical = strike-slip faulting.
Note that as stresses change, pre-existing faults may still represent the weakest plane, and hence may reactivate to accommodate displacement in unusual directions (i.e. oblique displacement)
Identifying faults in the field:
1) displacement of bedding surfaces
2) Striations on fault surface (scarp)= slickensides
These are the simplest cases- many faults are combinations of these types= oblique
Anderson was right except for unique cases of pre-existing features in rocks.
Ductile response to stresses
-rock could be weak, or stress could be applied slowly and for a long time.
Most common with thrust faults = crustal shortening
[[Draw original and then folded layers with anticline and syncline]]
Fold shapes
Anticline= fold opens down
Syncline = fold opens up
Axis = strike and dip of middle of fold plane
Hinge = apex of fold – this is a line. Has a trend and a plunge
For areas with significant lateral continuity of rock, all rock layers must be accounted for when you reconstruct the original sedimentary layers- you cannot introduce more of one layer than of another one without good reason.
F05.303.L19
F05.303.L19
Ch. 6: Weathering and soils
Sediments and soils are survival assemblages of mineral and rock from which they formed, plus new minerals that formed at the weathering site or the basin where the sediments collect.
The nature, intensity and duration of the weathering process influence the end product.
1) Weathering- the physical and chemical breakdown of rock
a) mechanical weathering: breaking into smaller pieces (small rocks rather than clay).
Increases the surface area of rock for chemical weathering
Note that mechanical weathering breaks down minerals that have cleavage, whereas those without cleavage will be less affected by it. This leads to quartz being a dominant component of parent rock that survives weathering.
How rocks fragment:
I) frost wedging: freeze-thaw cycle. Vol ice = 9% greater than Vol liquid
water. Produces talus
II) Unloading: pressure release as rocks approach surface leads to volume expansion at the top of the rock, and causes outside pieces of large rock bodies to flake off (e.g. exfoliation domes like Half Dome).
III) Thermal expansion: expansion-contract cycle leads to fractures, particularly is the temp change is rapid. Produces shattered rocks in desert.
IV) Biological activity
-roots expand and contract with water availability
- burrowing animals bring fresh rock to surface
-grazing animals- expose fresh rock
V) expansion caused by hydration of minerals (e.g. when mica alters to clay).
b) chemical weathering- creates a suite of new minerals that are stable in surface environment.
Principal agents: Water, CO2 and O2 (these occur naturally)
These can alter both the chemical and the physical composition of the rock.
I) Dissolution: affects ionic bonds (e.g. dissolution of halite). Polar water molecule has strong enough charge to break ionic bond between Na+ and Cl-. Other minerals also subject to dissolution: evaporates, carbonates
Calcite dissolution: CaCO3 + 2 [H+(H2)O] (aqueous acid) -> Ca2+ + CO2 + 3 H2O
II) Oxidation (particularly important for Fe). Oxidation state of Fe changed from Fe2+ (ferrous) to Fe3++(ferric). Note that most Fe in silicate minerals is reduced and oxidizes at the surface.
metal oxides Fe3O4 (magnetite)- partly oxidized
Fe2O3 (hematite)- all oxidized
FeO(OH)- limonite = all oxidized
sulfide mineral decomposition (e.g. pyrite):
4 FeS2 + 15 O2 + 14 H2O -> 4 Fe(OH)3 (yellow boy) + 8 SO42- + 16 H+
The H+ and SO42- will combine to form H2SO4 (sulfuric acid)
III) Hydrolysis (particularly important for feldspar decomposition) (H2O + CO2 + OH)
Hydrogen attacks silicates and replaces cations in the crystal structure
e.g. 2 KAlSi3O8 + 2 H2CO3 + H2O -> Al2Si2O5(OH)4 (kaolinite) + 2 K+(aq) + 2 HCO3- + 4 SiO2 (aq)
kaolinite is the main constituent of inorganic soil
SiO2 -> chert
K+ = important nutrient for plants; this makes granites and volcanic rocks good soils.
IV) Ion exchange: once minerals broken down to clay, they often undergo cation exchange:
e.g. Na replaces K
2 Na replace Ca (albitization)
V) Chelation: organic complexing. Lichens do this. Involves bonding metal ions with organic substances- removes cations from rock.
Products of weathering
1) Parent (source) rock residues- those minerals least likely to weather
2) Secondary minerals- clays, metal oxides & hydroxides
clay types:
smectite: immature soil
illite: immature soil
kaolinite: more mature
gibbsite:super mature- aluminum ores
diaspore: super mature- aluminum ores
3) soluble materials (shown in decreasing order of abundance):
HCO3- , Ca2+, H4SiO4 (silicic acid), SO4, Cl-, Na+, Mg2+, K+
Rates of Weathering affected by:
1) Rock composition- recall that for the silicate minerals, the Bowen’s Reaction series gives you some guidance about the rates of weathering. The bottom of Bowen’s (quartz) is most stable, whereas the top of the chart are minerals that crystallized at temperature that are most dissimilar to surface conditions. This means that a basalt should weather faster than a rhyolite if put next to each other.
2) Climate: weathering is more rapid in warm and humid environments. The rule of thumb about temperature is that a 10C increase doubles the chemical weathering rate.
A related note about climate is that climate differed through geologic time. Early Precambrian time had no oxygen and no plants, so different controls on weathering existed then. Also recall that glacial time would be different than interglacial time, and so the last several million years have largely been different climatologically than the present.
F05.303.L20
Soils
Soil- a combination of mineral + organic matter + H2O + air
air= delivers and removes CO2 and O2
water = delivers soluble nutrients necessary for plant growth
Represents an equilibrium state of the surface environment.
Soils are dynamic: if you change any component the soil will change
Contributing factors for Good Soil
1) Parent material= regolith: Influences the rate of soil formation and fertility (by composition)
Residual soil: forms in place (the survival assemblage after weathering). Slower soil formation because bedrock must be weathered
Transported soil: forms in place on loose sediment transported from elsewhere. Fast soil formation because occurring on material that is already partly weathered.
2) Time: For young soils, the parent material contributes strongly to the characteristics of the soil. For older soils, the effects of the parent material diminish and the other effects on soil dominate (climate).
3) Climate: temperature and moisture determine the rate of chemical weathering. The importance of chemical versus mechanical weathering will be determined by climate.
Rule of thumb: Increase T by 10C, double chemical weathering rate.
4) Organic material: constitutes 1-100% of soil
decomposition of organic material releases acids that hasten chemical weathering
also- organic material has high water retention
however, decomposition also uses oxygen, which can lead to anoxia if there is unsufficient movement of air or water through the soil. This can lead to concentration nd growth of sulfide minerals, commonly pyrite.
5) Slope:
steep slopes = immature soils. Slope allows water to drain away from rock, which limits the effect of chemical weathering, limits plant growth.
Low mechanical strength of soil versus rock causes soil to erode or suffer from mass wasting relatively quickly.
Parent material becomes very important for the quality of soil on steep slopes!
basins: thick, transported soils, but if poorly drained can lead to sulfide mineral deposition = bad! Parent material of upslope areas determines quality of soil.
Orientation: sunlight important for plant growth, which affects soil formation.
Soil formation is a top-down process
Five horizons of a soil in a Humid Environment (= idealized soil)
O = loose and partly decomposed organic material
A = minerals and humus (decayed remains of animal and plant)
O+A = topsoil
E = Zone of leaching and eluviation (washing out of fine components). Soil here is light in color because soluble components are leached out and transported away.
B = Zone of accumulation – material from E collects here, resulting in a higher clay content
O+A+E+B = solum = true soil
C = partly altrered parent material
Consider and arid environment with little plant cover
No formation of O or A horizon
No E horizon because no organic acids from above
No B horizon because no E horizon.
So you end up with a C horizon over parent material (regolith)
There are over 10,000 different types of soils, but they have been grouped into several categories
F05.303.L21
Having formed sediment, it is necessary to lithify them (turn to stone) to form a sedimentary rock.
~ 75 % of Continental outcrops are sedimentary Rock.
Diagenesis: The physical, chemical and biological processes that happen to sediments during and after lithification
includes:
compaction
cementation
recrystallization (to more stable minerals)
These can take variable amounts of time to lithify a rock: from days to millions of years.
At the coarsest level they are divided into
detrital (aka Clastic)and
chemical (aka nonclastic) sedimentary rocks.
Detrital sedimentary rocks are classified based on the
1) composition: minerals present, rock fragments: how much weathering has occurred
generic = sandstone (e.g. quartz sandstone)
feldspar + mafics = arkose (immature history)
graywacke= lots of rock fragments plus matrix (silt and clay filling voids)
2) grain size (gravel, pebble, sand, silt, clay= how much transport
[[Table 7.1 = grainsize chart]]
3) particle shape (e.g. rounding)= how much abrasion
4) fabric (packing of grains – matrix or grain supported, stacking pattern- imbrication?)= how deposited
5) Sorting= what separated the grains prior to deposition- particularly informative about the mode of tranport
discussed how weathering yields parent residues, secondary minerals and dissolved ions. It is this last category that makes chemical sedimentary rocks
Inorganic = ions concentrated without aid of biological processes (e.g. evaporation of water to concentrate sea salt
Organic = biological processes concentrate ions, generally produce exoskeletons
classified based on mineral composition, texture,
Limestone = 10% of all sedimentary rock
Both organic and inorganic kinds
Organic = corals, foraminifera, coccoliths
Inorganic = travertine, oolites
Chert = organic = chert, inorganic = chert nodules
forms concentrated deposits below the CCD
Coal = discussed as a separate topic,
tell you about the environment of deposition
dunes?
turbidity currents?
Streams?
Deltas?
[[7.19 – environments]]
There are so many environments that I don’t really want to spend the time going over them here.
Bedding- layers of sedimentary rock that have unifying characteristics that distinguish it from the rocks above and below.
Upper and lower surfaces= bedding planes
lamina < 1 cm thick
beds > 1 cm thick
Ripples
asymmetric = current ripples. Lee side is steep and downwind.
Symmetric = oscillation ripples- tidal ripples.
Crossbedding: beds inclined to the horizontal (happens in sand dunes, river channels and deltas.
Indicate direction of transport:
Two general shapes of cross beds:
1) tabular cross beds (sand dunes)
2) trough cross beds (meandering rivers)
Note that these refer to the shape of the contacts, not the shape of the foreset beds within the cross beds.
shape of foresets = planar or tangeantial.
Wet = high water table which stops the transport of sand= preservation of foresets.
Dry = low water table which doesn’t affect the transport of sand.
Graded beds: normal (fining upward) and reverse (coarsening upward)
-either can tell you about change in energy of environment.
Fining upward = decrease in energy of transport- can be from climate change in source area, change in the discharge direction (e.g. delta abandons one lobe for another), or distancing of source area by marine transgression.
Varves- alternating light and dark colored layers- indicates an anoxic environment (no bioturbation).
F05.303.L22
There are no notes for this talk. This was a look into the research world of DK. The talk was titled, "Should Rome Worry?" and was a presentation of volcanic hazards assessment for Rome.
F05.303.L23
Chapter 20: Shorelines
Topics:
Waves
currents
shorelines
Ocean circulation occurs because of heat imbalances. Same is true for wind. Poles are cold. Equator is hot. Hot goes to cold. But then there are the details.
Thermohaline Circulation : the thread for this lecture
Thermo = heat
Haline = salt
circulation = movement of the ocean water
There are two principal ways that ocean water circulates around the World:
1) Surface Currents
2) Deep water currents
1) Surface Currents
Surface currents are driven primarily by wind.
When you are sitting on the beach at the Equator on a windless day, the wind is really moving about 1700 km/hr; it just happens to match the speed that the ground is moving.
When you are sitting near the pole on a windless day, the wind really is not moving velocity = 0 km/hr!
The reason for this is that the wind is tied to the Earth by friction, but that coupling is not perfect…the wind can have different velocities than the ground beneath it.
[[Fig. 18.16 Wind directions of planet]]
[[Fig. 18.15 Generalized Hadley Cell circulation]]
Follow air form equator to pole to explain wind directions,
Bending of air masses= Coriolis forcing.
Air not really bending. It is the position of the ground relative to the air above it that is changing.
[[Draw Earth with equator and 2 longitude lines]]
Wind- generated by heat gradients.
Equator gets most heat. Air rises here and moves away from equator.
But it doesn’t flow away in a straight line.
Note Earth rotates counterclockwise in Northern Hemisphere (from west to east).
At equator, speed of rotation = ~ 40,000 km/24 hrs =~1700 km/hr.
Near the poles, the speed of rotation is less- virtually 0 km/hr at rotational pole.
[[Draw ground speed versus air speed]]
ground speed = angular speed
air speed = velocity
What this means is that air moving from equator towards poles has higher west-eastvelocity than ground beneath it. So it moves towards the east.
By 30 degrees N or South, the air is moving due east. This air descends and returns to the equator to replace the rising air.
THIS MEANS THAT AT GROUND LEVEL, THERE IS A STRONG EAST TO WEST FLOW OF AIR, AND THIS DRIVES OCEAN CURRENTS.
[[Figure 18.16- general wind pattern over Earth]]
just point out wind flow at equator is to the west
=TRADE WINDS.
We’ll talk about the rest of the air movement later, but for now it is sufficient to know that these winds drive the ocean surface currents.
[[Figure 15.2- average ocean surface currents]]
Note the GYRES = coriolis forcing
Note this means western sides of continents COLD
From Equator to pole we find the following temperature and salinity effect:
[[Figure 14.3- salinity and temperature by latitude]]
explain that temp is high at equator and low at poles
NOW LET”S LOOK AT THE OCEANS IN CROSS SECTION TO SEE HOW TEMPERATURE AND SALINITY VARY WITH DEPTH
[[Figure 14.4- temperature by depth]]
There is no temperature change at the high latitudes- the water is cold from top to bottom.
Near the equator, there is warm water near the surface, but this drops off rapidly by 1 km depth. This drop is called the thermocline.
Why is there a thermocline at the Equator? Surface waters are hot. This causes surface waters not to descend (hot = less dense).
At poles, the water is cold from top to bottom- no heat-induced density change
Density layering
[[Figure 14.5 pycnocline]]
Density is a combination of temperature and salinity
Salinity control
Oceans are salty
~ 35 ‰ salt
Relatively constant, but local concentrations change by the addition or removal of water
Addition of fresh water = runoff, melting of sea ice, melting of glaciers
Subtraction of fresh water= evaporation, production of sea ice, production of glaciers.
Sea ice affects the high latitudes salinity seasonally.
Ocean density also has the rapid change just like the thermocline. It is called the pycnocline.
[[Figure 14.6. Three layers of the ocean]]
1) surface mixed zone ~ 2% of ocean volume. Doesn’t reach the polar oceans.
2) transitional zone ~18% represents the thermocline and pycnocline region.
3) Deep zone ~ 80% of ocean.
This creates a very stable water column near the equator that doesn’t mix much vertically. However, at the poles, there is no density of thermal layering, and so small changes in density or temperature sends water from the surface to the deep ocean.
This process sets up one of the most important heat transport systems in the world- the thermohaline circulation.
[[Figure 15.6: Thermohaline circulation model]]
Formation of sea ice at poles causes density increase and water sinks to the bottom.
Cold water fills the ocean basins.
Warm water moves from equator to pole to replace lost water.
Occasionally this system breaks down.
End of last ice age = fresh water floods N. Atlantic. Sea ice does not increase salinity.
Thermohaline circulation stops.
[pictures of 8200-year event in North Atlantic]
[Picture of Day After Tomorrow]
1) Waves
In water, waves transfer energy rather than transferring material; a wave can traverse the ocean in a day (e.g. Tsunami) but water takes several years to circulate.
[[Figure 14.2- waves, oscillations ]]
Label: Crest, trough, wave height, wave length, period
In open ocean, waves are oscillatory waves- a cell of water follows a near-circular path
You feel this on a boat
the oscillatory motion decays with depth and by 1/2 wavelength the oscillatory motion is gone.
Note that the wave energy dissipates because the force that creates the waves is wind, which is a force at the surface.
Water does move a little bit in the direction of the waves, but most of the motion is in the wave (this is the transfer of energy rather than translation of water)
==wave base
at shoreline energy changes from wave of oscillation to wave of translation= the energy is transferred from one body to another.
How waves break on shore
[[Figure 15.10 ES]]
Note that the waves no longer follow a circular path; they drag due to friction at the bottom. Eventually the crest of the wave advances far enough ahead that it is no longer supported by the wave. It breaks.
[[Fig. 15.13- wave refraction toward headlands]]
Irregular coastlines cause wave energy to focus and diverge.
Focus on headlands
Diverge in bays = dissipation of wave energy causes particles to settle out of water- develops
2) Currents:
[[Figure 15.2 ES Global surface currents]]
Long-shore currents:
Waves do not usually approach the shoreline perpendicularly. Usually they approach at an angle and bend (refraction) as the waves feel bottom.
(we already talked about waves bending in the direction of lower velocities, but let’s review it for shorelines.)
In the surf zone where there is lots of turbulence, this moves sand down current.
AND
Longshore transport == Beach drift
- this occurs on the beach.
[[Figure 15.14- longshore transport]]
Inward trajectory= at a slight angle
Outward trajectory= perpendicular to shoreline= controlled by gravity rather than waves.
Sand accumulation or erosion:
Swash = inward movement of water
backwash = outward movement of water
When low wave activity, most swash soaks into beach and returns as groundwater flow to the ocean. This leads to accumulation of sand that is washed in by the waves. When greater wave activity, beach gets saturated and backwash above ground, taking sand back out with it.
[[Fig. of rip current from elsewhere]]
Outflow of water not uniform. Some areas concentrate on outward flow = rip current.
How to get out of rip current: swim parallel to shoreline: Rip currents usually less than 20 meters wide. You can swim safely to shore once you are out of the current.
Erosional Landforms:
wave-cut cliffs: erosion at base causes slides, hence steep slopes.
wave-cut platform: At wave base
marine terrace: uplifted wave-cut platform
sea arch: refraction leads to caves on both sides of headland; they meet
sea stack (fallen arch)
Depositional landforms
1. sandspit= extension of beach into a bay.
2. baymouth bar= spit crosses whole bay
West Coast problems: Damming the rivers eliminated a large portion of the coastal sediment, so beaches are thinning. This leads to less wave energy dissipation so headlands erode faster.
East Coast problems: Development on barrier islands, which naturally move.
Shorelines are the interface of the ocean with the land. To think that they should remain in n the same place is unrealistic. They will migrate as forces that affect the coastline change.
Seawalls- keep wave energy from eroding base of seacliff. Problem- the reflect wave energy outward, and so waves carry beach sediment offshore.
Groins: capture sand. Problems they starve places downcurrent and can cause erosion there.
[[Figure of groins with sediment trapped]]
Note downcurrent has depleted sediment
Jetties: extend out and block sand from filling a harbor entrance
[[Figure 14.16 = a jettie]]
Breakwater: barriers that cause wave energy to dissipate offshore. Problem that causes sedimentation at shoreline.
Beach nourishment: adding sand artificially
F05.303.L24
Ch. 16: Running water,: Erosion and sediment transport, river systems, meanders, baselevel
What can I say about running water and ground water? For most of the world running water and ground water are simply processes that sculpt the landscape.
In the western United States, running water and groundwater have a whole different meaning. We live in a semi-arid desert, and only through the largest irrigation project in the world’ history have we made the west habitable for us. So massive was this irrigation effort that even today most of us regard water as if it were an unending supply. But this isn’t true. Only one or two years of draught can cause massive crop losses and require major water-saving efforts from us all.
With regard to surface water, forecasts of long-term climate change predict that rainfall will not remain the same in the coming century as it was in the last one. Most scientists think that rainfall will change- at least in geography if not in total amount.
But surface water is only part of the Western US’s water story. A significant fraction of our water use comes from groundwater- the water found in pore spaces of loose sediment and in fractures and voids in rock.
[[Figure of 1990 irrigation- total groundwater withdrawl]]
The problem with groundwater supplies in the United States is that much of it came from non-renewable water sources:
[[Fig. 17.B Aquifers formed and later separated from their recharge sources: Ogallala Fm. recharge = Rocky Mtns.]]
and the melting glaciers during the last Ice Age
[[extent of pluvial lakes throughout US]]
and those glaciers are gone. So the water is not being replenished as quickly as it is being pumped from the ground.
As a consequence, the aquifers are collapsing, and the pore spaces that once held water are irreversibly lost; you can remove pore spaces by removing water, but you cannot pump the water back in if the pores are gone.
Added to these supply issues are contamination caused by sewage, leaking gasoline tanks, urban runoff, and salt, and you can start to see how troublesome water is becoming for the western US.
Many of you will go do work in the business of managing water resources.
Predictions of future climate change for California
[taken from talk to Sonoma County Leadership Institute, given 4/9/2005]
quick error estimate=
standard deviation from the mean =
mean = xbar = sum(xn)/n
stdev = sqrt[(sum(xi-xbar)2 /n]
Not that differences are squared because equal number of data are above and below the mean, so if not squared, they’d cancel to = 0.
Other water issues for California and the West
Salt may not be the first thing that comes to mind when you think about pollution, so let me give you some examples. The Great Salt Lake didn’t used to be salty. Neither did Mono Lake.
[[Figure 18.30- the extent of pluvial lakes in the Basin and Range]]
Let’s look back at the west of 10,000 years ago. Pretty remarkable huh? The west was much wetter back then., for two reasons. One was that the great ice sheets created their own weather and led to wetter conditions throughout the Basin and Range but also there was water draining off the glaciers, particularly as they retreated.
Salt is also a problem for agricultural land, and one way to keep salt from building up is to irrigate fields with lots of water so that the salts run off instead of accumulate in the soil.
Approximately 5% of cropland is affected by salt build-up in the Southwest US right now source= http://ic.ucsc.edu/~flegal/etox80e/SpecTopics/Salinity/
Many of you may have noticed when driving by farmland that it seemed like the farmers were wasting a lot of water. While that may be partially true, the farmers are not being totally irresponsible. They are making sure that salt does not build up in their soils.
The unfortunate consequence is that run off has lots of salt, and so if you rely on water sources downstream from agricultural land, your water will be salty; sometimes very salty.
As a consequence of all these problems, water is to today’s geologist what oil and gold were to the geologists of the last century. Many geologists are finding careers in planning for tomorrow’s water needs, and repairing the tremendous damage that has been inflicted on our water supplies. Whereas geologists 100 years ago were mining the coast range for mercury and other metals (and abandoning their mines with no reclamation plans undertaken), today’s geologists are working with other environmental scientists to identify these old contamination issues, model their transport through the surface and subsurface waters, and implement reclamation programs to clean up the messes left by our predecessors.
I would like to suggest a book for those of you who are interested in reading more about the water issues affecting the American west. The book is called “Cadillac Desert” by Marc Reisner, This book is a fabulous documentary into the history of water in the US, and all the good and evil that accompanied it.
Chapter 16: Running water
The movement of water from the oceans to the land is driven by the external heat engine- the Sun.
The return path is driven by gravity.
The water can flow at the surface= runoff
or infiltration into the subsurface= groundwater
Runoff is the most important process that sculpts the Earth’s surface.
[[Figure 4.1. Note that groundwater is the largest reservoir of free-flowing water on land.]]
Flow of water:
Laminar flow: Water flows in straight-line paths, little transfer of energy into stream channel
Turbulent flow- water takes irregular path, sometimes even flowing uphill (eddies).
Generally, laminar flow transports water downstream slower than turbulent flow.
To understand the return path of water requires us to introduce the term:
[[Figure 4.7A- the concept of base level- the level to which streams erode.]]
Base level. The level to which stream valleys erode. Gravity will cause water to flow back to the ocean or other large body of water. We call the level of the body of water the base level, and streams will not cut the valleys below this level.
Ultimate base level= the ocean
Temporary Base level = a lake, etc., that is above sea level. These are all temporary storages, and ultimately erosion will remove them.
Controls on stream velocity
1) Gradient- steepness of the stream
steeper gradient = faster flow
2) Channel characteristics- shape, size, roughness
shape= semicircle= least water/surface contact
size = bigger= less water/surface contact
roughness= less rough = less water/surface contact
A large, semicircular, smooth channel will transport water fastest
3) Discharge- amount of water
more discharge = faster flow
[[Figure 16.7-longitudinal profile along the length of a stream.]]
point out Head and Mouth
point out gradient steeper at head, gentle at mouth
Note that the stream youngens landward.
Mature through time
Narrow “V” shaped valleys- tend to be young valleys.- energy of the stream goes into eroding the stream channel, so it downcuts rapidly.
Wide Valleys- the downcutting to baselevel has occurred already so the energy of the stream goes into widening the stream course- the stream works laterally to erode the walls of the valley, and produce wide flood plains.
Graded Stream- one where the whole stream system is in balance. The stream carries the materials supplied to it- the channel does not deposit or erode.
Because graded streams do not maintain balance for long.
They’ve already cut down to baselevel, and then the stream continues to mature inland. This leads to more runoff and finer sediment. These factors increase stream velocity, and so since the steam cannot downcut any further, it adjusts its gradient to compensate.
streams cut the outside of the riven bend = cutbank and deposit on the inner bank = point bar.
Creates impermeable surfaces = more runoff
Creates smooth surfaces = faster runoff.
Leads to flooding.
[[show fig 04-A, which has hydrograph for undeveloped and developed land]].
1) Dissolved Load
2) Suspended load flow velocity -> settling velocity; most sediment transport
3) Bedload – move by saltation
Competence: largest particle that can be transported; scales with the square of the velocity
Capacity: Amount of sediment the stream can carry- scale linearly wit volume of water
94% of all liquid freshwater is groundwater
A bit misleading because runoff moves through the hydrologic cycle much quicker than groundwater.
[[Table 17.1 – residency time of water in hydrologic cycle]]
groundwater = 280 years ~ 100,000 days
runoff = 10 days
In some places the rocks have interconnected pore spaces that allow the water to infiltrate and flow below ground level.
[[Figure 17.12- shows the water table and also porosity and permeability]]
Water adheres to grains by capillary action- the electrical charge attraction, plus the surface tension of water
water table: the level of saturation- where all pore spaces are full of water
zone of aeriation: above the water table
Porosity: the spaces in the rock
Permeability: the interconnectedness of the pores which allows water to move
e.g. some rocks have lots of pores (like pumice) but these pores are not connected, and so water cannot flow through them.
Aquifer – transmits water well
Aquitard- prevents the movement of water
Perched Aquifer Impermeable layer makes local water table- Good in wet times, but not in dry times- the perched aquifer will be the first to dry out.
water flows from high to low areas- no duh. But this means that water flows from the ground INTO streams, lakes, ocean, as they are all located at the lowest elevational points in their areas. this leads to continuous flow of water into streams year-round.
[[Fig. 17.6- flow of water into streams when the water table intersects the surface]]
Springs- when the water table intersects the surface. Keeps streams flowing year-round.- water will flow into aquifer during winter and out during summer. These are known as gaining streams
Streams that lose water to the subsurface = losing streams.
Drawdown in wells. The removal of water exceeds the rate of replenishment, and so the water table is locally reduced.
Darcy’s Law: explains the discharge rate of groundwater = rate of recharge.
Hydraulic gradient = slope = rise/run = change in elevation/ distance
Hydraulic conductivity = K = terms that takes into account permeability and viscosity of water
Discharge (Q) = K A(h1-h2) / d
where h1-h2/d = hydraulic gradient
A = crossectional area of aquifer
Leads to subsidence
Note that some subsidence is natural compaction (see New Orleans at 8 ft below sea level)
However, other compaction is due to the removal of groundwater and collapse of pore spaces- air cannot provide the support that water does!
See subsidence of San Joaquin Valley by 9 meters.
Note that loss of groundwater is one effect, but flooding is another effect- lowering the floodplain in areas leads to more frequent floods in those places.
Chapter 18: Glaciers and glaciation
Glaciers: Meticulous scientists have named every bump, lump, and clump created by a glacier. I don’t want you to know all of them. I think there are a couple of interesting things to note about remnants left behind by glaciers in the United States.
Valley Glaciers: Occur in mountain valleys
Ice Sheets: Much bigger. Cover whole continents. Presently two exist: Greenland, Antarctica
First, glaciers flow a lot like water does. Albeit slower. Glaciers flow under the influence of gravity, and they are slowed by friction with the walls of the glacier valley. As a consequence, the middle of the glacier flows fastest (same goes for water in a stream).
[[Figure 6.5- flow of glaciers like liquid, only slower]]
Glaciers erode much better than liquid water because frozen water can transmit shear stress (recall that fluids do not).
[[Figure 6.9. Growth of V shaped valley into U shaped valley]]
[[Figure with notes- why does it form a U shaped valley>
Erosion not concentrated at stream
sides not influenced by angle of repose
sides supports by weight of glacier
Glaciers form in mountain valleys that are originally “V” shaped in cross section. What glaciers do to “V” shaped valleys:
1) widens (V becomes a U)
2) deepens (proportional to amount of ice- trunk glaciers and tributary glaciers
causes formation of hanging valleys like YOSEMITE
3) straightens: flowing ice won’t meander
[[show pictures of Yosemite- Bridalveil Falls- a perched valley]]
Erosional Features of glaciers:
1) striations: grooves in rock left by passing ice with rocks in it
[[Show pictures of striations]]
Glacial deposits
Till: Sediments deposited directly from the melting of ice. Unsorted material of many different types.
Erratics: large boulders in till
Moraines: layers or ridges of till.
Lateral- along the side of the glacier
Medial- where two valley glaciers join.
[[Show figures of lateral and medial moraines]]
End – at the terminus of the glacier. If the glacier melts as fast as it advances, then lots of till can deposit here.
[[Fig. 5.17: The Ronkonkoma end moraine from the last ice age.]]
Ground- while the glacier recedes, it lays down a layer of sediment
Some well-known moraines: Long Island, Martha’s Vineyard, Cape Cod, Nantucket
Loess: Rock Flour that washes out of glacier and transported away by wind
1930s Dust Bowl a consequence of remobilization of loess by loss of vegetation.
Great Missoula FLood: Glacial lake Missoula held behind ice dam. dam meltith, lake runneth away.
-Created the Channeled Scablands of Eastern Washington State.
Earth’s orbital variations
Kepler’s three laws of planetary motion
1) The orbits of the planets are ellipses, with the Sun at one focus of the ellipse.
2) The line joining the planet to the Sun sweeps out equal areas in equal times as the planet travels around the ellipse.
3) Period of orbit ^2 = radius ^3
Eccentricity =95, 125, 420 kyr
e=1-(b/a)
a = semi-major axis
b = semi-minor axis
Earth's eccentricity has varied over time between values of 0.005 and 0.0607 (currently the eccentricity is 0.0167).
Gravitational attraction of planets to Earth.
Arctic Circle= 66.33 degrees N Latitude.
From minimum to maximum obliquity, the area of the Earth that is exposed to sunlight during the winter changes by ~ 300 km (I think this is too big) by latitude, and the distance around the Arctic Circle is about 12,000 km. That means a potential change in ice coverage by 3.6 million km2. That’s kinda a lot!
Equatorial bulge pulled on by Moon and Sun (Moon about 2/3, Sun about 1/3); tries to upright Earth. But since Earth rotating, the response is for the Earth to precess.
Precession in opposite direction of Earth’s orbit, so the same spot on Earth points to the sun more frequently than 26 kyr.
Precession, combined with eccentricity, affects the delivery of sunlight to any latitude. For any given season, the precession will be modulated by the eccentricity.
periods = 19, 22, 24 kyr
with 24>22>19 in spectral power
F05.303.L26
There were no written notes for this lecture. This was another area of DK's research, this one dealing with theories of causes of glacial cycles. Read through the presentation.