F08.303.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 lecture 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.
Suggestion: This will become a long file during the semester. To move directly to a lecture of interest use the "find" command and identify the specific lecture number ((e.g.Find "F08.202.L2").
A regrettable problem: Superscripts and subscripts are not activiated when I cut and paste them here from Word. Please make sure that you understand the chemical formulas to the extent that I ask you to do so in class.
F08.303.L1
The only notes of importance here are for you to reference the syllabus for all administrative matters with this class, and that you should frequently check this webpage for class materials, especially if you happen to miss a class.
F08.303.L2
Topics (write on board):
Big Bang
subatomic particles = Big Bang Nucleosynthesis
Geochemistry – isotopes, radioactivity
Expanding Universe
Supernovae
Nebular Hypothesis
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:
Subatomic particles (when Universe too hot for atoms)
Quarks (6): up, down, top, bottom, charm, strange
Baryons = 3 quarks (e.g. 1/3B +1/3B+1/3B = 1B)
protons and neutrons are Baryons
Leptons (6) = Non-baryonic matter
electrons and neutrinos are Leptons
Not a subatomic particle per se, but some thing we deal with
photons = massless carriers of electromagnetic radiation
Big Bang Nucleosynthesis
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.
(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]]
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 memory
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.
Isotopes: Neutrons are dealt with separately. Neutrons control the stability
of the atom (radioactivity) and are not represented in the periodic table.
[[Table of isotopes]]
Valley of stability = stable isotopes. Unstabe isotopes fall into the valley of stability through B- decay (from lower right) or from
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.
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.
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.
F08.303.L3
Topics
Nucleosynthesis
Doppler effect
Mars-sized impactor
Earth’s layers due to density stratification
Stellar Nucleosynthesis- Where heavier elements formed
Center of stars: Nuclear fusion : He to Fe and Ni will form in stars, but heavier elements form from additional source of neutrons: Slow and Rapid processes of nucleosynthesis
Nuclei still can capture neutrons, provided that a source of neutrons is nearby.
[[see chart that has the nuclides]]
S = Slow accumulation of neutrons ~100,000 years per neutron.
This allows isotopes to increase in atomic mass until a radioactive isotope is obtained. Then the isotope will decay to another element, which will then climb up the mass range for that atom.
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 be faster than beta decay, so unusual isotopes can be created. These accumulate neutrons until they reach the magic number bottlenecks. But they reach the bottleneck with lots of neutrons- unstable!
Source: Neutron star.
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)
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
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.
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 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.
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 (Jacobsen, Science, v. 300, 6 June, 2003, p. 1513-1514)
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.
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!
Sources:
1) asteroid and comet impacts
2) Radioactive decay
3) Gravitational settling
4) Latent heat of fusion (outer-> inner core)
F08.303.L4
Topics
Age of Earth paper
Mineralogy
Bonding
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
Mineral Defined: Naturally occurring solid formed by geologic processes with a crystalline structure and definable chemical composition.
Naturally occurring- must be found in nature, though if same mineral is produced by humans, then I’d consider it a mineral (consider salt).
Solid- means that atoms are not moving around
Formed by geologic processes- can include biological processes (i.e. biomineralization). Note that the latter case often are unstable outside the influence of the organism.
Definable chemical composition: Chemical Formula = simplest representation of the elements that compose the mineral (e.g. SiO2 is the chemical formula of quartz, but this can be a little misleading- it is not equivalent to a molecule such as CO2)
Orderly arrangement of atoms- atoms arranged in a repeating pattern, known as the unit cell. For instance, quartz has a unit cell that is the silica tetrahedron…Darwin 128 has ball and stick models that show the orderly arrangement of atoms for some common minerals.
Polymorph- same chemical formula but different orderly arrangement of atoms (e.g. diamond is a tetrahedron and graphite is a sheet).
Isomorph = same crystalline structure but different chemistry (e.g. NaCl and FeS2 are both cubic minerals).
Pseudomorph = shape a mineral may inherit as it grows to fill the evacuated space left behind as an unstable mineral recrystallizes).
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.
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.
Hydrogen: Electric attraction caused by the presence of water, which is a polar molecule.
NOTE: Many minerals have a combination of chemical bonds. The relative strengths of these bonds will determine the mineral strength, as well as preferential zones of weakness (if the weak bonds align). This will be the reason why some minerals have cleavage (fracture planes caused by weaker bonds) whereas other minerals do not (if all bonds of equal strength).
General rule: Covalent stronger than ionic bonds
Metallic, Van der Waals, and hydrogen bonds are weaker still.
[[See chart of atomic and ionic radii]]
More electrons in a particular shell = larger ions
This means that cations = small anions = large
[[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.
How minerals form:
Consider how you’d destroy a mineral
1. From melt (solidification of liquid)
2. Precipitation from a solution (solution reaches saturation)
3. Solid-state diffusion – atoms migrate to form new minerals that are stable in new environment
4. Interface between biological organism and rest of world
5. Directly from vapor (e.g. fumaroles)
Mineral classes
Determined by anions
1. Silicates (SiO44- tetrahedron)
2. Oxides (O2-)
3 Sulfides (S)
4. Sulfates (SO42-)
5. Halides (Cl- F-)
6. Carbonates (CO32-)
7. Native metals
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
F08.303.L5
Topics (write on board):
Layered Earth
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 CMB. They accumulate to form D’’ layer. The physical process of these excluded silicates rising to the top of the outer core is believed to be the mechanism that causes the outer core to convect.
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?
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
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
F08.303.L6
Topics
Forming melt
crystallizing melt
Forming melt
Geotherm = the temperature versus depth.
Flatter (greater heat loss per km) near surface and steeper (less heat loss per km) at depth.
Geothermal gradients
Beneath continent = 15-50˚C in upper few km; about 10˚C per km in upper 10s of km below that point.
Beneath Ocean crust = 50-100˚C (?)
Pressure terms: 1 atm = 1 kg/cm2 which is 1.01 bar. Since rock density is about 3 g/cm3 this equates to about 350 cm column of rock = 1 bar
1 km of rock = 250 bar = 0.25 kbar
Wet magma = contains volatiles
Dry magma = contains no volatiles
If you add volatiles to the mantle you will lower the melting temperature of the mantle by hundreds of ˚C.
Magma compositions: While magma mostly originates from the melting of mantle, the evolution of that magma before it cools will change its bulk chemistry.
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
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
Magma- partly molten material
3 components
liquid
solid
volatile (H2O, CO2, SO2)
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.
To address this, I will introduce the:
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 C
Crystallization 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.
mostly composed of silicate minerals. Fortunately there are only a few.
Mafic (or ferromagnesian) minerals = top of Bowens Reaction Series. Rich in Mg and Fe
[[Figure 6.18- mineralogy of common igneous rocks and magmas from 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
assimilation of crustal material during ascent.
Location: Convergent plate boundaries
subduction HEATS material (primarily by geotherm, though a small amount of frictional heat also can be created)
subducted crust has lots of volatiles (from surface)
These both drive volatiles into the mantle wedge, causing it to melt.
Location Continental rifts
This involves a heat source beneath continents (e.g. East Africa Rift)
substantial melting and assimilation of continental crust to form very weird volcanic rocks.
Spreading ridge = MORB
HotSpot volcano= from deep!
Subduction volcanoes = andesite
continental rifting volcanoes = ultra potassic
subduction = 2nd generation melt
continental rifting = 3rd generation melting
Each of these concentrates certain elements.
e.g. K increases in each generation.
SiO2 increases, but strangely not so much in ultrapotassic volcanoes- probably related to the inclusion of some mantle and some continental crust.
Each cycle increases SiO2 and excludes more Fe and Mg. This causes densities to decrease, which is why each type of crust is more buoyant than the previous one.
average composition = granodiorite =
SiO2 quartz
CaAl2Si2O8
(K,Na)AlSi3O8 feldspar
Ca2(Mg,Fe)5Si8O22(OH)2 (hornblende)
Density = 2.7 g/cm^3
Thin= Oceanic crust
composition = basalt
CaAl2Si2O8
(Ca,Fe,Mg)2Si2O6
olivine (Mg,Fe)2SiO4
average density = 3 g/cm^3
F08.303.L7
Topics
crystallizing melt
phase diagrams
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 weren’t 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.
mostly composed of silicate minerals. Fortunately there are only a few.
Mafic minerals = top of Bowens Reaction Series. Rich in Mg and Fe
average composition = granodiorite =
SiO2 quartz
CaAl2Si2O8
(K,Na)AlSi3O8 feldspar
Ca2(Mg,Fe)5Si8O22(OH)2 (hornblende)
Density = 2.7 g/cm3
Thin= Oceanic crust
composition = basalt
CaAl2Si2O8
(Ca,Fe,Mg)2Si2O6
olivine (Mg,Fe)2SiO4
Density = 3 g/cm3
Note that several of these minerals have multiple cations that can fit into the crystal. Which ones go in depends on composition and temperature.
Experimentally determined how the chemical composition of the solid and liquid phases change when in equilibrium (the crystals that form in a magma are usually in equilibrium at the time that they form).
What this means is that you can take a real rock, work out the compositions of its components, and then determine what the physical conditions were at the time of its cooling.
Particularly useful for crystals that accommodate multiple cations = solid solution series.
T vs. Composition
shows the phases that exist:
Liquid
Solid + liquid
Solid
Lines = liquidus and solidus
Start with Forsterite-Fayalite diagram (Mg2SiO4 to Fe2SiO4).
Point out end members
Forsterite begins to crystallize first
Composition of the solid phase at the start of crystallization that is in equilibrium with the liquid phase is determined by passing a horizontal line from the liquidus to the solidus
LEVER Rule
Horizonal line though diagram shows the equilibrium compositions of melt and solid at the same temperature.
As the crystallization continues, the proportion of crystals to liquid can be determined by the relative length of the horizontal line segments to the left and right of the initial composition.
Note that as crystallization continues, the melt become enriched in Fe. The mineral will change along with it if there is enough time.
Sometimes cooling is fast, and you can work out the temperature of the eruption from the chemistry of the melt and the phenocrysts.
Anorthite-Albite phase diagram
These are the end members of the plagioclase feldspar solid solution series on the right side of Bowen’s Reaction Series.
Draw initial composition of An50Ab50. First crystals have composition of An80. Through time what this means is that plagioclase feldspars will develop albitized rims around anorthite cores.
Important point about plagioclase feldspars. This diagram shows that it’s nearly impossible to have igneous phenocrysts composed of pure albite (especially albite cores). When assessing phenocrysts for alteration (which happens at surface conditions because anorthite forms at very high temperature and therefore is out of equilibrium at surface conditions), the presence of albite cores or albite replacement of anorthite is one of the first indications that you have alternation.
Anorthite-Diopside phase diagram
Bowen’s Reaction Series shows that two minerals can coexist in melt. The anorthite-diopside phase diagram shows how the crystallization of these two minerals affects the composition of the melt.
In the case shown the initial composition is 75% An 25% Di. The An + liquid is reached first and forms crystals of An100. The melt evolves toward the left until is reaches the eutectic point. Here there are two mineral phases that now are in equilibrium with each other. At the eutectic the remaining liquid will crystallize out at the euctectic composition (this composition needs to be determined by the atomic weight of each mineral’s chemical formula:
CaAl2Si2O8 = 40+27+56+128 = 278
CaMgSi2O6 = 40+24+56+96= 216
Di = 216/216+278 = 42%
Not that we’ll be dealing with the Phase rule specifically (you’ll get that in Ig-Met), but in case you’re interested in it, here you go:
Phase rule F + P = C + 2
F = degrees of freedom
P = phases = solid or liquid
C = components = end member compositions
F08.303.L8 &F08.303.L9 (for which I was absent). Hand out 9/25/08
Students- we’ve fallen behind so I thought to catch up in part by giving you my lecture notes to study. They cover the essential materials that I’d like you to know. Soils are part of the information that one will be tested on when trying to become registered geologist.
Ch. 7: Sediments, soils and sedimentary rocks
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) (Physical (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).
End product is detritus or grains, which we will characterize in the sedimentary rocks lab in two weeks.
Grain size classification
boulders >256 mm
cobbles 64-256 mm
pebbles 2-64 mm
sand 1/16 -2 mm (grit can be seen)
silt = 1/256-1/16 (grit can be felt with mouth, but not seen)
clay <1/256 mm (grit cannot be felt with mouth)
Grainsize chart (show them)
Grain sizes controlled by duration of mechanical weathering and composition of parent rock. Note that initial grains will consist of many crystals (they will be rocks) but eventually the grains will be reduced in size to the size of individual crystals (consider mechanical weathering of granite versus rhyolite).
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, but affects all silicate minerals) (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
dissolved 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 (Al4Si4O10 (OH)8
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.
Soils vs. sediment
Soils form in place
Sediments are transported from elsewhere
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. So soils appear different from parent materials.
Soil- a combination of mineral + organic matter + H2O + air
air= delivers and removes CO2 and O2
water = delivers and removes 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
The nature, intensity and duration of the weathering process influence the end product.
PHYSICAL FACTORS AFFECTING 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 (i.e. young = A+C, but no B horizon). For older soils, the effects of the parent material diminish and the other effects on soil dominate (climate); lead to formation of B horizon. Overall, older = thicker soil.
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). Note these horizons form in place; they are not transport features as are sedimentary strata.
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. Deposition of Fe- and Al oxides + hydroxides form red to yellow coloration; dry climates also include calcite, gypsum and halite, as insufficient water exists to remove the dissolved ions (so they re-precipitate here). Formation of these evaporite minerals leads to lower porosity, decreased intrinsic permeability, and so lower hydraulic conductivity.
C = partly altrered parent material (i.e. disaggregated)
Unaltered Bedrock
O+A+E+B = solum = true soil = that part of regolith with distinct soil horizons
O through C = regolith (=blanket of rock = all unconsolidated materials)
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)
Soils are classified into 12 Orders
Alfisol = pedalfer = Al+Fe rich soil
Andisol = young soil on recently deposited volcanic ash
Aridisol = Desert soil
Entisol = lightly weathered soil on recently-deposited sediment
Gelisol = weakly weathered soil in permafrost regions
Histosol = wetland soil (organic-rich, oxygen-poor)
Inceptisol = Young soil with slightly developed reddish B horizon
Mollisol = Grassland soils
Oxisol = Rainforest soils (nutrient-poor due to high infiltration and loss of nutrients)
Spodosol= highly leached, low fertility soils in cold, humid regions
Ultisol = Highly leached, low fertility soils = consists of Fe- and Al- oxides (e.g. laterites)
Vertisol = From very clay-rich parent material, leads to high shrink-swell potential.
F08.303.L10
F08.303.L10
Topics:
Bedding
Stratigraphy
Facies: record of an environment
Walther’s Law
Contacts
Lithostratigraphic units
Bedding
Bed- a layer of sediment with an identifiable top and bottom surface.
Bedding plane = surface between two beds. Develop as a consequence of change in lithology, or by hiatuses in deposition (which may be accentuated by changes in cementation).
Strata – series of beds
Bedforms
Ripples – cm-scale can be unidirectional or bidirectional, the latter being causes by tidal action.
Dunes – 10 cm – 100+ m-scale
Ripples and dunes are essentially the same thing, but form from stronger or weaker currents.
Cross beds – the internal laminations preserved in ripples and dunes
Slip face = steep side = leeward side. The ripple or dune creates a shadow zone where materials drop out of suspension.
Migrating ripples and dunes create bedding plane in which parts of the cross beds are preserved.
Graded bedding – clast size changes from top to bottom of bed.
Normal grading = fining upward- Indicates decreasing current, such as with
turbidity currents (large-scale submarine sediment flows that mix with water…as the turbidity current flows over a more gentle slope the suspended material settle, largest pieces first and finer ones later. The rock nmes for the deposite from a turbidity current is a turbidite. Each turbidite represents a unique event. Tops of turbidites consist of shale, which represents quiet deposition in between the times of sequential turbidites.
Reverse grading- happens when sediment source grows closer to site of deposition, as in the case of a delta that is building out into a body of water (= prograding).
Bedding surface markings.- indicators of events that occurred between times of deposition- mud cracks, scour marks, fossils (usually trace fossils). Helpful when trying to decide which way is up!
Stratigraphy- the study of layered rocks as the record of Earth history
1) Time
1a) Relative (this all we had until start of 20th Century)
1b) Absolute
Radiometric dating of various kinds
Leads to the absolute calibration of other chronologic indicators
Geologic Time Scale
The law of original horizontality
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:
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.
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.
Principle of Faunal Succession
Principle of Cross-cutting relationships
Principle of Inclusions
Principles of unconformities (nonconformity, disconformity, paraconformity, angular unconformity)
Strata that occur in vertical succession also occur in lateral succession
Def: Facies that occur in conformable vertical succession also occur in laterally adjacent environments
Utility: We don't often get the chance to view lateral facies changes in geology. More often than not we get to view vertical columns of strata, and from these we get to interpret geologic history, Walther's Lab enables you to identify unformities in the rock record, which will be identifiable by finding two facies in succession in a stratigraphic column that do not occur in nature in adjacent depositional environments.
F08.303.L11
Absolute ages
Using radioactivity to measure time
Parent= the radioactive isotope
Daughter = the element following radioactive decay
by changing number of protons, element changes- you find elements that have no physical business being in the mineral, and this leads you to speculate that it is a radioactive daughter product.
Half life= the amount of time needed for half the radioactive material to decay
[[Fig. 10.14. showing exponential decay of radioactive element]]
[[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 (10^13 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 ∆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]
F08.303.L12
F08.303.L13 & F08.303.L14
F08.303.L14
Topics a Brief History of Geologic Events
Interlude E: Fossils and Evolution
Note: Content of Chapter12 largely covered through past several lectures
Precambrian broken into three Eras:
Hadean = 4.57-3.8 Ga
Archean = 3.8-2.5
Proterozoic = 2.5-0.54
[[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
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
F08.303.L15
F08.303.L16
Topics: (These are from many lectures, mostly lectures 14-16)
seismic waves
P, S, Surface
amplitudes
Magnitude scales and what they are based on
refraction (In air, ocean, rock)
California Earthquake shake computer models for 1906, 1989 earthquakes
view at: http://earthquake.usgs.gov/regional/nca/1906/simulations/movies/sf1906santarosa.mov
Plate Tectonics and the San Andreas Fault System
view at:
http://emvc.geol.ucsb.edu/animations/quicktime/sm02Pac-NoAmflat.mov
Faults (normal reverse, strike-slip
The relative influence of gravitational force in each.
restraining and releasing bends
Earthquake damage
Volcanic Seismicity (Video: Volcano’s Deadly Warning) – synopsis of video can be Googled if you missed the movie
Notes:
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
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.
MAGNITUDES
Richter Magnitudes:
Moment Magnitude
Modified Mercalli
[[Travel-time graph enables you to determine the distance.]]
Note that arrival times enable you to use a chart to determine distances to earthquake.
Amplitude of signal is the displacement of the seismograph drum relative to a stationary mass = the pen
Richter Magnitude = maximum amplitude of seismogram and S-P travel time delay (equates to distance). Connect these two values on the time-travel graph, and that line passes over the Richter Magnitude.
Moment magnitude- does not rely on maximum amplitude. Rather is it is measure of the total elastic energy released by an earthquake. = rock strength X fault rupture length X average fault offset. This is more difficult to measure and often requires field investigation to produce a final magnituide.
Mercalli Magnitude = qualitative measure of shaking based on observation. This is important for emergency response because shaking intensity is not always proportional to distance from the earthquake hypocenter- the density of the surface geology can severely amplify the shaking by slowing the passage of seismic waves.
[[ Typical Seismogram. ]]
[[ 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!”
Triangulation: Intersection of calculated distances from several seismograph locations determines the unique location of focus.
Western North America and San Andreas Fault specifics
Topics
California Earthquake shake computer models for 1906, 1989 earthquakes
view at: http://earthquake.usgs.gov/regional/nca/1906/simulations/movies/sf1906santarosa.mov
Plate Tectonics and the San Andreas Fault System
view at:
http://emvc.geol.ucsb.edu/animations/quicktime/sm02Pac-NoAmflat.mov
Transform Boundaries- mountains will form at restraining bends in the faults. The restraining bends form local convergence, which causes thrust faulting, and local thickening of the crust. (ex. Transverse Ranges in Southern California)
Topics
California Earthquake shake computer models for 1906, 1989 earthquakes
view at: http://earthquake.usgs.gov/regional/nca/1906/simulations/movies/sf1906santarosa.mov
Plate Tectonics and the San Andreas Fault System
view at:
http://emvc.geol.ucsb.edu/animations/quicktime/sm02Pac-NoAmflat.mov
Composition of subsurface geology and its effects on the transmission of seismic waves
[[Figure showing crystalline, sedimentary, non-lithified sediment, bay mud, and how they change amplitude of shaking]]
The density of rock determines how quickly seismic waves and transmit through it. More dense = faster transmission.
Porous materials slow down seismic waves, causing the seismic waves to increase in amplitude similar to the increase in water wave height as ocean waves approach the shoreline.
Amplitude of seismic waves = amplitude of ground shaking, so slower seismic wave transmission is a bad thing!
Computer shake models of Northern California during the 1906 earthquake. Note the propagation of P, S, and surface waves cross the landscape at different speeds, therefore the first waves you may feel could be a warning signal that larger waves will soon follow.
Rule of thumb about seismic waves arrival and distance to hypocenter of earthquake. For every second delay between the arrival of P and S waves, the distance to the hypocenter increases by about 15 kilometers. Therefore if you are alerted of an earthquake by the arrival of P waves, and then notice an increase in shaking 5 seconds later (at which point you might interpret the increased amplitudes to correspond to S waves), then you can estimate the distance to the hypocenter to be about 5 X 15 = 75 kilometers. Note also that if you cannot let go of the English system, the relationship would be 1 second delay corresponds roughly to 10 miles distance to hypocenter.
This information can help you gauge the seriousness of an earthquake right away, and can give you some sense of whether you are likely to be close to the areas most seriously affected, or whether you are likely to be in a position to lend assistance to others who are in greater need of help.
Plate tectonic context for the San Andreas Fault
Note that the San Andreas Fault is just one of several faults in California that accommodate the displacement between two major tectonic plates – North America and the Pacific Plate.
Note that this model (can be downloaded and viewed at: http://emvc.geol.ucsb.edu/animations/quicktime/sm02Pac-NoAmflat.mov
Note that this movie shows that 38 Ma the San Andreas Fault did not yet exist. The Pacific spreading ridge (in plate tectonic terms this is a divergent plate boundary, which causes normal faulting) existed offshore, with the eastern ocean plate called the Farallon Plate and the western ocean plate called the Pacific Plate. The white arrows show the direction of plate movement relative to a fixed North American plate. The Farallon plate collides with North America and subducts beneath it (in plate tectonic terms this is a convergent plate boundary). The collisional boundary is a fault where the footwall (Farallon Plate) moves down relative to the hanging wall (North America). This is a Reverse Fault (also called a thrust fault). The volatile-rich subducting plate produces magma once it reaches a depth of about 100 km, and hence is responsible for volcanoes all along the Coast of North America. Note that the Pacific Spreading ridge consists of spreading segments, each offset from its neighbors by strike-slip faults. Strike-slip faults that offset spreading center segments are called Transform faults in plate tectonic terms.
Note what happens at about 25 Ma. The Pacific Spreading ridge collides with North America, and so there is no more Farallon Plate to subduct. In fact, there is no more convergence along those parts of the coast where the spreading ridge has collided with North America. Additionally, there is no more divergence along these segments of the Pacific Spreading ridge, even though divergence continues to the north (we call this northern remnant of the Farallon plate the Juan de Fuca Plate) and south (we call the southern remnant the Cocos Plate). The fault that now exists where the spreading ridge meets the North American Plate is the San Andreas Fault. But as I mentioned above, a fault that connects two segments of a spreading ridge is a transform fault. Hence, in plate tectonic terms, the San Andreas Fault is a transform fault. The SAF continues to grow in length as more of the Pacific Spreading ridge collides with North America.
Liquefaction- shaking allows grains to settle in porous material (=densification). Water is forced by pressure to escape upward. If under building foundation…bye to the foundation.
Tsunamis (aka tidal waves). Produced by displacement of sea floor as crustal plates slip.
Note that waves slow as they approach shallow water. Friction causes them to slow, but to conserve energy, waves increase in amplitude
tsunami wavelength = 100-700 km – so can take half an hour to spill up the beach.
Video- Volcano’s Deadly warning
1. Measuring volcanic gas output
Fumaroles- gas escapes to the surface through these conduits
Stanley Williams- believed that physical measurements of gas needed to be used to identify immanent eruptions.
CO2 +SO2are gases that come from the magma, not the porewaters of the volcano (i.e. H2O)
Good for monitoring open volcanoes. But if magma seals the gas conduits, then low gas could be a sign of immanent eruption. On Galeras, it was interpreted as no gas to be measured = safe.
Video: Volcano’s Deadly Warning
Bernard Chouet realized that harmonic resonance caused by movement of magma increased significantly just prior to eruption.
A-type earthquakes – caused by rock fracture associate with magma injection into country rock. Te injection is done under pressure, and causes rocks to burst apart and magma to intrude.
Seismologically these appear very similar to tectonic earthquakes, with a sudden rise in seismic amplitude and a gradual decline.
Difference- these occur in clusters, rather than as aftershocks. Magnitudes are relatively small (M3-5).
B-type earthquakes- different pattern. Long period seismic waves that slowly builds up and slowly decreases. This is more like a symmetric sine wave.
Cause: Pressure caused by magma movement in magma chamber builds up, and B type seismic waves increase in number just prior to eruption (meaning within a few days).
Successful in predicting several notable eruptions, including:
Popocatepetl, Mexico
Colombia- Novado del Ruis- the signal was detected but people not warned. 1985- 25,000 people died by lahar- a volcanic mud flow
1989- Redoubt, Alaska – successfully predicted, saved oil workers.
Galeras, Colombia, 1991- Formed a lava dome (indication that magma is near surface. So long as gases continue to escape (open dome) from fumaroles, then pressure is not building. When they stop letting out gas (closed dome), then the pressure is building.
F08.303.L17
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 strength (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 empirically (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.
F08.303.L18
Chapter 4: Plate Tectonics
Topics
Paleomag.
Plate boundaries
Compelling story that launched modern thought:
The ferromagnetic minerals: Fe2O3 Fe3O4 FeS2 plus some others (griegite)
The ferromagnetic minerals: Fe2O3 Fe3O4 FeS2 plus some others (griegite)
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
Declination: the angle between the geographic north pole and the magnetic north pole, measured clockwise.
What causes remnant magnetization?
Natural remnant magnetization(NRM): fossil magnetism.
In igneous Rocks:
Curie temperature: the temperature at which magnetite seals in its magnetic signature ~540C for magnetite.
In igneous rocks this is known as: thermal remnant magnetization(TRM) If magnetite is in melt, the atomic-scale magnetic fields within the grain can align themselves with the Earth’s magnetic field (note that the grains do not physically rotate).
Other rocks can be heated above the Curie Temp. and so lose their NRM.
In sedimentary rocks: detrital remnant magnetization mineral grains are already below the Curie temperature and so the grains behave like bar magnets…if the grains are free to rotate, then the magnetic properties will rotate the grains until they are aligned with the magnetic field. Sometimes this is known as Depositional Remnant Magnetization(DRM).
Then, if the sediments compact, the grains will seal in the magnetic field.
Note that this is called the “lock in depth”. The interesting question for doing high-resolution paleomag work on the B-M reversal is addressing this lock-in depth question.
Growth of new minerals that record a different field of magnetization. (Chemical remnant magnetization (CRM).
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.
[[show quicktime movies of East Pacific 80 Ma,
Atwater model of San Andreas Fault
Examples:
[[Figure 19.17]]
Each plate is bounded by a combination of these three plate boundaries. These tell you whether the plate is going to grow or shrink.
For instance. Philippine plate surrounded on all sides by destructive margins- it’s doomed.
Juan de Fuca, Cocos and Nazca plates have a destructive and a constructive margin, but the destruction is happening faster than the construction.
Pacific Plate has been consumed to make room for the Atlantic Plate.
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
F08.303.L19
(some of these lecture notes are missing! I have misplaced them- probably at school. I will upload them probably on Monday)
Topics
Chapter 4 (continued)
Triple Junctions
Mechanisms for plate tectonics
Theories about what drives plate tectonics
Ridge-push, slab-pull model.
To first order plate tectonics is driven by gravity. Midocean ridges are topographically high, and have a outward-directed force that pushes lithosphere away from the ridge (ridge-push). Sinking lithosphere at subduction zones leads to the descending slab pulling the lithosphere down into the mantle (slab pull).
Secondary effect- mantle convection added and subtracting from the motion created by ridge-push slab-pull. If plate conveyance is in the direction of mantle convection then these add together to move a plate quickly. This generally causes plates to subduct with a low angle of the Benioff-Waditi zone, cause volcanism to occur at great distances from the trench. If the mantle convection is in the opposite direction, then the descending lithosphere gets deflect downward, creating a very steep subduction zone (and hence volcanism near the trench).
F08.303.L20
Topics
Ch. 18: Oceans
Interlude F
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. Wind directions of planet]]
[[Fig. 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- 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- average ocean surface currents]]
Note the GYRES = coriolis forcing
Note this means western sides of continents COLD
Density of sea water 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.
Thermohaline circulation.
[[Figure: 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]
F08.303.L21
Ch. 18: Oceans (continued)
Topics:
Coastal currents
Coastal landforms
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: waves move in 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
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.- 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: 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 - 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
F08.303.L22
Topics
Streams
Profile of a stream
Baselevel
Mature streams- why they meander
Climate causes of excessive rainfall
Streams- Transport water at the surface.
1) Gradient- steepness of the stream
2) Channel characteristics- width, shape, roughness
3) Discharge- amount of water
[[Fig.: Stream profile- concave upward]]
Sediment load
Capacity = the total amount of solid material a stream can transport
Competence- the largest particle a stream can transport
Dissolved load- dissolved ions
Suspended load- transported in suspension
bedload- moved along the streambed
To understand the return path of water requires us to introduce the term:
Baselevel- the level to which streams erode.
Both ultimate and temporary baselevels exist.
[[Figure: the concept of baselevel- the level to which streams erode.]]
Ultimate baselevel= the ocean
Temporary Baselevel = a lake, etc., that is above sea level. These are all temporary storages, and ultimately erosion will remove them.
Narrow “V” shaped valleys- tend to be young valleys.- energy of the stream goes into eroding the stream channel, so it downcuts rapidly.
Wide “U” shaped 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.
Note that the stream youngens landward- continued downcutting increases size of drainage basin.
Defined: "A deposit, partially subaerial, built by a river into or against a permanent body of water"
Importance: sediments accumulate fast and can contain large deposits of oil, coal, and gas, which are trapped by pinching-out layers
Interactions at base level (where river meets stationary body)
Density contrast
inflow > sea = sheet flow along base causing submarine fans
inflow = sea = rapid dispersal causes Gilbert-type deltas (lake)
inflow < sea = typicial marine environment- dispersal dependent on what's happening in the basin:
3 Main basic types of deltas:
1) River Dominated- rapid seaward progradation
birdsfoot or lobate
sand bodies perp. to shoreline
coarsening upwards sequences
Hi angle unidirectional X-beds
2) Wave Dominated deposits reworked
form stacked beach ridges
sand bodies parall. to shoreline
backed by lagoons or marshes
3) Tide Dominated delta deposited asymmetrically
high tide may back into the rivers
Lots of tidal flats - thickness may
be the only characteristic feature
Deltaic Cycles- (term only really valid for river-dominated deltas)
Constructional Phase: active progradation of the delta
Destructional Phase: abandonment of lobe for easier route
Rate of superposition depends on sedimentation, subsidence, tectonics, and compaction rate.
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.
Creates impermeable surfaces = more runoff
Creates smooth surfaces = faster runoff.
Leads to flooding.
[[show fig 04-A, which has the runoff profile for undeveloped and developed land.
Erosion is a consequence of the moving water. Water flows under the influence of gravity, and so low-lying areas are where water concentrates. The movement of water is a primary mechanism for both mechanical and chemical weathering, and so water works to carve drainages through rocks.
SALT
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 6.21- 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.
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.
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.
Why does flooding occur?
A: Increase in rainfall and/or poor land use planning.
Climate events that lead to increased rainfall in USA
Stalled Midlatitude cyclones
1993 Flood: Greatest Flood in US History
What happened? Early snow melt saturates ground
High pressure over East Coast prohibits Midlatitude cyclones from migrating East
Rainfall all occurs in the Mississippi and Missouri River Valleys
At St. Louis, Mississippi River above flood stage for 144 days.
[[Lots of photos about the damage]]
St. Louis – floodwalls create bottleneck, leads to upstream flooding
El Nino events
Normal climate pattern is for the Trade Winds to blow east to west across the Pacific, building up a pool of warm water in the western Pacific. This leads to increase rainfall for Indonesia and Australia. During El Niño events, the Trade Winds die off and the warm water moves eastward across the Pacific, bringing with it the intense rainfall to the North American Southwest. This leaves the west Pacific without its normal rain source, and so drought conditions develop there while we get soaked.
F08.303.L23
Chapter 16: Landslides and mass movements
Topics:
slope processes
human interaction
minimizing hazards
Physical factors that cause mass wasting
Angle of repose- the angle at which rock strength equals the gravitational force= the steepest slope a material can attain before becoming unstable.
Types of mass wasting:
1) FALLS: free fall of material off free face of cliff
2) SLIDES- movement of cohesive block of material
a) slump- soft, cohesive material moving along a curved slip face
3) FLOWS: movement of unconsolidated material
a) slow flow = creep
b) rapid flow= earthflow, mudflow, debris flow
Also discuss earthflows, mudflows, rockfalls, snow or debris avalanches
LAST TWO IMAGES: signs of mass wasting
Prevention tools: change the angle of repose and eliminate water.
Slumps- the most common type of mass wasting around here.
Slump = the downward sliding of a mass of material along a curved surface.
Forms a recognizable topographic feature= a scarp
[[Draw the plan view and cross section through a slump]]
Mass wasting types:
lahars
subsidence
natural and unnatural compaction
sinkholes
Prevention
Crescent-shaped cracks in hillsides
Tongue-shaped area of soil or rock on the hillside
Large boulders or talus at the base of a hill
Linear path of cleared or disturbed vegetation
Exposed bedrock with sedimentary layers parallel to slope
Irregular land surface (hummocky)
bent trees
tilted fenceposts
Pre-existing landslides (subsequent erosion can lead to reactivation of an older landslide)
Case studies: La Conchita (2005), 1995
Problems:
Very steep, high slopes. Uplift of coastal terrances at La Conchita is extreme. 40,000 year-old terraces at 180 m above sea level. That’s 4+ meters of uplift per 1000 years.
Presence of weak rocks. The “rock” at La Conchita is mostly loosely-lithified sediments, ranging from clay to silt to sand. It is typical beach material for California.
Presence of historic and prehistoric landslides. The landslide of 2005 occurred by reactions of the 1995 landslide; they knew it was coming, but they just didn’t want to believe it could happen again. Southern Pacific Railroad wrote reports in the 1800’s that stated they felt the railroad should not be built through this area for fear of slides.
Chapter 14: Energy and mineral resources
Coal
Oil and natural gas
United States
Middle East
California
CAFÉ = Corporate Average Fuel Economy. Note that the USA is the worst of the industrialized countries when it comes to fuel economy.
US consumption per day
1 barrel oil = 42 gallons
TOTAL = 20.0 mbl/d (million barrels/day)
motor gasoline = 9.0 mbl/d (44% of the total)
distillate fuel oil consumption was 4.1 mbl/d (20%)
jet fuel consumption was 1.6 mbl/d (8%)
residual fuel oil consumption was 0.8 mbl/d (4%)
Fossils fuels are nature’s way of storing energy from the Sun. Our burning these fossil fuels releases this energy. Plants and animals use the Sun’s energy to create complex molecules of H+C+O+N. Normally, these molecules break back down into their constituent parts. But if the organic material is buried quickly, then the molecules can end up in an anoxic environment and the molecules will not decay.
Often the minerals that grow in this environment are the anoxic minerals. Sulfide minerals can accumulate along side oil reservoirs and in coal beds. As a consequence, extraction of fossil fuels also brings out large quantities of metal sulfides, which oxidize as the surface to form metals and sulfuric acid (H2SO4), and burning fossil fuels also releases SO2. which combines with water to form sulfate (SO42-)
Fossils Fuel groups:
Coal
Natural gas (CH4)
crude oil
methane hydrates
20% of all energy consumption in the US.
Plant remains that do not weather (oxidize) because of environment of deposition
Peat
Lignite
subbituminous
bituminous
anthracite
Evaluated based on rank and sulfur content. These combined determine how much pollution will be caused by coal burning
[[Rank- how much energy is released when the coal is burned]]
[[Sulfur content = high medium low]]
[[Changing peat to coal]]
burial and heat change
peat -> lignite ->bituminous coal -> anthracite
Of these bituminous is the best to burn (most efficient)
But it also has the highest sulfur content of the coals
Sulfur the biggest environmental concern associated with coal burning.
SO2
can go into forming sulfuric acid (H2SO4) = acid rain
or sulfate aerosols= atmospheric cooling due to sunlight reflection (potentially a good side effect)
Sulfate emissions associated with fossil fuel burning = 3 main sources: United States, Eastern Europe, China.
Pattern of global cooling loosely associated downwind from sources of sulfate emissions
Suggested that volcanic eruptions a good analogy for sulfate emissions
Mt. Pinatubo, 1991 eruption: Release of huge quantities of sulfur (15 –30 million tons of SO2) into high atmosphere led to 2-3 years of global cooling of about 0.3 C.
[[Figure: coal reserves by continent]]:
US has about 25% of world’s coal
New Developments with regard to oil supply
But as prices increase other sources of hydrocarbons can be brought to market economically. The big one these days is oil sands, which yield bitumen.
Canada’s Oil Sands have an estimated 1.5 Trillion barrels of oil-equivalent bitumen, which can be refined into synthetic oil (not used for gas, but can substitute for diesel and heavier fuel oils).