S09.102.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 "S09.102.L2").
S09.102.L1
No notes. Review Syllabus for instructors' expectations.
S09.102.L2
S09.102.L2
Topics
Periodic Table Light
Chemical bonding
SiO44- tetrahedron
Silicate minerals
Cleavage
So, here’s a quick tour of why minerals form the way they do. We need to start with Chemistry- atoms and the periodic table of the elements.
[[ Planetary structure of atoms]]
This is a simplified model, but it works just fine for our purpose.
Atoms consist of protons and neutrons in the nucleus, and it is surrounded by clouds (simplified version = rings) of electrons.
The number of electrons equals the number of protons, resulting in a neutral charge.
The number of electrons that can fit into each ring (shell really) is:
[[Planetary orbital model]]
2 for inner ring
8 for second ring
8 for third ring
18 for fourth ring
Atoms with the outer ring full of electrons are very stable- this is a property of the electron rings.
[[PeriodicTable]]
This is all the periodic table of elements tells us. Each row represents a ring of electrons, and the position within the row tells you how full the outer ring of electrons is.
Note: the number of neutrons is not depicted in the periodic table; neutrons are dealt with separately.
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 clouds (or rings, shells, etc.)
4) Columns= similar bonding behavior (same number of electrons in outer ring)
5) Elements REALLY want their outer ring of electrons to be filled. They do this by bonding.
6) Distance from noble gas affects how they do it (ionic versus covalent).
7) Neutrons are not represented in Periodic table.
Bonding (how elements achieve filled outer shell)
ionic bonding = transfer of electrons- weaks bonds. Electrons spend most of the time around one atom, so it is easy to remove the other atom from the bond.
covalent bonding= sharing electrons. Electrons spend similar amount of time around each atom, so no atom is weakly bonded = strong! Si and O bond covalently, and so very difficult to break quartz bonds.
[[Examples: NaCl = ionic bonding; SiO44- tetrahedron = covalent]]
Note that the mineral formula does not mean that separate molecules form; the formula repeats over and over to build crystals.
Unit cell= the simplest repeat formula for the mineral
Silica tetrahedron:
Tetrahedron = 4 bases
Silica tetrahedron is the basic structure upon which the silicate minerals of Earth form.
Silicate minerals
Earth’s mantle and crust are mostly made of silicate minerals.
The SiO44- tetrahedron is a compound that has unsatisfied charges on the oxygen ions. How they satisfy those charges determines what mineral forms:
[[common silicate minerals]]
Note that the silica tetrahedra join to form more complex structures as you go down the chart. Also note the chemical formulas at left. They show the other elements that get involved to satisfy charges. These are all ionic bonds, and so they are the ones that will break. In other words, the shape of the silicate structure in each mineral determined how the minerals will cleave.
S09.102.L3
S09.102.L3
Topics: Context
Layered Earth
Moon-forming event
Earth’s heat
Rock Groups
Mineral stability
Igneous rocks
Sedimentary rocks
Metamorphic rocks
Moon-forming event
Moon now believed to have formed from a large asteroid (the size of Mars) colliding with early Earth. Moon is chemically very similar to Earth’s Mantle. But the Moon has no core! This means that the Earth had to be separated into a core and mantle before the asteroid hit us, so that the asteroid only excavated the outer part of the Earth. This material was ejected into orbit around Earth, where is then coalesced into the Moon.
Note: Moon too close to be a captured asteroid; Moon has been moving farther away from Earth through time, due to the gravitational drag of Earth’s water on the moon. This tidal drag causes the Moon to slow its orbit, and this has caused it to recede.
Earth’s Heat
Earth is still very hot! Center = 6000C! That’s as hot as the surface of the Sun!
Heat sources:
1) Radioactive decay (U, K, Rb)
2) Heat of impacts
3) Heat of gravitational settling
4) Latent heat of fusion (heat released as liquid metal outer core solidifies to form inner core).
Heat causes materials to melt, which allows heavier materials to sink to the cores (Fe, Ni) whereas the rocky materials (particularly rich in Si, O and Al) float to the top.
Earth is density-layered: we will see this again during the course.
Earth’s crust:
Oceanic crust (basalt) Density = 3 g/cc
Continental crust (granodiorite) Density = 2.7 g/cc
This is why the continents float higher on the mantle than do the oceans.
Mineral Stability: Minerals form in equilibrium with environment – tem., pressure, chemistry. If those conditions change then minerals will often become unstable.
S09.102.L4
S09.102.L4
Topics:
Rock Groups
Igneous rocks
Sedimentary rocks
Metamorphic rocks
Ch. 4: Igneous rocks
Bowen’s Reaction Series
How form magma
Where form magma
Rock Groups
Brief Intro- show large pieces of each rock group
Igneous = from root word “ignite” = from fire. These are rocks that form from magma.
Sedimentary = from “sedimentum” = to settle. Sediments are formed at the surface by weathering and transport.
Metamorphic = from “metamorphosis” = to change form. These are rocks that have changed under the influence of pressure and temperature; a new group of minerals will form in equilibrium with the new environmental conditions.
Igneous Rocks
Source of igneous rock = molten rock from the mantle= magma. When rock melts, it expands and so is less dense. It then rises toward the surface.
Ways to melt mantle:
1) decrease pressure
2) increase temperature
3) Add water (or more generally- volatiles = dissolved gases. this lowers melting temperature- allows ions to mobilize easier.
Several places where igneous rocks form
1) Midocean ridges = decompression melting
2) Hotspots = mantle plumes from core-mantle boundary start off hot and then undergo decompression melting during ascent to surface
3) Subduction zones = subducting ocean crust brings water into mantle which lowrs the melting temparature of ther mantle.
In magma chamber
Cooling leads to crystallization. This happens in a predictable way, and depends on the chemical make up of the magma.
Crystal settling- denser crystals can sink to bottom of magma chamber, this causes residual magma to change chemistry.
Igneous rocks: Extrusive if erupted onto the surface
Intrusive if solidified below ground.
Bowen’s Reactions Series
Q: What determines whether magma will produce extrusive or intrusive rocks?
A: Density to first order.
[[Put up Bowen’s Reaction Series]]
Bowen’s Reaction Series
NL Bowen conducted lab experiments with basaltic rocks to see how the melted and crystallized
Found order to crystallization:
Olivine Ca plagioclase feldspar
pyroxene
Amphibole
Biotite
K feldspar Na plagioclase feldspar
muscovite
quartz
[[Show Si-O bonding Figure for different silicate groups]]-
Si-O become enriched in the final stages of the crystallization process.
Mg, Fe, Ca become depleted in melt
What happens to Igneous Rocks at surface?
S09.102.L5
S09.102.L5
Topics:
Ch. 4: Igneous rocks
Bowen’s Reaction Series
crystal settling
residual magma
In magma chamber
Cooling leads to crystallization. This happens in a predictable way, and depends on the chemical make up of the magma.
Crystal settling- denser crystals can sink to bottom of magma chamber, this causes residual magma to change chemistry.
Igneous rocks: Extrusive if erupted onto the surface
Intrusive if solidified below ground.
Bowen’s Reactions Series
Q: What determines whether magma will produce extrusive or intrusive rocks?
A: Density to first order.
[[Put up Bowen’s Reaction Series]]
Bowen’s Reaction Series
NL Bowen conducted lab experiments with basaltic rocks to see how the melted and crystallized
Found order to crystallization:
Olivine Ca plagioclase feldspar
pyroxene
Amphibole
Biotite
K feldspar Na plagioclase feldspar
muscovite
quartz
[[Show Si-O bonding Figure for different silicate groups]]-
Si-O become enriched in the final stages of the crystallization process.
Mg, Fe, Ca become depleted in melt
What happens to Igneous Rocks at surface?
~ 75 % of Continental outcrops are sedimentary rock, but they only constitute about 5% of the continental crust.
Mechanical weathering- abrasion. Creates more surface area, and breaks apart minerals with cleavage more readily.
S09.102.L6
S09.102.L6
Topics
Mechanical weathering
Chemical Weathering
Soil formation
Mechanical weathering- abrasion. Creates more surface area, and breaks apart minerals with cleavage more readily. There are tons of ways to abrade rocks. Let’s not go through them all.
Chemical weathering- rocks attacked by water, oxygen and acid, which dissolves rock and separates ions from rock (i.e. in silicate rocks, the Fe, Mg, Ca, K are carried away as dissolved ions, or oxidized to form new minerals).
{{Diagram showing Sedimentary Rock Cycle- weathering and transport}}
End product of Weathering
Unaltered parent material
dissolved ions
new stable minerals.
~ 75 % of Continental outcrops are sedimentary rock, but they only constitute about 5% of the continental crust.
2 Types of Sedimentary Rocks:
1) Detrital (aka clastic): clays = weathered feldspars, quartz = durable
Sediment undergoes compaction and cementation = LITHIFICATION
2) Chemical Sedimentary Rocks (aka non-clastic)
Soluble ions undergo chemical weathering ->transported to oceans
2a) Inorganic = concentration by evaporation
halite, gypsum
2b) Organic = concentrated by biological processes
CaCO3 = limestone, SiO2 = chert
Features of sedimentary Rocks:
layers= strata or beds
Separated by bedding planes
Each separates a period of sedimentation
Soil Profile
Soil = mixture of minerals, organics, air and water
Know the soil profile for a humid environment
O = Organic material
A = mineral matter + humus
E = Zone of eluviation = attacked by organic acids from above layers- washing out clay
B = Zone of accumulation (clay from above)
C = partly altered parent material
But what happens in an arid environment?
Arid Environment= no organics, so no eluviation, so no accumulation
Left with partly altered parent material.
S09.102.L7
S09.102.L7
Chapter 5: Sediment, Soil and Sedimentary Rock
Chapter 5: Not covering Sections 5.5-5.7
Chapter 6: Metamorphism
Agents of metamorphism
Facies
Texure- foliated or nonfoliated
Topics
Soils
Sedimentary Rocks
Textures (2)
Origins (4)
Metamorphism
Agents of…
Facies
Textures
Factors that affect soil formation:
1) Climate – availability of water to chemically leach rocks. Also increase in temperature by 10??qC doubles the rate of chemical reactions.
2) Substrate composition- Are minerals in equilibrium with surface conditions?
3) Slope steepness- Steeper slopes erode more quickly, resulting in thin, immature soils. Areas at base of slope will accumulate sediment, resulting in continued burying of soil. Soils can be residual material or transported material.
4) Wetness- what is the local effect of water migration through the substrate? Is it water-logged (which leads to anoxic conditions) or not?
5) Time- the longer the duration of soil-forming processes, the more mature will be the soil.
6) Vegetation type- contributed to acidity of soil, plus breaking up soil or creating cohesion.
Example-
Arid Environment= no organics, so no eluviation, so no accumulation- Left with partly altered parent material No O, A, B and only weak B horizon.
~ 75 % of Continental outcrops are sedimentary rock, but they only constitute about 5% of the continental crust.
Processes involved in forming sedimentary rocks
1) Weathering of parent material
2) Transportation o detritus by water, wind, gravity
3) Deposition- when transport mechanism weakens
4) Lithification- involves compaction and cementation.
4 Categories of sedimentary rocks
1) Clastic- also known as detrital or fragmental: clays= weathered feldspars, quarts = durable so stays around through many cycles of weathering.
Sediment undergoes compaction and cementation = lithification.
Ways to describe clastic sedimentary rocks
A) Clast size- function of mechanical weathering and function of crystal sizes in parent material.
B) Clast composition= indicates degree of mechanical and chemical weathering.
C) Angularity = abrasion knocks off the sharp corners to cret rounded grains.
D) Sorting = reflects the transportation mechanism. Wind = very selective and so leads to very well sorted sediments. Water less selective. Mud or ice = not selective.
E) Character of cement= reflects the environment of deposition. Calcite and silica are the most common forms of cement. Note that the deep oceans (>4 km deep) are not saturated with calcite and so sedimentary rocks from this depth or greater will only have silica cement.
2) Biochemical = concentrated by biological processes
CaCO3 = limestone
3 types- fossiliferous (visible fossils), micrite (generally recrystallized foraminifera- animals), chalk (coccoliths- plants)
SiO2 = chert. Forms from radiolaria (animals) or diatoms (plants).
Which dominates is a function of water depth
Carbonate compensation depth (CCD) water below about 4 km not saturated with calcite, so calcite debris falling from above dissolves rather than accumulates.
3) Organic = Carbon-rich relicts of plants = coal and oil shale
Requires anoxic bottom water conditions. If oxygen were present, organic material would decay. Instead, it de-waaters (de-volatilizes) to form carbon. Metal Sulfide minerals- also concentrate in organic sediments. When we burn fossil fuels one of the byproducts is oxidized metal sulfides – yields sulfuric acid (H2SO4) plus dissolved metal ions, both of which are toxic environmental hazards.
4) Chemical Sedimentary Rocks - Soluble ions undergo chemical weathering ->transported to oceans, then concentrated by
Evaporites- concentrated by saltwater evaporation. Notable mierals = halite, gypsum, calcite
Condensates – usually from hot water cooling off leadng to oversaturation with minerals- Travertine (calcite) forms around geysers from this process.
Features of sedimentary Rocks:
layers= strata or beds
Separated by bedding planes
Each separates a period of sedimentation
S09.102.L8
S09.102.L8
Chapter 6: Metamorphism
Texure- foliated or nonfoliated
Facies
Chapter 7: Volcanic eruptions
Magma viscosity
Volcanic edifices
Metamorphic grades (known as facies)
Interpreted by presence of INDEX MINERALS
Metamorphic facies to know:
1) Subduction zones = low temperature and high pressure = blueschist facies
2) Around intrusive igneous bodies = high temperature and low pressure= hornfels metamorphic facies = contact metamorphism. This also is the environment for hydrothermal metamorphism, which forms serpentine and black smokers around midocean ridges.
3) Deep in mountain belts = regional metamorphism = high temperature and high pressure. – creates foliation texture as new minerals grow under influence of directed pressure. Increasing metamorphic grade passes through several facies.
S09.102.L9
S09.102.L9
Chapter 7: Volcanic eruptions
Topics
Magma Viscosity
Volcanic edifices
Types of deposits
Volcanic edifices
1) Convergence (subduction volcanism)
2) Divergence (mid-ocean ridge basalt)
3) Intraplate volcanism (hotspot volcanoes)
[[Mt. St. Helens]]
[[pyroclastic fall and flow images]]
and some are gentle
[[slides of Pahoehoe, aa lava = fluid, gentle eruptions]]
What determines eruptive behavior?
1) Composition
2) Temperature
3) Volatile content (dissolved gases)
These affect magma and lava viscosity
viscosity = stickiness (resistance to flow)
Factors affecting viscosity
SiO2 content
Basalt= ~ 50%
Intermediate ~ 60%
Felsic ~ 70 %
Silica forms long chains in unstructured magma- increases viscosity.
Temperature:
Ranges from about 1100 C in basaltic eruptions to 900 C in felsic eruptions. Higher Temp= lower viscosity.
can be 1-6% of the total weight!
Help to clear a conduit to the surface- erosion
water > Co2 > N2 > SO2 > Ar
Volatiles are dissolved until near the surface and then expand to fragment the magma
Volatiles increase in the upper part of the magma chamber-
[[Figure 4.12- profiles of volcanoes]]
Note scale!
1) Shield Volcanoes
2) Composite Volcanoes
3) Cinder Cones
Shield Volcanoes
[[Figure volcano type- shield volcano- many thin flows
roof collapse common after large eruptions- form central caldera
Final stages- often more silicic and more pyroclastic in nature.
Basaltic composition, low volatiles, high eruption temperatures, HOTSPOTS
Cinder Cones-
lots of scoria- builds up higher angle of repose
short lived eruptions (usually one eruptive episode and then done)
Composite volcanoes
along subduction zones- steady supply of magma causes many eruptions and so a large volcanic structure develops.
Pyroclastic Eruptions
pyro=fire clast = fragment
high volatile content leads to the break up of lava
Hotspots =Intraplate Volcanism
Different magma source than plate-tectonic-related volcanism
Mantle Plume- source of heat is at the CMB probably
[[Figure 7.something- Hawaiian Hotspot trace across the Pacific Ocean]]
Hawaii hotspot- source of heat has remained stationary while lithosphere has conveyed over it- for 80 Million Years!
Mantle ~ 150 C hotter at hotspots than surrounding mantle.
Hotspot volcanoes often have really large calderas associated with them.
[[Figure of Crater Lake caldera]]
[[Figure of how crater Lake caldera formed]]
[[Yellowstone Hotspot]]- probably eastward migration of hotspot that formed the Columbia River Flood Basalt.- starts to melt the continental crust??
Long valley Caldera- California’s Caldera-
S09.102.L10
S09.102.L10
Topics
Faults
Elastic Rebound Theory
Waves
Body waves = P and S waves
Surface waves = Love and Rayleigh waves
Refraction
[[Figure 6.11- Distribution of earthquakes with magnitudes > 5 from 1980-1990.]]
Earthquakes are a part of our planet. Earthquakes occur in many regions of the world, but most of the earthquakes are concentrated in narrow regions that are the edges of large pieces of the Earth’s crust. These large pieces of crust move as one and we call them plates.
[[Figure 16.3: Focus and Epicenter]]
An Earthquake is a vibration caused by the rapid release of energy. Usually caused by slippage along a fault.
Focus: The point in the ground where the earthquake occurs.
Epicenter: The point at the surface directly above the focus. This can be hundreds of km from the focus!
[[Figure 16.5- elastic rebound]]
Elastic Rebound Theory: Rocks can store elastic energy like the bending of a stick. Once the elastic energy overcomes the frictional resistance (“glue”) that holds the rock together, it will rupture. This releases the elastic energy that was held in the rocks.
[[snap a stick demonstration]]
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.)
Body Waves:
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 (at the speed of light = 300,000 km/sec), but the thunder travels at the speed of sound, which is about 0.3 km/sec (= 1 million times slower). So it takes about 5 seconds for the sound of thunder to travel about one mile.
Earthquakes also produce different kinds of waves with different velocities, and these can be used to tell you the distance to the earthquake focus. The two waves used for this are the P and S waves.
[[Figure 17.2 transmission of P and S waves]]
P waves= primarywaves (the first to arrive at a seismograph…these are the fastest 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 p waves pass through.
S waves- secondary waves (these arrive after the primary waves, hence they are slower)
Secondary waves are 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:
While not used to measure the distance to the earthquake, another kind of earthquake wave is concentrated at the Earth’s surface, and is measured by seismographs.
These are the surface waves.
They are the slowest waves
Largest amplitude of displacement
These produce the most damage
Actually two kinds of surface waves:
Love waves- create side-to-side shear of ground. This must be the manifestation of the S wave that is traveling parallel to the surface.
Strangely, these move faster than Rayleigh waves
Rayleigh waves- must be a combination of the P and S wave that are traveling perpendicular to the ground surface.
Faults
= where elastic stresses exceed the rock strength, leading to fracture.
Fault types:
Named in associate with the following terminology (which comes form mining).
Dip = inclination of the fault plane relative to horizontal.
Shallow dip = shallow slope of fault plane into the subsurface.
Steep dip = fault inclined steeply relative to the surface.
Strike = the direction (relative to north) of a horizontal line drawn on the fault plane.
Dip – slip faults – displacement on fault plane is in the direction of the dip
Normal fault = dip slip in which the hanging wall slides down relative to the footwall.
greatest force acting on normal faults is gravity (the downward force of gravity causes the rocks on the hanging wall to slide downward).
Reverse fault = dip-slip fault in which the hanging wall moves up relative to the footwall.
greatest force acting on reverse faults is tectonic forces (the sideways force of tectonic plates colliding). In reverse faults the force of gravity is weaker than the tcctonic forces, and so the fault pushes the hanging wall up into the air (i.e. in the direction of the gravitational force).
Strike-slip fault = displacement moves in the direction of the fault plane strike. This means that both the strongest and the weakest forces are within the plane of earth’s surface, and thus are due to tectonic forces. The gravity force is intermediate, and so the crust does not move either up or down as it does in dip-slip faults.
Based on the relative importance of the gravity force, it appears that reverse faults are displaced with the greatest buildup of tectonic forces, whereas normal faults are displaced with the lowest buildup of tectonic forces. This plays out with relative strengths of earthquakes, which are smallest with normal faults ane greatest with reverse faults.
S09.102.L11
S09.102.L11
Topics
Waves
Body waves = P and S waves
Surface waves = Love and Rayleigh waves
Refraction
[[Figure 6.11- Distribution of earthquakes with magnitudes > 5 from 1980-1990.]]
Earthquakes are a part of our planet. Earthquakes occur in many regions of the world, but most of the earthquakes are concentrated in narrow regions that are the edges of large pieces of the Earth’s crust. These large pieces of crust move as one and we call them plates.
[[Figure 16.3: Focus and Epicenter]]
An Earthquake is a vibration caused by the rapid release of energy. Usually caused by slippage along a fault.
Focus: The point in the ground where the earthquake occurs.
Epicenter: The point at the surface directly above the focus. This can be hundreds of km from the focus!
[[Figure 16.5- elastic rebound]]
Elastic Rebound Theory: Rocks can store elastic energy like the bending of a stick. Once the elastic energy overcomes the frictional resistance (“glue”) that holds the rock together, it will rupture. This releases the elastic energy that was held in the rocks.
[[snap a stick demonstration]]
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.)
Body Waves:
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 (at the speed of light = 300,000 km/sec), but the thunder travels at the speed of sound, which is about 0.3 km/sec (= 1 million times slower). So it takes about 5 seconds for the sound of thunder to travel about one mile.
Earthquakes also produce different kinds of waves with different velocities, and these can be used to tell you the distance to the earthquake focus. The two waves used for this are the P and S waves.
[[Figure 17.2 transmission of P and S waves]]
P waves= primarywaves (the first to arrive at a seismograph…these are the fastest 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 p waves pass through.
S waves- secondary waves (these arrive after the primary waves, hence they are slower)
Secondary waves are 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:
While not used to measure the distance to the earthquake, another kind of earthquake wave is concentrated at the Earth’s surface, and is measured by seismographs.
These are the surface waves.
They are the slowest waves
Largest amplitude of displacement
These produce the most damage
Actually two kinds of surface waves:
Love waves- create side-to-side shear of ground. This must be the manifestation of the S wave that is traveling parallel to the surface.
Strangely, these move faster than Rayleigh waves
Rayleigh waves- must be a combination of the P and S wave that are traveling perpendicular to the ground surface.
[[ 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!”
S09.102.L12
S09.102.L12
MAGNITUDES
Richter Magnitudes: [[Table 6.2]]
Modified Mercalli Intensity Scale: [[Table 6.1]]
[[16.11 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
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.
S09.102.L13
S09.102.L13
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
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Lecture canceled due to illness.
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Topics
Chapter 2: Plate Tectonics
Topics
Divergent Boundaries
Convergent Boundaries
Transform Boundaries
1) Divergent Plate Boundaries- constructive plate margins – new lithosphere created here. Normal Faults dominant. Ultimately becomes a passive margin as the edge of a continent moves away from ocean spreading ridge.
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
[[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
Driving Mechanism behind Plate Tectonics
Still being worked out.
What we know:
1) Convective flow in the mantle.
2) Mantle convection and plate tectonics are coupled.
3) Uneven distribution of heat in earth’s interior; plate tectonics gets rid of it.
Tectonic plate boundaries cause most mountain ranges
[[Figure of World’s mountain ranges]]
- most active mountain ranges occur in two environments:
1) Ring of Fire = circum Pacific mountain ranges cause by ocean-continent collisions and subduction
2) Eurasian plate versus Africans, Arabian and Indian plates = continent-continent collision
Older mountain ranges, such as the Appalachians in the eastern US are due to ancient collisions. Appalachians formed 300 Ma when all the continents collided to form Pangaea…those portions of the US east of the Appalachians were part of the Africna continent prior to this collision.
What is the main difference in the mountain ranges around the Pacific Ocean versus those that occur between the Africa, Arabia, and Indian plates and Eurasia?
A: Circum Pacific have volcanoes due to subduction, whereas the other collisional belt has continent vs. continent, so no subduction.
Fault types
dip-slip
A) Normal faults = extensional tectonic settings (e.g. Basin and Range of western US). DOn’t form large mountains because of crustal thinning.
B) reverse faults ( a.k.a. thrust faults) = collisional tectonic settings- lead to thickening of crust, and folding of rock layers (e.g. Himalayas).
involves shortening and thickening of continents, and so these build very high mountains.
strike-slip = transform fault boundaries. (e.g. an Andreas fault). These do not change the thickness of crust, and so do not have mountains associated with them.
Folds: Anticlines and synclines…which will trap oil?
Anticline folds open downward
Synclines open skyward
Anticlines form a natural trap for oil. Find the anticline, and sink a well into it.
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S09.102.L16
Topics
Ch. 9: Mountain building
Mountains associated with plate boundaries
Faults and folds
Mountains
Tectonic plate boundaries cause most mountain ranges
[[Figure of World’s mountain ranges]]
- most active mountain ranges occur in two environments:
1) Ring of Fire = circum Pacific mountain ranges cause by ocean-continent collisions and subduction
2) Eurasian plate versus Africans, Arabian and Indian plates = continent-continent collision
Older mountain ranges, such as the Appalachians in the eastern US are due to ancient collisions. Appalachians formed 300 Ma when all the continents collided to form Pangaea…those portions of the US east of the Appalachians were part of the Africna continent prior to this collision.
What is the main difference in the mountain ranges around the Pacific Ocean versus those that occur between the Africa, Arabia, and Indian plates and Eurasia?
A: Circum Pacific have volcanoes due to subduction, whereas the other collisional belt has continent vs. continent, so no subduction.
Fault types
dip-slip
A) Normal faults = extensional tectonic settings (e.g. Basin and Range of western US). DOn’t form large mountains because of crustal thinning.
B) reverse faults ( a.k.a. thrust faults) = collisional tectonic settings- lead to thickening of crust, and folding of rock layers (e.g. Himalayas).
involves shortening and thickening of continents, and so these build very high mountains.
strike-slip = transform fault boundaries. (e.g. an Andreas fault). These do not change the thickness of crust, and so do not have mountains associated with them.
Folds: Anticlines and synclines…which will trap oil?
Anticline folds open downward
Synclines open skyward
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S09.102.L17
Topics
Chapter 10: Deep Time
Absolute Dating
Precambrian events
Early atmosphere
BIFs
Snowball Earth
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?
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S09.102.L19
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S09.102.L21
Chapter 13: 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:
[[classification in Keller Table 5.1]]
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.
Periodic rainfall. The added weight and wetting of sediment causes them to move.
S09.102.L22
Chapter 13: Landslides and mass movements
Topics:
slope processes
human interaction
minimizing hazards
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:
[[classification in Keller Table 5.1]]
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.
Periodic rainfall. The added weight and wetting of sediment causes them to move.
Topics
Streams
Profile of a stream
Baselevel
Mature streams- why they meander
Streams- Transport water at the surface.
1) Gradient- steepness of the stream
2) Channel characteristics- width, shape, roughness
3) Discharge- amount of water
[[Fig. 5.6: Stream profile- concave upward]]
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.
S09.102.L23
Topics
Streams
Stream charactristics
Mature versus braided streams- why they meander (or don’t)
Streams- Transport water at the surface.
1) Gradient- steepness of the stream
2) Channel characteristics- width, shape, roughness
3) Discharge- amount of water
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.
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.
Braided Streams
Loess
Dust Bowl
El Niño
1993 Flood
Braided Streams- sediment load too large to be moved by water; channels filling up continually and so main channel cannot develop.
Braided streambeds are largely exposed to the air and so the fine silty material can be blown away by the wind.
Braided streams flow from the melting edge of glaciers. The windblown dust from these streams is called loess (pronounced luss). The ancient Mississippi River braided stream that flowed off the North American ice sheet 10,000 years ago was the source of much of the midcontinent’s soil. In the 1930’s when this land became dry due to drought and poor farming practices, the loess was again exposed to the wind and developed into what has become known as the Dust Bowl.
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.
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Chapter 16: Groundwater
In some places the rocks have interconnected pore spaces that allow the water to infiltrate and flow below ground level.
[[Figure - shows the water table and also porosity and permeability]]
Water adheres to grains by capillary action- the electrical charge attraction, plus the surface tension of water
water table: the level of saturation- where all pore spaces are full of water
zone of aeriation: above the water table
Porosity: the spaces in the rock
Permeability: the interconnectedness of the pores which allows water to move
e.g. some rocks have lots of pores (like pumice) but these pores are not connected, and so water cannot flow through them.
Aquifer – transmits water well
Confined aquifer- separated from the surface by an impermeable layer- this impermeable layer procts the water quality in the aquifer.
Unconfined aquifer- no barrier between the aquifer and the surface. these aquifers are prone to contamination.
Aquitard- prevents the movement of water
Perched Aquifer Impermeable layer makes local water table- Good in wet times, but not in dry times- the perched aquifer will be the first to dry out.
water flows from high to low areas- no duh. But this means that water flows from the ground INTO streams, lakes, ocean, as they are all located at the lowest elevation points in their areas. This can lead to continuous flow of water into streams year-round.
[[flow of water into streams when the water table intersects the surface]]
Drawdown in wells. The removal of water exceeds the rate of replenishment, and so the water table is locally reduced.
Leads to subsidence
Note that some subsidence is natural compaction (see New Orleans at 8 ft below sea level)
However, other compaction is due to the removal of groundwater and collapse of pore spaces- air cannot provide the support that water does!
See subsidence of San Joaquin Valley by 9 meters.
Note that loss of groundwater is one effect, but flooding is another effect- lowering the floodplain in areas leads to more frequent floods in those places.
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Drawdown in wells. The removal of water exceeds the rate of replenishment, and so the water table is locally reduced.
Leads to subsidence
Note that some subsidence is natural compaction (see New Orleans at 8 ft below sea level)
However, other compaction is due to the removal of groundwater and collapse of pore spaces- air cannot provide the support that water does!
See subsidence of San Joaquin Valley by 9 meters.
Note that loss of groundwater is one effect, but flooding is another effect- lowering the floodplain in areas leads to more frequent floods in those places.
The Ocean Floor
Bathymetry
Start with Sandwell Image
Show how sea-floor topography (Bathymetry) is determined
Old way = throw a weight over the side
Now: Satellites measure deflection of ocean surface
Show picture of Sandwell map, describe many of the features that the students are now familiar with:
Ocean ridges,
There is a lot of topography on the ocean floor!
Note that there are broad shallows adjacent to continents-
Continental Shelves.
Concentrate on the Atlantic margin (= passive continental margin- plate tectonics occurs at a great distance from edge of continent))-
cross section through crust from NA to Africa.
Shelf= submerged continental crust. Thins seaward by continental rifting.
Slope- transition to ocean crust.
Rise: accumulation of sediment at the base of the slope = turbidites
Abyssal Plain= pure ocean crust
Then rises up to form ridge
Turbidity currents- how a lot of sediment is transported from the continental margin to the abyssal plain.
Pulses of sediment delivered by storms or earthquakes.
These are near the edges of the continents.
Pacific margin by comparison (= active continental margin- plate tectonics occurs right at the edge of the continent)
continental shelf
slope
trench – with accretionary wedge
Ocean basin (abyssal plain)
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S09.102.L26
Topics:
Chapter 15: Oceans
Coriolis Effect
Ocean currents
Surface currents
Deep currents
Ocean Circulation
Ocean circulation occurs because of heat imbalances. Same is true for wind. Poles are cold. Equator is hot. Hot goes to cold. But then there are the details.
Thermohaline Circulation : the thread for this lecture
Thermo = heat
Haline = salt
circulation = movement of the ocean water
There are two principal ways that ocean water circulates around the World:
1) Surface Currents
2) Deep water currents
1) Surface Currents
Surface currents are driven primarily by wind.
When you are sitting on the beach at the Equator on a windless day, the wind is really moving about 1700 km/hr; it just happens to match the speed that the ground is moving.
When you are sitting near the pole on a windless day, the wind really is not moving velocity = 0 km/hr!
The reason for this is that the wind is tied to the Earth by friction, but that coupling is not perfect…the wind can have different velocities than the ground beneath it.
[[Fig. 18.16 Wind directions of planet]]
[[Fig. 18.15 Generalized Hadley Cell circulation]]
Follow air form equator to pole to explain wind directions,
Bending of air masses= Coriolis forcing.
Air not really bending. It is the position of the ground relative to the air above it that is changing.
[[Draw Earth with equator and 2 longitude lines]]
Wind- generated by heat gradients.
Equator gets most heat. Air rises here and moves away from equator.
But it doesn’t flow away in a straight line.
Note Earth rotates counterclockwise in Northern Hemisphere (from west to east).
At equator, speed of rotation = ~ 40,000 km/24 hrs =~1700 km/hr.
Near the poles, the speed of rotation is less- virtually 0 km/hr at rotational pole.
[[Draw ground speed versus air speed]]
ground speed = angular speed
air speed = velocity
What this means is that air moving from equator towards poles has higher west-eastvelocity than ground beneath it. So it moves towards the east.
By 30 degrees N or South, the air is moving due east. This air descends and returns to the equator to replace the rising air.
THIS MEANS THAT AT GROUND LEVEL, THERE IS A STRONG EAST TO WEST FLOW OF AIR, AND THIS DRIVES OCEAN CURRENTS.
[[Figure 18.16- general wind pattern over Earth]]
just point out wind flow at equator is to the west
=TRADE WINDS.
We’ll talk about the rest of the air movement later, but for now it is sufficient to know that these winds drive the ocean surface currents.
[[Figure 15.2- average ocean surface currents]]
Note the GYRES = coriolis forcing
Note this means western sides of continents COLD
From Equator to pole we find the following temperature and salinity effect:
[[Figure 14.3- salinity and temperature by latitude]]
explain that temp is high at equator and low at poles
This process sets up one of the most important heat transport systems in the world- the thermohaline circulation.
[[Figure 15.6: Thermohaline circulation model]]
Formation of sea ice at poles causes density increase and water sinks to the bottom.
Cold water fills the ocean basins.
Warm water moves from equator to pole to replace lost water.
Occasionally this system breaks down.
End of last ice age = fresh water floods N. Atlantic. Sea ice does not increase salinity.
Thermohaline circulation stops.
[pictures of 8200-year event in North Atlantic]
[Picture of Day After Tomorrow]
Topics
Finish Ch. 15
Waves
Shorelines
1) Waves
In water, waves transfer energy rather than transferring material; a wave can traverse the ocean in a day (e.g. Tsunami) but water takes several years to circulate.
[[Figure 14.2- waves, oscillations ]]
Label: Crest, trough, wave height, wave length, period
In open ocean, waves are oscillatory waves- a cell of water follows a near-circular path
You feel this on a boat
the oscillatory motion decays with depth and by 1/2 wavelength the oscillatory motion is gone.
Note that the wave energy dissipates because the force that creates the waves is wind, which is a force at the surface.
Water does move a little bit in the direction of the waves, but most of the motion is in the wave (this is the transfer of energy rather than translation of water)
==wave base
at shoreline energy changes from wave of oscillation to wave of translation= the energy is transferred from one body to another.
How waves break on shore
[[Figure 15.10 ES]]
Note that the waves no longer follow a circular path; they drag due to friction at the bottom. Eventually the crest of the wave advances far enough ahead that it is no longer supported by the wave. It breaks.
[[Fig. 15.13- wave refraction toward headlands]]
Irregular coastlines cause wave energy to focus and diverge.
Focus on headlands
Diverge in bays = dissipation of wave energy causes particles to settle out of water- develops
2) Currents:
[[Figure 15.2 ES Global surface currents]]
Long-shore currents:
Waves do not usually approach the shoreline perpendicularly. Usually they approach at an angle and bend (refraction) as the waves feel bottom.
(we already talked about waves bending in the direction of lower velocities, but let’s review it for shorelines.)
In the surf zone where there is lots of turbulence, this moves sand down current.
AND
Longshore transport == Beach drift
- this occurs on the beach.
[[Figure 15.14- longshore transport]]
Inward trajectory= at a slight angle
Outward trajectory= perpendicular to shoreline= controlled by gravity rather than waves.
Sand accumulation or erosion:
Swash = inward movement of water
backwash = outward movement of water
When low wave activity, most swash soaks into beach and returns as groundwater flow to the ocean. This leads to accumulation of sand that is washed in by the waves. When greater wave activity, beach gets saturated and backwash above ground, taking sand back out with it.
[[Fig. of rip current from elsewhere]]
Outflow of water not uniform. Some areas concentrate on outward flow = rip current.
How to get out of rip current: swim parallel to shoreline: Rip currents usually less than 20 meters wide. You can swim safely to shore once you are out of the current.
Erosional Landforms:
wave-cut cliffs: erosion at base causes slides, hence steep slopes.
wave-cut platform: At wave base
marine terrace: uplifted wave-cut platform
sea arch: refraction leads to caves on both sides of headland; they meet
sea stack (fallen arch)
Depositional landforms
1. sandspit= extension of beach into a bay.
2. baymouth bar= spit crosses whole bay
West Coast problems: Damming the rivers eliminated a large portion of the coastal sediment, so beaches are thinning. This leads to less wave energy dissipation so headlands erode faster.
East Coast problems: Development on barrier islands, which naturally move.
Shorelines are the interface of the ocean with the land. To think that they should remain in n the same place is unrealistic. They will migrate as forces that affect the coastline change.
Seawalls- keep wave energy from eroding base of seacliff. Problem- the reflect wave energy outward, and so waves carry beach sediment offshore.
Groins: capture sand. Problems they starve places downcurrent and can cause erosion there.
[[Figure of groins with sediment trapped]]
Note downcurrent has depleted sediment
Jetties: extend out and block sand from filling a harbor entrance
[[Figure 14.16 = a jettie]]
Breakwater: barriers that cause wave energy to dissipate offshore. Problem that causes sedimentation at shoreline.
Beach nourishment: adding sand artificially