th
Read Lab 2 pp 39-72. The aim of the lab is:
A.
To investigate some aspects of the plate tectonic model.
B.
To interpret rates and directions of plate movement over different geological scales & times.
C.
To analyze data and associated geological processes & features.
The theory of plate tectonics is a powerful and far-reaching theory encompassing many aspects of geology. The theory explains such diverse phenomena as mountain building on continents and rifting plate margins, the evolution of ocean basins, magma chemistry, the long-term: migration of plants and animals, climatic change and the movement of continents.
Turn to Lab 2 in the lab manual and read the introduction to each Part, then answer the questions in the manual in the corresponding spaces provided below.
1.1.a
Read p 39-40. What was Alfred Wegener’s observation that led him to propose the Continental
Drift Hypothesis _________________________________________________________________. (1)
1.1.b
Why was this rejected?________________________________________________________. (1)
Why did anti-drift scientists oppose this and what was their evidence or bias? __________________
_______________________________________________________________________________. (1)
1.1.c
What alternative hypothesis did Bernard Lindemann (1927) and Otto Hilgenberg (1933) propose?
_______________________________________________. (1)
1.1.d
What evidence led them to this conclusion? _________________________________________
_______________________________________________________________________________. (1)
Instantaneous Motions and GPS Tracker Arrays:
2.1
The GPS (Global Positioning System) satellite network was built for US Military navigation. It allows ground, sea or air based GPS receivers. The ephemeris (altitude) component of the satellite position is intentionally corrupted for civilian use. This reduces the immediate precision, but fixed stations with redundant receivers, long data records and subsequent decryption allows precision of cm rather than metres for a single reading. GPS arrays are coming to be the mainstay of geodetic surveying.
This allows for precise tracking of station positions over the course of months or years. Do the on-line,
GPS Homework Exercise from p.43-45, forms from p.57-58 & the website: http://sideshow.jpl.nasa.gov/post/series.html
(not slideshow!) from the Jet Propulsion Lab. We will do this exercise for the location of Camosun College. Examine the vectors for North America relative to the
Juan de Fuca and Pacific Plates near Vancouver Island and the Queen Charlotte Islands (Haida Gwaii).
Select station AB-49 by clicking on the Vancouver Island station then select its name AB49 on the list to get a page sized data set.
A.
What plate is Camosun College on? _________________________________________________ (1)
B-1.
What is the name in this data base for the closest station? ______________________________ (1)
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B-2. Longitudinal component of vector motion: circle : (East or West) & velocity _________ mm/yr (2)
B-3.
Latitudinal component of vector motion: circle : (North or South & velocity __________ mm/yr (2)
B-4. Describe the azimuth direction (degrees east of north) ___ ° and general compass heading ____ (2)
B-5.
The _________________________ plate (and this station) is moving at_____________ mm/yr. (2)
B-6.
Present Station Latitude _____________ ° N and Present Station Longitude ____________° W. (2)
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B-7.
The UTM map below goes from -180° to +180° Longitude where 0° Longitude is Greenwich
England, and from -80° to +80° Latitude with 0° running along the Equator. Plot a dot on the station we just used and draw a vector showing its direction of motion, labeled with the velocity. (2)
C.
Look again at the JPL UTM world map. Draw a dot with initials and a vector showing the general
(average) direction of motion for how: South America=SA, Africa=AF, Europe=EU, Australia=AU,
Asia=AS and North America=NA are currently moving. (6)
D.
This GPS data gives an instantaneous (synoptic) view of plate motion on the surface of the Earth.
Continents, Ocean Basins and plate sizes that hold pieces of them are vast (thousands of kilometres across). Write a paragraph relating these directions and the size of the domains they represent to Plate
Tectonic theory and mantle convection cells. What concerns should we have about using GPS data alone to understand long term geological time scale plate motions? (4)
A.
Analyse Figure 2.2, p.44 and figure 2.4 p.48 to see how fault types relate to styles of stress and to tectonic boundaries. We have now mapped the Earth in far greater detain than in the early 20 th
century.
For changing size, either mass or heat has to increase for expanding volume, or decrease for shrinking volume. Where might this new mass or heat come from or go to. Keeping those old hypotheses in mind,
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Choose the dominant: faulting type and plate boundary type for:
2.2-A-1.
Expanding Earth ______________ & __________________ (2)
2.2-A-2.
Contracting Earth ______________ & __________________ (2)
2.2-A-3.
Sheared, no size change __________ & __________________ (2)
2.2-B.
Complete the Table noting the features & directions in Figs 2.2 (p.42), 2.4 (p.45) (12)
Boundary
Type
Major Stress Relative Plate
Motion
Fault Type & Other geological features or processes
(applied force)
Divergent
Convergent
Transform
2.2-C.
On Fig 2.5, p.48, the major lithospheric plates (about 12 or 13 depending on how small you decide qualifies as a major plate) and 3 plate boundary types are shown. Divergent boundaries may be either of 2 types: MOR (Mid Ocean Ridge) or CR (Continental Rift) and they are active over their entire length. Convergent boundaries can occur 3 ways: between 2 Oceanic plates (Ocean-Ocean), an Ocean and a Continental plate (Ocean-Continent) or between 2 continental plates (Continent-Continent) and they are active over their entire length. Transform Faults similarly may occur between 3 different types of plate pairs (O-O, O-C, or C-C) but the active shearing part that generates earthquakes can only occur between active spreading ridge segments, as beyond either ridge or rift, both plates are moving the same direction and there is no more shear or strain build up so they become inactive fracture zones. Use the table below for estimates to calculate total length and percentages of each type boundary. Note that the ridge spreading segments are in bold red and the transform offsets are grey. Trenches/subduction zones have bold black lines with teeth on the upper (over-riding) plate. Keep in mind that not all rates are equal, even along a particular boundary so the notion of ridge = transform for length alone is only approximate.
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2.2-C-1.
Complete the following table: for % take 100xtotal convergent/all boundary total length etc. (6)
Plate Boundary Type
Ocean-Ocean Convergent
Ocean-Continent Convergent
Length (km)
17,499
51,310
Continent-Continent Convergent 23,003
Continental Rift divergent
Mid-Ocean Ridge divergent
27,427
67,338
Oceanic Transform
Continental Transform
47,783
26,132
Total length (km) % of all boundaries
2.2-C-2a. The evidence from the figures, including your completed 2. 4 and Q’s 1-3 suggests that
Earth’s size is: ( Circle your answer ) and justify it from the data and calculations above.
Expanding Contracting Staying the same (1)
Explain: _____________________________________________________________
____________________________________________________________________
____________________________________________________________________ (2)
2.2-C-2b.
Do you think that Lithospheric Plate Tectonic motions are caused by changes in earth’s size and justify your answer. (2)
____________________________________________________________________________________
____________________________________________________________________________________
___________________________________________________________________________________
2.2-C-3.
Earth’s total surface area is 510,000,000 km
2
and according to Peter Bird’s calculations, the net rifting or creation rate of new basaltic crust is 3.4 km
2
/yr, and the rate or destruction or subduction of oceanic crust is the same at 3.4 km
2
/yr. From this information alone, how long would you predict it should take to entirely recycle all of Earth’s lithosphere? ___________________________________ (3)
Show your calculation here:
2.2-D.
Reflect on the value and validity of the last answer you gave. What is the age of the oldest seafloor remaining in the ocean basin’s today. _ __________________________________ my. (2)
Is all of Earth’s lithosphere recycled or are their older rocks yet than those found at the convergent edges of the ocean basins? If you answer was yes, where are those rocks found and how old are the oldest rocks? Where: ___________________________, How old: ____________________________ my. (2)
Is your answer in C-3 too fast too too slow and why? ______________________________________
_______________________________________________________________________________ (2)
Discuss how do you think your rate in C-3 relates to Mantle convection rates and why. __________
________________________________________________________________________________
_______________________________________________________________________________ (2)
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Among the states of matter: solids are dense and retain their own form and are strong, liquids attain the shape of their container but are still dense, gases are very low density and assume the shape of their container. Fluids (like mushy liquids) are somewhere between liquids and gases in their behavior. The minerals that make up the Earth’s solid, thin, cold, brittle, outer crustal rocks and solid (but more like a strong plastic), thick, hot, plastic mantle rocks (both Mesosphere and Asthenosphere) at short time scales and fast strain rates behave like elastic solids. Elastic solids are firm, retain their shape, vibrate and pass sound waves, resume their initial form after the force is removed. This behavior describes rubber balls, vibrating silver tuning forks, and even solid minerals like quartz crystals. Plastic materials usually have finite yield strength such that a small force doesn’t make a dent but sufficient force, results in permanent and irreversible deformation and flow. Hot rocks and minerals can gradually flow and change shape, size and position. This is especially true of hot mantle peridotite close to its solidus temperature (initial melting point). Temperature in any matter is a measure thermal vibration of atoms and their electron bonds. The hotter matter gets, it expands and the weaker it gets. While a directed pressure, differential pressure or shear will cause matter to flow, isostatic pressure (the same in all directions) opposes thermal vibration and makes matter stronger. In the mantle, the peridotite rock gets stronger with depth and increasing pressure, even though temperature also increases with depth. Mantle rocks can expand with heat and buoyantly rise, shrink as they cool and sink due to higher density. Only at the lowest pressures in the uppermost Asthenosphere, are the pressure and melting point low enough, and the temperature high enough to permit a few percent partial melt to form. Otherwise the Mantle is a hot plastic solid. We will examine 2 models of mantle behavior using silly putty and lava lamps.
“Panta rhei.” Simplicius of Cicilia, 560CE.
The aphorism of Simplicius: “Everything flows.”
Rheology is the science of flow . Ideal fluids have zero strength and zero viscosity and strain rate is independent of shear stress (horizontal axis). Newton studied common fluids like water and air and found them to flow in proportion to the applied pressure at fixed temperature. For these Newtonian fluids , shear stress=shear strain x viscosity (slope of line). Linear Newtonian behaviour also applies to common substances like glass (supercooled liquid) and magma with no crystals or bubbles. Rheological fluids ( Rheids ), instead of deforming elastically and springing back upon application of force, respond non-linearly and they flow. There are 2 ways that viscous or rheid behavior can deviate from linearity.
Thixotropy is time dependent shear thinning like drilling muds and quick-clays. Ideal Bingham Plastic is linear after a finite yield strength is overcome. Most real plastics and pseudo-plastics (no yield strength) have decreasing (non-linear and thus non-newtonian) viscosity, as do ketchup, silly putty and mantle rock under the right conditions of stress and rates of applied force. Shear thickening ( dilatant ) behavior is an increase of viscosity with applied force or stress like: printers ink, plaster slurry, cornstarch paste,
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wet sand and peat (run on it and it is hard, stand on it and you sink). Rheopectic behavior describes shear thickening such that with continued application of force the viscosity increases over time to even become solid. Many substances are complex mixtures or suspensions of other things such as rigid solid particles, compressible gas bubbles or even other immiscible fluids in a host fluid like magma, water, ice cream, foams. Depending on whether the solids bear the force or the fluid flows and propels the mixture the deviation from Newtonian behavior for these can go either way. Rocks are mixtures of solid mineral particles of different hardnesses, shear strengths, cleavage properties and maybe some fluids like magma, water or gases. Behaviours can vary with proportions, forces, strain rates and temperatures. In magmas viscosity thins as temperature goes up but proportional to T to the 4 th
power! Rheology and natural flows are complex business!
2.3-A1.
Play with a piece of silly putty to perform the following experiments and determine which conditions and rates of strain, shear, or applied force make it behave like an strong elastic solid and which make it behave like a weak plastic fluid (semi-liquid, rheid) . Perform each test and briefly describe the amount of force or strain rate (how fast you applied it) and put an x in the box that best describes the behavior you observe for your silly putty when forces are applied in that fashion.
Solid-like behaviour Fluid (rheid) behavior Silly Putty Test/Experiment
1. Roll a ball and bounce it.
2. Make a bar & pull both ends slowly
Force or Strain rate
3. Make a bar & pull both ends quickly
4. Roll a ball and press with your thumb
5. Roll a ball and let it sit a few minutes
2.3-A2.
Define what a Rheid material is and describe in what way or ways your silly putty is a rheid. (3)
2.3-A3. Reflect and discuss in what ways the rocks of the uppermost lithosphere and those of the asthenosphere (upper mantle) below can behave like Silly Putty under different forces, times and conditions. Give 2 different examples for the lithosphere and 2 for the mantle. ______________ (4)
_____________________________________________________________________________
_____________________________________________________________________________
_____________________________________________________________________________
2.3-B.
Observe a lava lamp for a few minutes and notice the shapes, changes, motions, directions and speeds of the 2 fluids. Gently lift the glass of the lava lamp and note where the light bulb is. Lift the cap and see what the top of the lamp and cap are like.
B-1 Sketch, label and describe in words, the motions of oily and waxy fluids in a lava lamp over at least one cycle. (3)
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B-2 Why does “waxy-lava” rise and where does the heat come from? _______________
_______________________________________________________________________
_______________________________________________________________________ (2)
B-3 Why does “waxy-lava” sink and where does the heat go to? ____________________
_______________________________________________________________________
_______________________________________________________________________ (2)
B-4 See p14, fig 1.6. When energy, heat, mass and momentum all move together in a confined space,
(like the lava lamp, Earth’s atmosphere, Oceans, Mantle or Outer Core, or a lava lake), the cyclic process of doing work while rising and sinking is called: _________________________ (2)
Part C: Compare Plate boundaries in Fig. 2.5 p.48 to the seismic tomography map made for 80 km depth in the Asthenosphere with its Red and Blue regions in Fig. 2.6 on p.49. These are false colours derived from seismic wave behaviours for earthquake waves that travel through the level of the upper mantle shown in the false colour map. Red corresponds to seismic P-wave slowness (warmer rocks at this particular depth) and Blue to fastness (cooler rocks at this particular depth) from earthquake arrival times. Because the interior of the Earth is not uniform in density and temperature, sometimes the waves from distant large earthquakes arrive either too soon or too late compared to an average Earth. Don
Anderson and his students at Cal Tech spent more than a decade measuring and mapping this type of
“seismic tomography” of the Earth’s interior in a place too deep to ever drill or mine. Medical doctors were so intrigued that they then applied this kind of imaging to the human body inventing ultrasound,
CAT scans and MRI imaging. This type of open interaction is when we need to do research and publish it widely. Unimaginably good and useful things come from the open interaction of new ideas and different minds. Recall what you learned from the different kinds of crust & isostasy in Lab 1.
How is Earth’s mantle in terms of: state of matter, heating, cooling, rising, falling and the process of heat and mass transfer:
C-1 Like a lava lamp? _____________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________ (3)
C-2 Different from a lava lamp in terms of the physical processes involved? What is the state of the mantle? How fast does it move? How long does one cycle take? ___________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________
_______________________________________________________________________ (3)
D.
Examine Fig 2.6 for the seismic tomography and note where the red (hotter mantle) is located compared to the plate boundary types in Fig 2.5. Now compare where the blue colder mantle is with respect to the plate boundaries on Fig 2.5. Finally look back at the plate vectors you saw from the JPL website and your map in Q 2.1-7.
D-1 When you compare the plate Tectonic map to the tomography, what types of surface tectonic features sit on top of the slower, warmer, less dense mantle rocks? _________________
_______________________________________________________________________ (2)
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D-2 When you compare the plate Tectonic map to the tomography, what types of surface tectonic or geological features sit on top of the faster, cooler, denser mantle rocks? _____________
_______________________________________________________________________ (2)
E. From
Fig’s 1.5(p.11), 2.2(p.42), 2.5 & 2.6 and your sketch of the lava lamp, Provide a simple, labeled sketch showing a global cross section through the mantle like Fig.2.2 but deeper to the Outer
Core. Label depth in km & the different layers from the outer core to the crust including the upper mantle, its driving mechanism with arrows to show convective motions of solid mantle rocks & tectonic lithospheric plates . Be sure to show where ridges and trenches sit with respect to mantle convection cells. Hint: There is corner flow in the wedge above subducting plates or it would curl up or flatten out against the crust. Convection cells are usually paired under ridges. Do you think there is convection beneath the mantle in the outer core? Discuss your drawing, inferences and reasons. (10)
When basalt lava cools to solid rock, some magnetite crystals form and inside them, their electrons align with the compass direction of the Earths dipole (N-S) magnetic field. Periodically the main field reverses but rebuilds as (S-N). For rocks formed now (normal field) they add to the Earth’s field strength and have higher amplitude magnetic anomalies (bumps, strong spots). These normal times are coloured on your map like Figure 2.2 in your manual. During reversed times, rocks have magnetic vectors that point the opposite way, hence they subtract from the Earth’s modern normal total field at that location and make a lower field strength or a negative anomaly. These reversals are all coloured white. Basalt magmas continue to erupt, cool and form new seafloor at a rate of several cm/yr while the fields remain stable and also when they flip. We can date the basalts using radiometric clocks to know when each magnetic anomaly stripe formed. This anomaly age is in your book and on the key next to the stripes.
2.4-A1.
On the anomaly map on the left below, label the following tectonic boundaries and features:
JDF-Juan de Fuca Ridge, GR-Gorda Ridge, CSZ-Cascadia Subduction Zone, PP-Pacific Plate, JP-Juan de Fuca Plate, GP-Gorda Plate, NP-North America Plate, BFZ-Blanco Fracture Zone (Transform Fault),
QCF-Queen Charlotte Fault. Add half arrows to the transform faults. Put teeth on the upper plate edge of the Subduction Zone Fault. (11)
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2.4-A2.
What is the rate of seafloor spreading from B-A west of the Juan de Fuca Ridge ___ km/Ma. (1)
2.4-A3. What was the average spreading rate B-C, east of JDR? _________ km/Ma (1)
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2.4-A4. Explain or account for why are all rocks (magnetic stripes) east of the JDR (on the
Juan de Fuca and Gorda Plates) younger than 10 Ma, while west of the JDR (on the Pacific
Plate) they continue to far older ages. What happened to those on the JDF & Gorda Plates
(see line segment C-D)? ________________________________________________________ (1)
2.4-A5a. In your mind, take a brief trip in a submersible, to point C on the last map. Note this location on Figure 2.2 in your manual. What is this location called, in tectonic terms.
Draw a cross section and label or describe the contrasting geology, rock types and local terrain you might expect to see at this location on the seabed. (3)
2.4-A5b. Name the lithospheric plate to the EAST of point C. ______________________ (3)
2.4-A5c. Name the lithospheric plate to the WEST of point C. ______________________ (3)
2.4-6. Look above at the last 2 figures in this worksheet. The Cascades Volcanic Arc sits about 260 km inboard from the trench/continental slope boundary. Intermittently active volcanoes from Mt. Lassen in California through Mount Meagher in B.C. are strung out about 90 km apart. If you were to stand on one of them, and look north or south, one of the nearby neighbours would tend to erupt every century. Local eruptions are several centuries apart on each volcano. What sequence of geological and plate tectonic events cause the
Cascades Volcanic Arc to form and to erupt intermittently?
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________ (3)
Look back at the configuration of the ridge system on Figures 2.2 and 2.5. Note that they are made up of a series of short linear segments less than a few hundred km long which are cleanly offset by a series of perpendicular transform faults. This is because when things fail in tension or in simple shear, they tend to break along straight lines. When ridges develop one of these offsets, they tend to propogate and separate the ridges further and further apart with time. Transform faults are only active, and only generate earthquakes between adjacent ridge segments. The true plate boundary is either ridge or transform in this region.
Past the ridge offset, fracture zones are healed up cracks and because the lithosphere on 2 sides is going the same direction, there are no more earthquakes in these plate interiors.
Keeping these ideas in mind do the exercises below.
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2.5-A1. On the map below sketch in the Mid Atlantic Ridge with a RED pencil or pen. (2)
2.5-A2. On the map below sketch in the largest transform faults which cut the Mid Atlantic
Ridge with a BLUE pencil or pen. (2)
2.5-B . Points B and C were together exactly 100 million years ago at the end of the Early (lower)
Cretaceous Period. How can you tell that the Mid Atlantic Ridge was spreading symmetrically and the same on both sides for that time period? ______________________________________________
_______________________________________________________________________________ (2)
2.5-C.0
How far are points B and C today in kilometers? ______________________________ km (1)
2.5-C.1
Calculate the average full spreading rate for the last 145 Ma since the start of the Cretaceous?
Show your work. __________________________________________________________km/M.y. (3)
2.5-C.2 Convert your answer to mm/yr. _________________________________________ mm/yr (3)
2.5-D. From your answer and the map above, when was the last time that North America and Africa were both part of the same supercontinent Pangea? (hint: you’ll have to use your spreading rate calculation from C1 and the positions of D and E. this was the last time you could have walked from
Halifax, Nova Scotia to Rabat, Morocco without getting wet feet!). Show your work. ________ M.y.
______________________________________________________________________________ (3)
2.5-E.
Let’s switch this one to a Canadian question! How much further apart have Canada and Africa become since we burned down the Whitehouse in Washington D.C during the war of 1812 (which
Canada/British North America won)? Use your spreading rate in mm/yr from C.2 above and your wizard like math skills to subtract 1812 from 2014. Show your work and give your answer in ________ m &
___________________________________________________________________________ mm (4)
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2.3-A Use the map of earthquakes in the eastern Pacific and South America region to locate plate boundaries. Refer to figure 2.3 for help in finding the boundaries and plates. On your map, use a red for ridges, black for transform faults and blue for subduction zones, showing where these plate boundaries occur on the Earth’s surface. Don’t forget the transform faults, you should be able to find at least 10!
After doing this, label the following plate boundaries: the EPR -East Pacific Rise, GR -Galapagos Rise,
CR -Chile Rise, MAT -Middle America Trench, PCT -Peru Chile Trench. Now label the plates represented on this map: Antarctic , Caribbean , Cocos , Nazca , Pacific & South American and show their relative directions of motion with small arrows. Do this work on the following map provided. (18)
2.6-B.
Cross section of Nazca-South America subduction zone (Peru Chile Trench through Andes). Plot each Earthquake as a dot and each Volcano as a triangle. Note the volcanoes are on top of the land! Do not connect the earthquake dots as there is no significance to the order or locations in which they happen. After plotting them, draw in the South America Plate, The Nazca Plate and the mantle wedge.
Note that the Volcanoes lie above the hydrated mantle wedge and are caused by flux melting of the mantle not the subducting slab! The Slab is way too cold to melt but is the source of water for flux. (25)
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2.6-B.1
Which of the 3 possible types of plate boundary is outlined by the earthquake positions on your cross section? Circle one: a. Convergent b. Divergent c. Transform (1)
2.6-B.2
Draw a general line just above the lower earthquakes in your cross section to show the top of the descending Nazca Plate. (Do this on the map above in pencil!) (2)
2.6-B.3
Lightly shade in the region of earthquakes that indicate where cool, brittle lithospheric plates are located and label 2 plates: Nazca and South America on your cross section graph above. (2)
2.6-B.4
Note the positions for the region of active volcanoes in the Andean Arc on the upper surface of the South America plate in your cross section. Look back at figure 2.2 and note where the mantle wedge is & the region of partial melting. Write MELT on your cross section where the magmas come from. (1)
2.6-B.5
Circle the deepest earthquake on your cross section and explain what this one might mean in a few words. Explain why there are no deeper earthquakes from what you know about the pressure and
14

temperature and strengths of the subducted slab versus the mantle asthenosphere surrounding it. ___
________________________________________________________________________________
_______________________________________________________________________________ (3)
Examine the simplified geological map and position of the San Andreas Fault on the Left.
The map on the right shows the ages of all the bedrock geology. On the left, the rocks labelled K are the Coast Range
Batholith and are Cretaceous intrusive igneous rock. In the map on the Right these came rocks are coloured pink and labelled Pre-Cenozoic crystalline rock. The are the roots of the old extinct Farallon volcanic arc. On the left, Miocene (epoch) in dark grey are sedimentary and they are really a bit older at
25 Ma (Oligocene Epoch). On the map to the right rocks of this age are called Tertiary (Period) and coloured Blue for those same Oligocene sediments or Brick Red-Brown for slightly younger (23.5 Ma,
Earliest Miocene) capping volcanic rocks. These 2 small blobs of Miocene Volcanics are barely visible on the map on the right and occur near 36° 30’ to the west of the San Andreas Fault, and just to the south of 35° and the Purple ultramafic rocks on the East Side of the San Andreas Fault. In the new 10 th edition these Tertiary rocks are labelled Os for the Sediments and Ov for the volcanics. Once, the volcanics were much closer together as were the slightly older 25 Ma sedimentary rocks. Currently the 2 cretaceous batholiths are close together, but about 125 Ma they were very far apart. At that time the green blob on the outer coast was relatively further enough south to be about where Acapulco Mexico is today. The San Andreas Transform Fault has been creeping along, and generating earthquakes for a really long time!
A.1. a Using the map on p 66 in your lab book find the orange Ov blobs or the map on the right above.
Locate the red-brown Tertiary Volcanics (23.5 Ma). Calculate the average rate of strike slip faulting across the fault for the past 23.5 Ma, in cm/yr by measuring the modern distance in km between the 2 separated volcanic units and assuming they have been gradually and steadily separating since they were deposited. Be sure to convert your units if you measure distance in km and divide them by millions of years! Show your work. _____________________________________________________cm/yr (3)
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A.1.b
In 1906 there was a devastating earthquake centered near San Francisco that killed about 700 people in all and nearly bankrupted the US Economy from all the fire insurance collected (there was no earthquake insurance). During that ~7.8 magnitude earthquake the 2 sides of the fault jumped a further
5 m apart with the Pacific Plate side moving NW and the North America Side moving SE. Assume that the entire distance between the midpoints of the 2 Os basins (Miocene on the map on the left above) occurred since 25 Ma in individual jumps of 5 m apiece. Calculate how many earthquakes have happened in 25 Ma. ____________________________________________________________ # quakes
The rate of these big 7+ earthquakes is there total number divided by the 25 Ma. Expresse this rate in quakes per 25 Ma. ________________________________________________________ #Quakes/Ma
How often (how long between) must these big earthquakes be in years? _______________ years (6)
B.
Instead of using the separation of really old geological formations to calculate the rates of motion across the San Andreas Fault, we can now use GPS arrays to see what the rate of deformation is per year. This tell us what is going on now and may be of more use to predicting earthquakes than som long term trend or average. It also lets us see the absolute motion of both sides of the fault in a global reference frame relative to the known satellite constellation. Go back to the JPL website you used for the
GPS earlier: http://sideshow.jpl.nasa.gov/post/series.html
. Manipulate the map image so it centers on
Southern California, then control the scale bar on the left until you get about Santa Barbara to Tijuana
Mexico. Notice how many more stations there are than your map here from the lab book. The general picture remains that inland (East) from the San Andreas Fault the vectors are smaller and the opposite as you cross the fault to the West and head towards the coast. Both the JPL website and this simpler map give the same synoptic (instantaneous, real-time) view of strain.
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2.7-B.1
Notice that all of the vectors show directions of motion towards the NW. This is the direction the Pacific Plate is moving, towards Japan and the Kurile-Kamchatka Subduction Zone. The scale on the map is in mm/yr. Calculate on average: how fast the Pacific Plate (& Los Angeles to San Francisco) is moving to the NW expressed as ________________________________________________ cm/yr (2)
Repeat the same estimate for the East side (Nevada) ________________________________ cm/yr (2) then calculate how many times faster the West side is moving than the East (X.XX times faster) ___
______________________________________________________________________________ (2)
2.7-B.2
If you zoom out on the JPL site map, you’ll see that this part of Southern California is being dragged along with the Pacific plate motion towards the NW, while the rest of North America further to the East is actually moving in a SW direction. On the map above, had half arrows to indicate the relative sense of motion across the San Andreas Fault. When you do this make your arrows smaller than the vectors already shown, colour them Red and circle them on the map. ________________________ (1)
2.7-C. Pick an average sized vector on each side of the fault, measure them and subtract them to get the difference and put it here in ____________________________________________________ cm/yr (2)
Now divide this difference by 2, because the strain is really split evenly between the 2 sides of the fault.
Write that value here _________________________________________________________ cm/yr (2)
Discuss what is happening here across the San Andreas Fault. If it were very weak and not connected at all, it would behave like 2 sides of a button up shirt, if you were to offset the buttons on one side from the holes on the other. The vectors describe what is really happening. Discuss this here and make a statement about the difference between “relative” plate motion and “absolute” plate motion.______ (4)
__________________________________________________________________________________
__________________________________________________________________________________
__________________________________________________________________________________
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Look at Figure 2.7 below. On the left the Hawaiian Islands occur at the red dot at the southwest end of a long NW trending seamount chain (row of submerged extinct volcanoes). Near ~40My. (Midway
Island) a new chain of undersea volcanoes called the Emperor Chain is shown with its Northerly trend.
On the right, a close up (smaller scale) map shows the trend for the individual Hawaiian Islands. For the purpose of this exercise, treat the position of the islands as being at the peak of their respective volcanoes and treat their age as being a single instant in time. In reality the volcanoers last several million years and the hotspot is really a region of upper mantl that is warmer than its sourroundings and several hundred kilometers across.
Dr. J. Tuzo Wilson, a geophysicist at the University of Toronto, developed the Hot Spot hypothesis to explain the progression in ages along the Hawaii-Emperor seamount chain. The idea is that a hot, long lived place in the Mantle continues to upwell and partially melt, as the lithosphere of the Pacific Plate slid over this. When this hot spot was considered together with 20 or more other major long lived hot spot volcanoes around the world (Iceland, Yellowstone, Kilimanjaro, Reunion, Azores etc. ) it permitted a “hot spot reference frame” to form a reference frame, against which to measure global plate motions. This was extensively used by other researchers including David C. Engebretson, Alan Cox and
R. D. Gordon (1985) to track plate motions and make a global plate model back through the Cretaceous.
Other approaches for calibrating plate tectonic motions involve the measurement of magnetic stripes on the seafloor due to past magnetic reversals and creating “Global Plate Circuit Models” like those pioneered by Joanne Stock and Peter Molnar (1983).
2.8-A.1.
In general words, describe the relationship of the Emperor Seamount chain in terms of their relative ages and orientations. _______________________________________________________ (2)
_______________________________________________________________________________
_______________________________________________________________________________
2.8-A.2.
Treat the whole Emperor seamount chain as if it were a single object on the northern Pacific
Plate. Describe its absolute motion during the period between 40 M.y. & 20 M.y. in terms of direction.
Which way was this line of extinct volcanoes moving (and the whole Pacific Plate along with it). Give its compass direction and rate of motion in cm/yr. ________________________________________
______________________________________________________________________________ (2)
2.8-A.3.
Consider the Pacific plate that the Hawaiian Islands are built upon. What was its compass direction of motion and rate in cm/yr between 4.7 and 1.6 Ma (Mega annum = Million years) ___ (2)
_______________________________________________________________________________
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2.8-A.4
What was the direction of motion and rate in cm/yr for the Pacific Plate under the Hawaiian
Islands since 1.6 Ma? _____________________________________________________________ (2)
2.8-A.5a.
Go back to the JPL website again http://sideshow.jpl.nasa.gov/post/series.html
and locate station NPOC on Hawaii. The green dot is the station location and the yellow arrow is its vector direction of motion as per the scales of the map. Compare the real time GPS vector direction to that of the map from the volcano ages and positions over geological rates and times. How does the modern GPS rate compare to the long term rate on your map here in the lab exercise for the last 40 Ma? ________ (2)
________________________________________________________________________________
________________________________________________________________________________
2.8-A.5b.
Use the NPOC GPS station data with the Latitude component of its vector motion of: +1.4825 mm/yr and Longitude component of its vector motion of -5.1612 mm/yr as given here. Use the formula
(template) on Figure 2.3, calculate the absolute velocity in mm/yr and its vector direction in degrees east of north as per the template. Your answer should look like -X.XXXX mm/yr ~Y.YY° East. ______ (3)
________________________________________________________________________________
2.8-B.
Describe in your own words how the motion of the Pacific Plate in the general region ahs changed between 60 Ma and now. Speculate as to where former ridge positions might have been or former subduction zones might have been at different spans of time over this interval. Keep in mind the
Big Source right now is well to the SE at the East Pacific Rise and the Big Sink for Pacific Plate seafloor right now is the Japan-Kurile-Kermadec Subduction Zone. __________________________ (5)
________________________________________________________________________________
________________________________________________________________________________
________________________________________________________________________________
2.8-C Use the map on p 69 in the lab manual or the 2 maps below to see the age progression of the
Yellowstone Hotspot trace from past Caldera positions and outcrop patterns over the past 13.8 Ma based on the work of Mark Anders. Then go back to http://sideshow.jpl.nasa.gov/post/series.html
and check out the GPS vector motion direction like station GTRG near Craters of the Moon about midway along this trend.
2.8-C.1. Discuss the geological and plate motion evidence that this trend of NE younging
Calderas from Steens Basalt on the Oregon-Nevada border to Yellowstone in the NW corner of
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Wyoming is a hotspot trace. _______________________________________________________ (2)
________________________________________________________________________________
________________________________________________________________________________
2.8-C.1. From the caldera ages and positions on the map above or in the manual, calculate the rate in cm/yr and the direction of the North America Plate with these piles of volcanic rocks burned up through it and sitting on top of it! _____________________________________ cm/yr
_____________________________________________________________________________ (3)
2.8-C.1. Discuss where hotspots occur relative to plate margins and how we can use them to discover the directions and rates of plate motion over geological spans of time up to > 10 Ma.
________________________________________________________________________________
________________________________________________________________________________
______________________________________________________________________________ (2)
Examine the Pressure (Depth) versus Temperature diagram in Figure 2.8 below. This is a phase diagram for peridotite in the upper mantle. The bold line is the Dry Peridotite Solidus (100% solid mineral crystals) and the Dry Basalt Liquidus (100% melting). The dashed line is the Dry Peridotite
Liquidus. The Dotted Line from Azuza et al (2009) shows the Wet (Saturated) Peridotite Solidus = Wet
Basalt Liquidus. Water becomes a better solvent at higher pressure. The fainter solid lines to the left show the Continental versus Oceanic Geothermal Gradients. The 2 vertical scales show the equivalence between depth & pressure. 100 km equals 30kbar or there is a pressure gradient of 3.3 km/kbar.
W e s t o lid u s
A. Examine Fig. 2. 7 on p. 41 provided. Refer to figures 2.1, 2.5 and 2.6 and read this graph to find the numerical values and physical form of:
1. Rocks at 80 km depth beneath a Continent would be at: _____________________ °C (1)
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2. Rocks at 80 km depth beneath an Ocean Basin would be at: ___________________ °C (1)
3. The Peridotite at X would be: (Choose and circle one answer):
solid / solid + liquid / liquid (1)
Explain why the peridotite at X will be in this state? ___________________________________ (1)
________________________________________________________________________________
4. What would happen to the peridotite X at a constant depth of 80 km if it were heated to 1750°C ?
____________________________________________________________________________ (1)
Why? _______________________________________________________________________ (1)
5. What would happen to the peridotite X at a constant pressure (isobaric) of 25,000 atm or depth of 80 km if it were heated to 2250°C ? __________________________________________________ (1)
Why? ________________________________________________________________________ (1)
B. Refer to Figure 2.8 above. The Peridotite at X is currently at 25 kbar.
1. If the peridotite at X were uplifted At constant heat content ( adiabatic conditions):
by tectonic rifting or rapid erosion, at what depth level and pressure would it begin to melt if it kept its
1200°C temperature? i.e. When does it hit the dry peridotite solidus?
___________________Depth (km) ______________________Pressure (atm) (2)
2. The partial melting caused by uplift (constant heat) is ___________________ decompression (1)
Hint read B.1 above again!
3.a. What tectonic process of mantle motion leads to this kind of uplift ____________________ (1)
3.b. This type of melting occurs under within-plate hotspots like Hawaii, Yellowstone or Iceland, however, it occurs more frequently beneath which tectonic setting ________________________ (1)
C. While most of the upper mantle is too cold to melt near its top, and too high a pressure to melt lower down, nonetheless basaltic magmas form from small percentages of partial melting and arise from depths like point X for 2 dominant reasons or processes. Simplify in your own words, your conclusions from problems A and B. For the peridotite at X to begin to melt:
1. What would have to happen to the temperature (or heat content)? ______________________ (1)
2. What would have to happen to the pressure ? _______________________________________ (1)
D-1.
Refer to Figure 2.8
and examine the demonstration with the hotplate and sugar cubes. 1. Which cube melted first? (Circle one) The wet one / The dry one (1 )
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2. The flux was ______________________________________________________________ (1)
3.a. Refer to my version of fig 2.8 & note the position of the Wet Peridotite Liquidus (my dashed line).
What would happen to the peridotite at X if the peridotite was suddenly wetted? ______________ (1)
_______________________________________________________________________________
3.b Is this (hotter than) above or (colder than) below the Oceanic geotherm? __________________ (1)
4.a
At which plate tectonic environment could cold water most easily enter the mantle (refer to fig 2.2)?
________________________________________________________________________________ (1)
4.b
What was the physical material that held this water before it was carried down into the mantle? ___
________________________________________________________________________________ (1)
E. Refer to Figures below on the left or p 71 in manual.
1 . What type of plate boundary is pictured? __________________________________ (1)
2 . Which of the melting processes produced the magma?_______________________ (1)
3. Describe the sequence of tectonic and mantle processes that led magmas to form here?
____________________________________________________________________ (3)
_____________________________________________________________________
F. Refer to Figures above on the right or p 70 in manual.
1. What type of plate boundary is pictured? ______________________________ (1)
2. Which of the 3 melting processes produced the magma?__________________ (1)
3. Describe the sequence of tectonic and mantle processes that led magmas to form here?________________________________________________________________
_____________________________________________________________________
___________________________________________________________________ (4)
4.
Describe 2 different and realistic ways that basalt can form in the upper mantle and relate this to the local tectonic setting and the associated processes in the mantle below. ___________________ (2)
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