In one sense or another, we are all Living on an Island. The continents are buoyant rock masses that are floating in the Earth’s mantle-asthenosphere and surrounded by water at the surface. Earth’s surface is in constant motion and the ocean basins are continually evolving. The image of the Red Sea on this page is an example of some of the most recent change—an ocean basin is forming! We’re going to spend Lesson 2 exploring the Origin of the Ocean Basins and learning about how Sea Floor Morphology relates to the processes that have shaped the current ocean geometry. As I suspect you know, this all starts with Plate Tectonics and Sea Floor Spreading.
By the end of this lesson you should have a deeper understanding of: plate geometry and kinematics, the role of earthquakes, hot spots and how they relate to volcanic edifices, and continental margins. One of the things to think about this week is how you might develop a teaching module on Plate Tectonics.
By the end of Lesson 2, you should be able to:
The chart below provides an overview of the requirements for Lesson 2. For assignment details, refer to the lesson page noted. Due dates are listed in this table and in Canvas.
REQUIREMENT | LOCATION | SUBMITTED FOR GRADING? |
---|---|---|
Activity 1: In five easy parts... | see "activity 1" in lesson 2 menu | Yes - You should put a file with your answers in the Canvas dropbox, due June 16th |
Activity 2: Make map images and comment in a discussion forum. | see "activity 2" in lesson 2 menu | Yes- You should put a file with your answers and comments in the Canvas dropbox |
Activity 3: Hotspots, background reading and discussion | see "activity 3" in lesson 2 menu | Yes- Discussion responses will be graded. |
If you have any questions, please post them to our Questions? discussion forum (not e-mail), located in the Discussions menu in Canvas. I will check that discussion forum daily to respond. While you are there, feel free to post your own responses if you, too, are able to help out a classmate.
We need a basic understanding of plate tectonics in order to appreciate how the ocean basins have evolved over geologic time and how they will evolve in the future. I suspect that many, if not all of you, are experts on plate tectonics, and the M. Ed. program at Penn State has a course devoted to the Solid Earth (Earth 520), so we won’t cover things in an exhaustive/comprehensive fashion. Instead, we’ll pick a few representative activities that will highlight the connections we need for Oceanography and help you to see what additional background would be useful for you.
The notion that plates move has been around since at least early 1900's. Alfred Wegener, a German Meteorologist, proposed the theory of Continental Drift. He used a variety of observations to argue that the continents had moved and broken apart –including the shapes of coast lines, palaeontological and botanical data, and geological data. But he lacked a credible theory for motion. Physicists of his day dismissed the notion that the continents could move because they thought Earth’s interior was solid and rigid. Nevertheless, various people worked on the theory and proposed modifications. One such was Alex Du Toit, a south American geologist who collected geologic observations from both sides of the south Atlantic and published them in the 1930’s. A number of discoveries in the 1950’s and early 1960’s, including age dating of rocks and magnetic signatures in rocks, led first to the theory of ‘sea floor spreading’ and then to the theory of plate tectonics. A naval captain, Harry Hess, proposed sea floor spreading on the basis of bathymetric profiles he made in the pacific. His data showed that ocean depth increased systematically and symmetrically from a long, axial ridge. Hess’ data were combined with other key observations (including magnetic stripes, heat flow, seismicity along plate boundaries) and ideas (mantle convection) to form the theory of plate tectonics.
Tenets of the theory:
The Earth’s internal structure is made up of three layers called the crust, mantle, and core. The crust is the outermost layers made up of solid rocks mostly silicon and aluminum. The mantle is the layer beneath the crust and made up of mostly silicon and magnesium. The mantle has two layers called the upper mantle and the lower mantle, collectively called the lithosphere. The oceanic and continental plates are in the mantle. The core is the innermost layer of the Earth and is made of solid iron and nickel. The outer core is a liquid layer beneath the mantle and the inner core is the center of the Earth.
Note that there is a distinction between features defined by chemical composition (Crust, Mantle, Core, as shown in the sketch) and those defined by rheology (Lithosphere, Asthenosphere, Mesosphere --not shown here).
A few cool facts:
There are three types of plate boundaries:
In the parlance of structural geology: Divergent Boundaries correspond to normal faults, Convergent Boundaries correspond to thrust (or reverse) faults, and Transform Boundaries correspond to strike slip faults .
If you’d like more background on Faulting, see the page on "Faults" [6] from Prof. Eliza Richardson’s course, EARTH 520: Plate Tectonics and People.
Okay! The main thing is that it's roughly spherical (NOT flat!). The average radius of Earth is 6,371 km and the radius is nearly 22 km larger at the equator than at the poles (it's an ellipsoid, rather than a perfect sphere!). The lithospheric plates move on the outer surface of a sphere, so it's convenient to describe plate motion in terms of a rotation about a point on Earth's surface (or a rotation vector that hypothetically extends from Earth's center to the surface point). This point is called the pole of rotation, or just the rotation pole.
Let's start with a useful formula for the radius:
Earth is not a perfect sphere: its ellipticity can be written:
where Re is the equatorial radius=6378.14 km, and Rp is polar radius=6356.75 km. To first order, the variation in radius with latitude, , can be written:
Want more background or do you need more help with plate boundaries and plate names? Take a look at This PBS Learning Site [8]
Can you identify the type of faulting occurring at each plate boundary in the map below? What type of faulting is depicted between the Nazca and South American plates?
Let's do something with Plate Tectonics and ask how fast plates move relative to one another? The answer can be found by using plate rotation vectors. Stick with me for a minute or two. This looks more complicated at first blush than it really is. For our purposes, we just need the ability to plug numbers into an equation --so we need to follow the parameter definitions and the equation.
See "Simple Euler Poles [11]."
The motion of a point on one tectonic plate relative to another plate can be described by the relative velocity vector v. The velocity v has magnitude and direction and is given by the cross product of the angular velocity vector ω and the plate rotation vector r . The equation looks like this, where the "x" means cross-product. The reason we can't just use distance=rate*time is because we are describing the motion on the surface of a sphere as opposed to making the assumption that it's all flat and distances are simply linear.
v = ω x r
For example, according to one of the accepted models for plate motion (NUVEL 1), the velocity of the North American Plate relative to the Pacific Plate is given by the rotation pole at: 48.7° N 78.2° W and angular velocity 7.8x10-7 degrees/year (that is: 0.00000078 deg/year.) Therefore, a point on the Pacific plate near Parkfield California, which is at 35.9° N 120.5° W, is moving at 47.6 mm/yr relative to the rest of North America. How long will it take for this point to reach the present location of San Francisco?
How does this calculation work? Download this pdf file for the details [12]. That file contains some useful background. The last page is the example above.
NOTE: Parkfield CA is the site of a National Science Foundation project called EarthScope [13] that has drilled into the San Andreas Fault. See SAFOD Observatory [14] for more details on the drilling project.
Note the three types of plate boundaries (compare to the figure on the previous page) and the definitions of lithosphere, asthenosphere, and mesosphere. Lithosphere means the rigid part and thus the bottom of it is defined by an isotherm (do you know why?). The base of the lithosphere is typically taken as 1300° C. Note that the plate thickens as it moves away from a divergent spreading center. Mid-ocean ridge systems are hot (they are volcanoes!) and thus ridges are relatively buoyant, which means that they have relatively higher elevation than regions around them. Ocean depth increases systematically with distance away from mid-ocean ridge systems. We'll look at this more closely in Activity 3.
Note in the sketch below that the Earth and its plates are portrayed as a block instead of a sphere. If you think spherical geometry is difficult to work with, you are right. It's hard to visualize in your head and not so easy to sketch, either.
You can always use vector algebra to calculate linear velocity v from the position vector r and the angular velocity vector ω, but there's an easier way to get the magnitude of the velocity by using the solid angle between the pole of rotation and the location of interest (see below). The solid angle can be obtained using spherical trigonometry:
cos a = cos b cos c + sin b sin c cos A
where a is the solid angle of interest, b is the co-latitude of the location on Earth's surface, c is the co-latitude of the plate rotation pole and A is the surface angle between the pole and the location (that is: A is the difference between the longitude of the pole and the longitude of the location).
To work with plate motion vectors, and to calculate the linear velocity of points on Earth's surface, we need to know the distances between various points on the globe. A useful analogy is that of linear and angular velocities associated with Earth's daily rotation. That is, the angular velocity is the same everywhere on Earth. All points rotate through 360° (2 pi radians) in 24 hours. But the linear velocity, on Earth's surface, depends on where you are relative to the rotation axis. If you're at the North Pole, then you cover only a small distance, whereas if you're at the equator, then you cover a distance equal to Earth's full circumference in 24 hours (2 pi R). As Earth rotates each day, the linear velocity of points at the Equator is much larger than points near the poles. The same type of thing happens with plate motions. Points that are close to the pole of rotation move with lower linear velocity than points that are farther from the pole. So, we need to calculate the distance between each point and the pole. These next two figures will help show how this works. Remember, for our purposes, we just need to be able to plug numbers into an equation, so we need to follow the parameter definitions and the equation.
In the diagram above, upper case letters refer to surface angles and lower case letters refer to solid angles, measured between lines that extend from the Earth's center to the surface. For a point X at, say, latitude 20° N, the angle b is 70°, because b is measured from the north pole along a line of longitude. In the calculation, it's standard to use the 'co-latitude' b and c. Note that it's easy to get b and c, based on their latitudes. But the same is not true for the solid angle a. That's why we need spherical trig. Surface angles are perhaps more familiar. They are obtained from latitude and are therefore nothing more than a larger-scale version of the angle between the first-base line and the third-base line on a baseball diamond.
Here's an example that will help to fix ideas. Do you follow? If not, please post a question on Canvas.
Click for text description of the spherical trigonometry example image.
What is the magnitude of the linear velocity of the Eurasian plate w.r.t State College?
State College PA: 40.8°N, 77.9°W (-77.9°)
Use spherical trig identity
In our notation:
Check: Can you verify 73.3 deg. for this example? If not, make sure you're using co-latitude and that your answer is in degrees. Still having trouble? Then have a look at this [15].
Once you have the angular distance between the points (Δ), you can get the linear velocity using v = ω R sin Δ. See the last page of this pdf file for a worked example [16].
NOTE: For this assignment, you will need to record your work on a word processing document. Your work is best submitted in Word (.doc), or PDF (.pdf) format so I can open it.
NOTE: To start, you can enter just the latitude and longitude of the point of interest and hit submit. You'll get an answer, with default parameters. Hmm, does it work to just copy/paste in the numbers above? What happens if you write 122.5 W vs. -122.5? Can you include the ° symbols?
For our example problem, you should set the Reference to "PA Pacific" when doing the Golden Gate case, and "North America" when doing Hollywood (what happens if you choose Pacific for the Hollywood case?). You can also try NNR (no net rotation). Play around with this a bit. It's useful!
Under "Model" select "All of the above" so that you can see the range of predictions. Tell me your thoughts on why there are differences in the predicted rates of motions. What happens if you use a different frame of reference?
L2_activity1_AccessAccountID_LastName.doc (or .pdf).
For example, student Elvis Aaron Presley's file would be named "L2_activity1_eap1_presley.pdf"—this naming convention is important, as it will help me make sure I match each submission up with the right student!
Upload your paper to the "Lesson 2 - Activity 1" dropbox in Canvas by the due date indicated on our Course Schedule.
See the grading rubric [22] for specifics on how this assignment will be graded.
NOTE: For this assignment, you will need to record your work on a word processing document. Please submitted in Pages, Word (.doc), or PDF (.pdf) format so I can open it. Also, this doesn't have to be long. What I want is the figures I mention and some text to go with them that describes what you did and what you concluded. If we were in a classroom together I imagine this would be like an in-class or in-lab exercise, meaning I'm expecting you could do a reasonable job in an hour or two, depending on your reading speed and any prior experience with Google Earth.
L2_activity2_AccessAccountID_LastName.doc (or .pdf).
For example, student Elvis Aaron Presley's file would be named "L2_activity2_eap1_presley.doc"—this naming convention is important, as it will help me make sure I match each submission up with the right student!
Upload your document to the "Lesson 2 - Activity 2" dropbox in Canvas (see Dropboxes folder under the Assignments tab) by the due date indicated on our Course Schedule.
See the grading rubric [22] for specifics on how this assignment will be graded.
Now that we are experts on Plate Motions, let's think about how Volcanic Island Chains work and how they can help to understand plate tectonics and ocean process.
The dots on the map below show locations of major Hotspots on Earth's surface.
I made the image below with Google Earth. It shows the Hawaiian Island Chain and the Emperor Seamount Chain. Follow the linear track to the northwest from the Hawaiian islands (yellow lines show island coastlines). The features that are not outlined in yellow are below sealevel; they're called seamounts. The Hawaiian chain connects to the Emperor Seamount Chain, which has a more northerly trend. The seamounts are extinct volcanoes. Each one of them was once located over the Hawaiian Hotspot.
Hotspot tracks on the ocean floor were one of the first smoking guns for the theory of plate tectonics, and they were also one of the conundrums. Early evidence showed that hotspots were more or less fixed in space; they did not seem to move relative to one another. This led to the idea that they originated at great depth. But how could a narrow plume of heat, or low viscosity material, rise through the convecting mantle without being offset? Early researchers pointed to the analogy of smoke rising through the atmosphere: on a windy day, the smoke plume was offset, and when the wind changed direction, so did the plume.
The images below shows a basic idea of how hotspots and linear island chains work.
You will be graded on the quality of your participation. See the grading rubric [22] for specifics on how this assignment will be graded.
Want to explore these topics more? Here are some resources that might interest you.
Various Web site with links to resources aimed at teachers and students:
Reading the technical/scientific literature:
Have another Web site on this topic that you have found useful? Share it in the Comment area below!
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You have finished Lesson 2. Double-check the list of requirements on the first page of this lesson ("Lesson 2" in the menu bar) to make sure you have completed all of the activities listed there before beginning the next lesson.
If you comments about anything feel free to post your thoughts below. For example, what did you have the most trouble with in this lesson? Was there anything useful here that you'd like to try in your own classroom?
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Links
[1] http://www.slideshare.net/rahul/photos-of-earth-by-sunita-williams
[2] http://www.physicalgeography.net/fundamentals/images/earthcut.jpg
[3] http://pubs.usgs.gov/gip/interior/
[4] https://www.e-education.psu.edu/earth520/content/l4_p2.html
[5] http://pubs.usgs.gov/publications/text/Vigil.html
[6] https://www.e-education.psu.edu/earth520/content/l7_p3.html
[7] http://www.nasa.gov/centers/goddard/earthandsun/earthshape.html
[8] https://wpsu.pbslearningmedia.org/resource/ess05.sci.ess.earthsys.boundaries/tectonic-plates-and-plate-boundaries/#.WxA_VVMvwRE
[9] http://pages.uwc.edu/keith.montgomery/ribmtn/tektos.htm
[10] http://volcano.oregonstate.edu/vwdocs/volc_images/tectonic_plates.html
[11] https://sites.northwestern.edu/sethstein/simple-euler-poles/
[12] https://www.e-education.psu.edu/earth540/sites/www.e-education.psu.edu.earth540/files/540PlateTectonics.pdf
[13] http://www.earthscope.org/information
[14] http://www.earthscope.org/about/observatories
[15] http://www3.geosc.psu.edu/~cjm38/540/sphericaltrigExample.html
[16] https://courseware.e-education.psu.edu/courses/earth540/540PlateTectonics.pdf
[17] https://courseware.e-education.psu.edu/courses/earth540/GraderPennStatebaseballs.ppt
[18] https://www.e-education.psu.edu/earth540/sites/www.e-education.psu.edu.earth540/files/file/linearVelocityPlateMotionExample.mp4
[19] https://www.unavco.org/software/geodetic-utilities/plate-motion-calculator/plate-motion-calculator.html
[20] http://www.unavco.org/instrumentation/networks/status/pbo/overview/SBCC
[21] https://www.e-education.psu.edu/earth540/sites/www.e-education.psu.edu.earth540/files/SBCC.csv
[22] https://www.e-education.psu.edu/earth540/node/1704
[23] http://www.geol.ucsb.edu/faculty/macdonald/ScientificAmerican/sciam.html
[24] http://earth.google.com/
[25] https://www.e-education.psu.edu/earth540/sites/www.e-education.psu.edu.earth540/files/image/Lesson2/OceanBasin.png
[26] https://www.ngdc.noaa.gov/mgg/image/crustageposter.jpg
[27] http://jules.unavco.org/Voyager/Earth
[28] http://pubs.usgs.gov/gip/dynamic/hotspots.html
[29] http://pubs.usgs.gov/gip/dynamic/hotspots.html#anchor19620979
[30] https://www.e-education.psu.edu/earth540/sites/www.e-education.psu.edu.earth540/files/hotspotsChristensenNature1998.pdf
[31] https://www.e-education.psu.edu/earth540/sites/www.e-education.psu.edu.earth540/files/StockScience2003.pdf
[32] https://www.e-education.psu.edu/earth540/sites/www.e-education.psu.edu.earth540/files/StockScience2006.pdf
[33] http://www.ucmp.berkeley.edu/geology/anim1.html
[34] http://www.soest.hawaii.edu/GG/HCV/haw_formation.html
[35] http://www.unavco.org/edu_outreach/
[36] http://pubs.usgs.gov/gip/dynamic/Wilson.html
[37] https://courseware.e-education.psu.edu/courses/earth540/priv/clear_as_mud.pdf
[38] https://courseware.e-education.psu.edu/courses/earth540/priv/clearasmudSchall.pdf