Earth 540: Essentials of Oceanography for Educators
Published on Earth 540: Essentials of Oceanography for Educators (https://www.e-education.psu.edu/earth540)

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Lesson 2

Living on an Island: Origin of Ocean Basins and Sea Floor Morphology

About Lesson 2

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Source: Sunita Williams, http://www.slideshare.net/rahul/photos-of-earth-by-sunita-williams.

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 at right 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.

What will we learn in Lesson 2?

By the end of Lesson 2, you should be able to:

  • Explain the basic tenets of plate tectonics including relative and absolute plate motion.
  • Use spherical trigonometry to calculate linear and angular distances on Earth’s surface. Employ this knowledge to work with plate motion vectors
  • Use on-line resources to calculate rates of plate tectonic motion.
  • Explain hot spots at a basic and sophisticated level, including how they can be used to determine relative plate motion, past theories of their origin, the concept of “Mantle wind,” and current ideas about their origin.
  • Describe the Wilson Cycle and related concepts of Sea Floor Spreading.
  • Discuss Earth’s internal structure, including the relationship and distinctions between chemical classifications (Crust, Mantle, etc.) and rheologic classifications (Lithosphere, Asthenosphere…).
  • Readily point out examples of the three basic plate boundaries and their tectonic, seismic and volcanic manifestations at Earth’s surface.
  • Use on-line resources to construct maps of the ocean floor

What is due for Lesson 2?

The chart below provides an overview of the requirements for Lesson 2. For assignment details, refer to the lesson page noted. See the Course Schedule (located in the Resources menu to the left) for assignment due dates.

REQUIREMENT

LOCATION

SUBMITTED FOR GRADING?

Activity 1: In five easy parts...

page 2 Yes -  You should put a file with your answers in the Angel dropbox, Try to finish by Friday Feb 3

Activity 2: Make  map images and comment in a discussion forum.

page 3 Yes- You should put a file with your answers and comments in the Angel dropbox

Activity 3: Hotspots, background reading and discussion

 

page 4

Yes-  Discussion responses will be graded.

Questions?

If you have any questions, please post them to our Questions? discussion forum (not e-mail), located under the Communicate tab in ANGEL. 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.

Overview

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.

A brief background on Plate Tectonics and Continental Drift

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:

  1. Plates are rigid
  2. Earth is a perfect sphere (and points on it can be specified with a radius vector)
  3. Plates move tangential to radius vector.
  4. Material created and destroyed at plate boundaries.
  5. One such boundary, Transform faults, form small circles to poles of rotation.

Three important issues for plate tectonics

Issue #1:  Earth’s Internal Structure

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Sketch of Earth’s Internal Structure showing the main features.
Source: http://www.oso.tamucc.edu/labs/lab4/Lab4_files/image004.jpg

Note the distinction between features defined by chemical composition (Crust, Mantle, Core) and those defined by rheology (Lithosphere, Asthenosphere, Mesosphere).

Check this out!

A few cool facts:

  • The density contrast between the Lithosphere (roughtly 2.5 to 3 gm/cc) and the Atmosphere above it (roughly 0) is not nearly as big as that between the Outer Core (roughly 11 gm/cc) and the base of the Mantle (roughly 6 gm/cc).
  • Good place to learn more about Earth Structure. [1]
  • Better place to learn more about Earth Structure. [2]
  • Look at how thin the Crust is!   Challenge: Find a better Earth analog than a Major League Baseball (see Activity 1)
    Contact the instructor if you have difficulty viewing this image

Issue #2: Plate Boundaries

There are three types of plate boundaries:

  1. Divergent,
  2. Convergent, and
  3. Transform.

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 .

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Artist's cross section illustrating the main types of plate boundaries.
Source: Cross section by José F. Vigil from This Dynamic Planet—a wall map produced jointly by the U.S. Geological Survey, the Smithsonian Institution, and the U.S. Naval Research Laboratory. http://pubs.usgs.gov/publications/text/Vigil.html [3]

Want to learn more?

If you’d like more background on Faulting, see the page on "Faults" [4] from Prof. Eliza Richardson’s course, EARTH 520: Plate Teconics and People.

Issue #3: Earth is an oblate sphere

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. 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:

Contact the instructor if you have difficulty viewing this image

 

Want to learn more?

Read about changes in Earth’s shape due to climate events in the following NASA article: "Most Changes in Earth's Shape Are Due to Changes in Climate [5]."

Map view symbols for plate boundaries

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Want to learn more?

Want more background or do you need more help with plate boundaries and plate names? Take a look at "Tectonic Plates and Plate Boundaries [6]" from the Teachers Domain Web site.

Try this!

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?

Contact the instructor if you have difficulty viewing this imageBased on a map prepared by the U.S. Geological Survey.
Source: http://volcano.oregonstate.edu/vwdocs/volc_images/tectonic_plates.html [7]

Plate Motion and Activity 1.

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.

Want to learn more?

See "Simple Euler Poles [8]."

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 : 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 7.8e-7 deg/year (note that you should read 7.8e-7 as 7.8 times ten raised to the negative 7, that is: 0.0000007.8 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 [9]. 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 [10] that has drilled into the San Andreas Fault. See their page on the SAFOD Observatory for more details on the drilling project. [11]

One of the first sketches to show tectonic plates

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 is 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.

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Isacks, B., J. Oliver, and L. R. Sykes (1968), Seismology and the New Global Tectonics, J. Geophys. Res., 73(18), 5855–5899.

Plate motion vectors

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).

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Plate Motion Vectors

Some background on Spherical Trigonometry

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 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 should help to see 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.

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Spherical Trigonometry

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°. 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. This is 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?:

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Magnitude of linear plate velocity

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 [9]

Activity 1

NOTE: For this assignment, you will need to record your work on a word processing document. Your work must be submitted in text (.txt), Word (.doc), or PDF (.pdf) format so I can open it.

Directions

  1. Find a suitable analog for Earth's internal structure and measure the relative thicknesses of the layers. Here is an image that goes along with mine (a major league baseball), see details of my numbers on the previous page:Contact the instructor if you have difficulty viewing this image
    (Are you a fan? Here's a short set of ppt images showing how baseballs have changed over the years.) [12]
  2. Using the formula given on the previous page, calculate Earth's radius at your location. Note, this is just a sort of 'warm up' for us. We could expand on this and account for differences in radius as a function of position, but we won't go that far here. I thought you might like to see how much the radius changes as a function of latitude.
  3. Now, we're ready for a real problem. Calculate the rate of motion between the Pacific Plate and the North American Plate at the following two locations. I expect that you will show enough of your work so that I can follow how you did the calculation. At the least, you should list all of the parameters and factors used in your calculation.
    • 37.8 N 122.5 W (the golden gate bridge)
    • 34.1° N 118.3° W (Hollywood, CA)
  4. Check your answer using the plate motion calculator [13] at the UNAVCO Web site.

    NOTE: You can enter just the latitude and longitude of the point of interest. You'll get an answer, with default parameters. 

    For this 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?)

    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?

  5. Go to the UNAVCO Web site [14] and determine a plate motion rate using their GPS data for the Mission Viejo, CA station. You'll have to click on the Mission Viejo, CA (SBCC, upper left side of the page) and download the data. Then, you can fit a line to each of the components (North, East --note that the site has absolute values of position and also deviation; I would suggest using absolute values, but you can use either because we just want the rate of change) and use that rate to determine a rate along the orientation of the plate boundary (which is roughly: 300° cw from North). How does this value compare with the prediction from the Plate Motion Calculator for this latitude?
  6. Save your document as either a text, Microsoft Word, or PDF file in the following format:

    L2_activity1_AccessAccountID_LastName.txt (or .doc or .pdf).

    For example, student Elvis Aaron Presley's file would be named "L2_activity1_eap1_presley.txt"—this naming convention is important, as it will help me make sure I match each submission up with the right student!

Submitting your work

Upload your paper  to the "Lesson 2 - Activity 1" dropbox in ANGEL by the due date indicated on our Course Schedule.

Grading criteria

See the grading rubric [15] for specifics on how this assignment will be graded.

Motion Under the Ocean, Hot Spots

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 location of major Hotspots on Earth's surface.

Contact the instructor if you have difficulty viewing this image

http://jules.unavco.org/Voyager/Earth

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 coastline). 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.

Contact the instructor if you have difficulty viewing this image
Google Earth

Want to learn more?

  • More information about hotspots, see "'Hotspots': Mantle thermal plumes. [16]"

Activity 2

NOTE: For this assignment, you will need to record your work on a word processing document. Your work must be submitted in text (.txt), Word (.doc), or PDF (.pdf) format so I can open it.

Directions

  1. First, read this short paper. It's important background for mid-ocean ridges and the bathymetry around them:
    • Macdonald, K.C. and P.J. Fox, The mid-ocean ridge [17], Scientific American 262:72-79, 1990.
  2. Download Google Earth [18] and spend enough time with it to make an image of the ocean floor. In particular, look at the bathymetry around a mid-ocean ridge spreading center. Paste your image into a word processing document as noted above. Note that you should look at parts 3 and 4 before you complete this.
  3. In your document, comment briefly on how seafloor depth and morphology varies with increasing distance from a mid-ocean ridge spreading center. For example, I'm hoping you'll produce an image in part 1 that will tie-in with the article noted above. Think big, don't limit yourself to a few km from the spreading ridge; consider seafloor morphology at the scale of an entire ocean basin, like this example image [19].
  4. Look at the information on this web site [20] Take a screen shot or use another approach to make a map of the ocean floor showing seafloor age. Add one or two other features to your map, and describe in a few sentences whether you could use a Web site like this in one of your classes. Paste your images into the word processing doc along with your comments.
  5. Save your document as either a text, Microsoft Word, or PDF file in the following format:

    L2_activity2_AccessAccountID_LastName.txt (or .doc or .pdf).

    For example, student Elvis Aaron Presley's file would be named "L2_activity2_eap1_presley.txt"—this naming convention is important, as it will help me make sure I match each submission up with the right student!

Submitting your work

Upload your document to the "Lesson 2 - Activity 2" dropbox in ANGEL (see Dropboxes folder under the Lessons tab) by the due date indicated on our Course Schedule.

Grading criteria

See the grading rubric [15] for specifics on how this assignment will be graded.

Activity 3

Hotspots and Ocean Floor Morphology

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.

Contact the instructor if you have difficulty viewing this image
http://pubs.usgs.gov/gip/dynamic/hotspots.html#anchor19620979

Activity 3

Directions

  1. Read each of these short papers:
    • Christensen, Fixed hotspots gone with the wind, Nature, 391, 739, 1998. [21]
    • Stock, Hotspots come unstuck, Science, 301, 1059-1060, 2003. [22]
    • Stock, The Hawaiian-Emperor Bend: Older Than Expected, Science, 313, 1250-1251, 2006. [23]
  2. Post your responses to the following questions in the discussion area below (not on ANGEL).
    • Why do hotspots form distinct volcanic islands, rather than a continuous linear axis?
    • What major change occurred recently in our thinking about Hotspots, and what were the data that led people to rethink the way hotspots work?
    • Many textbooks say that the change in trend of the Hawaiian Islands and the Emperor Seamounts represented a major shift in the direction of the Pacific plate motion ~ 43 Ma. Is that still thought to be true?
  3. Read the postings made by other EARTH 540 students.
  4. Respond to at least two other postings, per question, by asking for clarification, asking a follow-up question, expanding on what has already been said, etc. You should have at least six responses to other postings.

Submitting your work

  • I have begun this discussion activity by posting each of the 3 questions to the "Comments" space below. Simply click on the "reply" link that follows each question in order to post your response. To respond to another student's posting, use the "reply" link that follows their posting.

    Don't see the "Post a Comment" area? You need to be logged in to this site first! Do so by using the link at the top of the left-hand menu bar. Once you have logged in, you may need to refresh the page in order to see the comment area below.

Grading criteria

You will be graded on the quality of your participation. See the grading rubric [15] for specifics on how this assignment will be graded.

 

Additional Resources

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:

  • Animations of Plate Motion and Continental Drift, UC Berkeley Site [24]
  • Hawaiian center for Volcanology [25]
  • University NAVSTAR Consortium is the data warehouse and central facility for GPS and other forms of geodetic data [26]
  • Early history of plate tectonics: Discovering transforms and hotspots [27]

Reading the technical/scientific literature:

  • Clear As Mud : A concise and stimulating summary of accessibility in scientific literature. Perfect for a discussion of how to write, and how to read, science. [28]
  • Summary of Clear As Mud and suggestions for how to read scientific literature. [29] Written by the former director of a Penn State writing center.

Tell us about it!

Have another Web site on this topic that you have found useful? Share it in the Comment area below!

Don't see the "Comment" area below? You need to be logged in to this site first! Do so by using the link at the top of the left-hand menu bar. Once you have logged in, you may need to refresh the page in order to see the comment area below.

Summary & Final Tasks


Reminder - Complete all of the lesson tasks!

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.

Tell us about it!

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?

Don't see the "Comment" area below? You need to be logged in to this site first! Do so by using the link at the top of the left-hand menu bar. Once you have logged in, you may need to refresh the page in order to see the comment area below.

Authors: Michael Arthur and Chris Marone, Department of Geosciences, College of Earth and Mineral Sciences, The Pennsylvania State University.

Creative Commons License

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This courseware module is part of Penn State's College of Earth and Mineral Sciences' OER Initiative.
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Source URL: https://www.e-education.psu.edu/earth540/content/c2.html

Links:
[1] http://pubs.usgs.gov/gip/interior/
[2] https://www.e-education.psu.edu/earth520/content/l4_p2.html
[3] http://pubs.usgs.gov/publications/text/Vigil.html
[4] https://www.e-education.psu.edu/earth520/content/l7_p3.html
[5] http://www.nasa.gov/centers/goddard/earthandsun/earthshape.html
[6] http://www.teachersdomain.org/asset/ess05_int_boundaries/
[7] http://volcano.oregonstate.edu/vwdocs/volc_images/tectonic_plates.html
[8] http://www.earth.northwestern.edu/people/seth/demos/BRICK/brick.html
[9] https://courseware.e-education.psu.edu/courses/earth540/540PlateTectonics.pdf
[10] http://www.earthscope.org/home
[11] http://www.earthscope.org/observatories/safod
[12] https://courseware.e-education.psu.edu/courses/earth540/GraderPennStatebaseballs.ppt
[13] http://sps.unavco.org/crustal_motion/dxdt/model/
[14] http://www.unavco.org/edu_outreach/data.html
[15] https://www.e-education.psu.edu/earth540/grading_rubric_problemsets
[16] http://pubs.usgs.gov/gip/dynamic/hotspots.html
[17] http://www.geol.ucsb.edu/faculty/macdonald/ScientificAmerican/sciam.html
[18] http://earth.google.com/
[19] https://www.e-education.psu.edu/files/earth540/image/Lesson2/OceanBasin.png
[20] http://sos.noaa.gov/datasets/Land/sea_floor_age.html
[21] https://courseware.e-education.psu.edu/courses/earth540/priv/hotspotsChristensenNature1998.pdf
[22] https://courseware.e-education.psu.edu/courses/earth540/priv/StockScience2003.pdf
[23] https://courseware.e-education.psu.edu/courses/earth540/priv/StockScience2006.pdf
[24] http://www.ucmp.berkeley.edu/geology/anim1.html
[25] http://www.soest.hawaii.edu/GG/HCV/haw_formation.html
[26] http://www.unavco.org/edu_outreach/
[27] http://pubs.usgs.gov/gip/dynamic/Wilson.html
[28] https://courseware.e-education.psu.edu/courses/earth540/priv/clear_as_mud.pdf
[29] https://courseware.e-education.psu.edu/courses/earth540/priv/clearasmudSchall.pdf