Essentials of Oceanography



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 the radius vector.
  4. Material is created and destroyed at spreading ridges and subduction zones, but not at transform boundaries.
  5. Transform faults form small circles to poles of rotation.

Three important issues for plate tectonics

Issue #1: Earth’s Internal Structure
Earths internal structure from inside out: inner core, outer core, mantle, crust
Sketch of Earth’s Internal Structure showing the main features.

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).

Check this out!

A few cool facts:

  • The density contrast between the Lithosphere (roughly 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.
  • Better place to learn more about Earth Structure.
  • Look at how thin the Crust is! Challenge: Find a better Earth analog than a Major League Baseball (see Activity 1)
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    Click for text description of image

    Text Description of Common Ellipsoids and Spheres Table

    Common Ellipsoids and Spheres
      Radius Thickness/ Radius: Crust Thickness/ Radius: Mantle Thickness/ Radius: Outer Core Thickness/ Radius: Inner Core
    Radial Thickness (km)
    6371km 0.0055
    35 km
    2855 km
    2260 km
    1221 km
    Moon 1738 km - - - (?)1
    Titleist Golf Ball 21.3 mm 0.1315 0.2253 - 0.6432
    Baseball (Major League) 36.3 mm 0.0716 0.4738 0.1680 0.2865
    Baseball (Little League) 35.6 mm 0.0702 - - 0.9298
    Foam Baseball 30.8 mm -- - - 1
    Softball (Worth, Blue Dot) 46.9 mm 0.0490 - - 0.9510
    Your_favorite_ellipsoid (?) _______ _______ _______ _______ _______

    Radius of volume equivalent sphere. Earth is an oblate spheroid; it's polar radius is 6357 km and it's equatorial is 6378 km. Foam Baseball's radii across and with the seams are 30.2 and 31.4 mm, respectively.

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.

Want to learn more?

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

Issue #3: Earth is an oblate sphere, which means it elliptical!

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 ellipse, 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: it's ellipticity can be written: f= (R e R p)/ R e

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, λ, can be written: R( λ )= R e ( 1f sin2 λ )

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."

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" 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?

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Based on a map prepared by the U.S. Geological Survey.