Lecture 6- Isostasy and Plate Tectonics
David Sandwell
(Plate Tectonic notes by Lisa Tauxe)

A number of questions were posed in the last two lectures but the following three were never answered:

How does the oceanic crust form?
Why don't continents subduct?
The histogram of elevation shows two peaks, why?

This first part of this lecture will answer these questions:

New questions:

How old is the oldest oceanic crust and where is it located?
How old is the oldest continental crust?
What is a typical spreading rate?
What are two methods for measuring spreading rate?
What is a triple junction?

Actually I won't answer two of these questions this time either but you can look in the book or wait for future lectures. The second part of the lecture will be:

The Plate Tectonic Theory (notes by Lisa Tauxe)

  1. The crust and outermost mantle form a thick (100 km) strong coat of armor that floats on a weaker layer that may be partially molten. This outer layer is called the lithosphere and the underlying soft layer is the asthenosphere.
  2. The lithosphere is broken into a number of plates, that are in constant motion with respect to one another. There are three types of plate boundaries. Where plates move apart is called a divergent boundary and new oceanic crust is generated along ridges formed by chains of submarine volcanoes. Where plates move toward one another is a convergent boundary. There, one plate overrides the other. The down-going plate is consumed at a subduction zone. This process also generates volcanoes which form island or continental arcs. Places where plates slide by one another are transform boundaries and zones of diffuse deformation where the plate interactions are complicated are called plate boundary zones - a fourth type of plate boundary.
  3. Plates move according to the rules of rotations on a sphere developed by Euler.
  4. Imagine that you place your hands on a globe with your thumbs tucked under your palms and your index fingers touching. Now move your hands apart while keeping the tips of your index fingers in contact. Where your fingers touch is the pole of rotation. Where your fingers were is the spreading center. Note that the farther from the pole of rotation you are, the faster your hands are moving in space, even though the angular rate of rotation is constant.

  5. The grinding of the plates as they move with respect to one another is the principal cause of earthquakes and indeed, most earthquakes are located along plate boundaries:
  6. earthquakes

  7. When new crust forms, it is hot. Therefore it is less dense, more buoyant and it "sticks up" higher than the surrounding, older colder material. This is why ridges are higher than the surrounding crust. As the oceanic crust is pulled away from the spreading center, it cools and sinks. Also, the farther from the spreading center, the older the crust.
  8. The Earth's field is a great magnet which switches its polarity often and without warning. As crust forms, it retains a record of the magnetic field at the time. Thus, the ocean crust forms a gigantic tape recording of the "music of the Earth":
  9. Formation of marine magnetic anomalies at a mid-ocean ridge. Black and white parts of the oceanic crust (Layer 2) represent normal and reversed magnetizations respectively. New lithosphere is created at the central ridge and is spreads away. Crust acquires the magnetization of the prevailing field and blocks of alternate polarity add or subtract from the ambient Earth's magnetic field resulting in lineated stripes of anomalously high and low values of the total magnetic field.

  10. The magnetic field averages to approximately the rotational axis of the Earth. If one plots the location of the magnetic pole as viewed from a continent that is drifting, that pole appears to move. Thus, the apparent polar wander paths of continents record continental drift.

Evidence for the plate tectonic theory

The theory of plate tectonics as described above makes a number of testable predictions. Here are a few:

Data from earthquakes can be used to infer the sense of motion of the plates involved. Plate tectonic theory predicts that the plates will slide by one another as shown in the figure below. Without plate tectonics, you might guess that the motion would be the other way!

Earthquakes also should mark where the subducting slab is penetrating the mantle:

Earthquake data in fact strongly support the plate tectonic theory.

Seafloor Bathymetry
As already mentioned. The new crust is hot and buoyant and it sinks as it cools. The actual depth of the ocean as a function of age can be predicted very well from thermal models of a cooling slab in contact with cold sea water.
Age of the oceans
If the timing of the reversals of the magnetic field is known from other sources, the age of the sea floor can be determined by matching of the magnetic anomalies. These anomalies should match the reversal pattern determined from sediments such as that shown here:

Here is a picture of the age of the sea floor as determined from magnetic anomalies (by Dietmar Müller):

One could then go and drill the seafloor to check if the predicted ages matched the observed ages. This has been done with the aid of research drilling ships such as the JOIDES Resolution and the agreement is excellent.

Matching coastlines - matching APWP
Many people have had an urge to put Africa and South America together like pieces of a puzzle. If the idea of continental drift is correct, doing so should line up the apparent polar wander paths (see above). Here are the data and I think you will agree, that the APWP for North America and Europe agree much better after the two have been slipped back together (right diagram) than in their present coordinates (left).

Other evidence
In the early part of this century, a meteorologist named Wegener presented a large body of evidence in support of continental drift. He proposed that all the continents were once part of a single huge continent called Pangaea. He noted among other things that fossils of plants and land animals that could not travel across a huge ocean were similar on many of the continents of Pangaea as shown here:

Please check out this link for a great summary of continental drift.

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Lisa Tauxe