Lecture 4- Continental Drift and Plate Tectonics: Don't Believe it Yet!

David Sandwell

The Theory

Plate tectonic theory provides an overall framework in which to understand almost all aspects of the solid Earth. This lecture and video will discribe the various components of the theory. You will be shown an elegant theory with plates sliding thousands of kilometers across the surface of the Earth and large thick slabs of cold lithosphere falling into the hot squishy mantle. Should you believe this story? NO! Not based on what will be shown in the lecture.

Why should the Earth behave in such a strange manner? The other planets in our solar system don't appear to have plate tectonics; why should the Earth? Perhaps Prof. Sandwell and other geologists have created some weird, convoluted theory because they don't really understand how the Earth works.

There are lots of people in the world who still don't believe the plate tectonic story. The most common alternate hypothesis is that the Earth is expanding. Some argue that since cretaceous time (about 100 million years ago), the radius of the Earth has increases by a factor of two. Basically these alternate theories explain the spreading ridges, transform faults, marine magnetic anomalies and many other aspects of marine geology. This lecture and video will claim that radius and area of the Earth have remained relatively constant over the 4.6 billion year lifetime of the Earth. Thus subduction zones are required to distroy the extra surface area that is created at the spreading ridges. However, if you believe that the Earth is only 100 million years old and that it is expanding then you don't need subduction zones.

Here is a good problem. Suppose we measure the present opening rate of the mid-Atlantic spreading ridge to be 40 millimeters per year and assume that this rate has been constant in the past. Assume futher that there is only one ridge on the Earth, and there are no subduction zones. Under these assumptions the circumference of the Earth must be increasing at a uniform rate with time. How long ago was the Earth 1/2 the radius of the present-day Earth. (The present circumference of the earth is 40,000 km or 40,000,000,000 millimeters.) Hey this would be a good exam question!

Does plate tectonic theory explain the observations?

As you read the book and listern to the lectures always think to yourself; Is plate tectonics really true? Are these observations consistent with plate tectonic theory? Is there an alternate theory that can explain all of the observations?

Later in the course, we will address the issues of plate tectoincs again and provide supporting evidence for the theory. Recent exploration of the ocean basins as well as geodetic and seismological data provide very strong confirming evidence for plate tectonics. By the end of the course, I hope that you believe that the plates really do slide thousands of kilometers across the top of the slippery mantle and then fall back onto the Earth when they are old, cold, and heavy. This plate tectonic activity provides and efficient mechanism for releasing the radiogenic heat that forms in the interior of the Earth.

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Dave Sandwell