Professor of Geology, Califonia Institute of Technology
E.A. Flinn Award 1993
The earth’s surface is thought to be composed of a dozen or so rigid plates. The sites where these plates come into contact are the sites of most of the interesting geological phenomena that exist, including earthquakes. The theory describing the action of these plates, termedplate tectonics, has been utilized for over 40 years, but may be limited in its utility.
Joining us today to discuss this issue is Prof. Jean-Philippe Avouac. Prof. Avouac is a Professor of Geology at the California Institute of Technology and was previously head of the Laboratoire de Télédétection et Risque Sismique and is a recipient of the prestigious E.A. Flinn Award from the AmericanGeophysical Union.
Prof. Jean-Philippe Avouac (JPA) talks with Charles Lee (CL) about extending the theory of plate tectonics.
CL: This is certainly a very fascinating issue especially for those of us living in California. I’m curious if you can first explain what is the theory of plate tectonics.
JPA: Plate tectonics is the theory that describes motion at the ground surface at the global scale. So,the idea is that the outer shell of the earth can be divided into a number of plates, and these plates are rigid over the long term so that deformation over the long term actually occurs along the boundaries of these plates. So the idea is that would explain the seismicity over the global scale and volcanism. And this theory that was proposed in the 60s has done wonders for reconciling a number ofgeological and geophysical observations that have been made over the last decades.
CL: And, what types of issues did it resolve?
JPA: You may remember the theory of continental drift, this idea that continents used to be welded together and then split apart. This idea was proposed in the early 20th century by Alfred Wegener and it was based on the observation that you would find the samefossils on different continents, so suggesting that at some time these continents used to be together. Also, it was based on the observation of glaciations that spread over several continents that were far away, and in order to explain this pattern of glaciations you needed to have them together. At that time, this theory was not accepted by most people, because there was no explanation for howcontinents might move on the surface of the earth. And, plate tectonics came in the 60s with the idea that the oceanic floor would be created along the mid-oceanic ridges. So, actually you don’t need to have the continents plowing their way along the oceanic floor, but actually it’s the oceanic floor that is moving. It is created along the ridges and disappears along the mantle subduction zone. So, inparticular, it could reconcile the old continental drift theory with a number of observations that were made after World War II.
CL: What is actually driving the motion of these plates?
JPA: The general idea today is that everything is driven by temperature. The earth is a huge thermal engine, and heat is transferred either by diffusion through the medium, or by advection, meaning that heat istransferred because the medium is moving. We know that in the earth, advection is the main mode of heat transfer. So, what happens is that there is a lot of heat from the initial formation of the earth and the decay of various radioactive elements, which is heating the earth. And, to cool down, heat is transferred outward by motion in the mantle, which we call convective motion. It’s just the sametype of convective motion you would have in a pan of heated water. And, what happens is that convection may be driving plate tectonics. But actually, we don’t know the exact relationship between what we see at the earth’s surface and what is going on in the mantle. One way to address that is to do some physical modeling of this to get a better view of the entire earth from various techniques, and...