Plate Tectonics

Plates explained


"The world's earthquakes are concentrated in narrow zones. Why is this? And why are volcanoes and mountain ranges also found in these zones, too?

The theory of plate tectonics combines many of the ideas about continental drift (originally proposed in 1912 by Alfred Wegener in Germany) and sea-floor spreading.  This notion of plates moving and grinding against each other has completely changed thinking about volcanoes and earthquakes in the last 10 years.

Most of the earthquakes are confined to narrow belts and these belts define the boundaries of the plates. We know that each year about 140 earthquakes of magnitude 6 or greater will occur within this area which is 10 percent of the Earth's surface.

The interiors of the plates themselves are largely free of large earthquakes, that is, they are aseismic. Thus, we can predict the general regions on the Earth's surface where we can expect large earthquakes in the future. 

There are notable exceptions to this. An obvious one is the 1811-1812 earthquakes at New Madrid, Missouri, and another is the 1886 earthquake at Charleston, South Carolina. As yet there is no satisfactory plate tectonic explanation for these isolated events; consequently, we will have to find alternative mechanisms.

Can plate tectonics help in earthquake prediction? Earthquakes occur at the following three kinds of plate boundary:

  • ocean ridges where the plates are pulled apart
  • margins where the plates scrape past one another, and
  • margins where one plate is thrust under the other.

On a worldwide basis we cannot say with much accuracy when these events will occur. Plate movements have been going on for millions of years. Averaged over this interval, plate motions amount to a several millimeters per year. But at any instant in geologic time, for example, the year 1977, we do not know exactly where we are in the worldwide cycle of strain buildup and strain release. Only by monitoring the stress and strain in small areas, for instance, the San Andreas fault, in great detail can we hope to predict when renewed activity in that part of the place tectonics arena is likely to take place.

In summary, plate tectonics is a blunt tool that tells us where 90 percent of the Earth's major earthquakes are likely to occur, but it cannot tell us much about exactly when they will occur. Perhaps the most important role of plate tectonics is that it is a guide to the use of finer techniques for earthquake prediction." 

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