Understanding Plate Tectonics

As little as a hundred years ago, earthquakes were thought to be spawned by demons or deities. The mere thought that something as solid and obviously stationary as the ground we stood on could actually be moving was considered ludicrous.

It had been noticed by the late 1500’s that certain landmasses such as Africa and South America seemed to fit together, yet despite this, scientists believed that the landmasses were stationary.

In the late 19th century, the presence of radioactive elements was discovered in the crust, and the heating abilities of the elements caused a shift in thinking. Using the new information, in 1912, Alfred Wegener put forth a new theory, the one called Continental Drift.

Continental drift suggests that at some time in the past, all the continents and landmasses were together, forming one giant super continent. As time went by, the giant continent broke up and the pieces floated away from each other. The mechanism for this, other that saying that it was because of the heat released by radioactivity was not explained. As a consequence, few scientists took the theory seriously.

The first supporting evidence for the theory of continental drift came from early seismology. When earthquake waves move through solid rock, they do so at a relatively constant rate. But when they encounter rock that is elastic rather than solid, each of the waves slows down and lessens in intensity since the elastic rock can give under the pressure.

From this information, scientists were able to deduce that not only was the rock under the crust in a semi-fluid state, but that there were areas where this elastic rock approached the surface or actually broke through to the surface. The bands where this happens occur mostly in the ocean, particularly in a ring located around the Pacific basin, which is called the Ring of Fire, since this is also an area that is very active volcanically.

In the last fifty years, the theory of continental drift has evolved into Plate Tectonics, as more information has been learned. It has been found that there are three main kinds of boundaries between the tectonic plates.

In constructive plate boundaries, the plates are pulling away from each other, allowing magma to rise to the surface from the mantle to create new rock. An example of this occurs roughly in the middle of the Atlantic Ocean and includes Iceland, where geologists can actually study the phenomenon.

In destructive plate boundaries, plates move together, with one sliding over the top of the other. This process is called subduction. The deepest points in the oceans occur in subduction zones. An example is located in the North Pacific near the Aleutian Islands of Alaska. In one area, the depth is over 35,000 ft (10,600 m) below sea level. When plates move together, they can also cause the formation of mountains as has happened with Himalayas, when the Indian plate smashed into the Asian plate about 50 million years ago.

The third kind of boundary is where two plates slide past each other. The point where this happens is called a transform fault. These occur in many areas, but perhaps one of the best known and studied is the San Andreas Fault in California.

Also, periodically, the polarity of the earth switches, so that North becomes South, and South becomes North. This has been known for a long time. Interestingly, as magma reaches the surface in a constructive plate boundary, it is magnetized. By taking core samples of rock on either side of the constructive plate boundary, it is then possible to measure the speed of the spreading, since geologists have a good notion of when the last polarity reversal took place.

From this information, we know that the rate of drift varies from .5 inch (.8 cm) a year in the North Atlantic, to 4 inches (6.4 cm) a year in the Pacific. This has also allowed geologists to figure out the relative movement of almost all the plates.

Plate tectonics explains not only that the continents move and how they move, but in what direction and how fast. It even explains why earthquakes and volcanoes occur and what the mechanism is for their occurrence. No doubt the theory will continue to evolve, and as it does, new important data will be found. This benefits us all in so many ways that it would take another article to explain them all.

The next time you hear of an earthquake or volcano occurring somewhere, at least now you know the underlying cause.