Introduction
Turkey's tragedy
Long time shaking
Plate tectonics
Finding faults
Proving the theory
Perilous predictions
Safe buildings

1999-2004 Activity

Courtesy USGS.

Photo courtesy of the Appalachian Cultural Museum, Appalachian State University in Boone, N.C..

One glance at a globe shows the curious, interlocking shapes of South America and Africa. The question of why these coastlines fit together so nicely emerged when improved maps were developed in the 19th century.

To most geologists who thought about it, the answer was apparently "chance." But not Alfred Wegener, a German meteorologist who had plenty of time to dwell on such ponderables while spending the winter recording weather in Arctic huts. In 1915, Wegener published a book called "The Origin of Continents and Oceans," suggesting that all the continents had once been part of a giant super-continent called Pangea.

Arguing that the continents had moved gradually to their present positions over time, Wegener called his revolutionary theory "continental drift."

Revolutionary, shmevolutionary.
Unfortunately, neither Wegener nor anybody else could figure out what kind of a horse and buggy could move something as massive as a continent, and his theory was scorned as the junk science of his day. Only after half a century -- what we might call a geologic epoch -- did an updated version of continental drift become geology's accepted wisdom. We'll look at why scientists changed their minds shortly, but first let's check out today's understanding of how continents actually move. (Hint: It's not with an 18-wheel moving van).

A moving story
Earth's crust -- the top five to 40 kilometers or so -- is composed, at the present time, of seven huge and about 10 smaller, rigid "tectonic" plates that "float" on the semi-solid rocks of the upper mantle, allowing them to move independently. ('Tectonic' is seismo-jargon for the processes that deform the Earth's crust.) Movement of the surface is a response to movement in the mantle of convection currents shown schematically by the arrows in the figure above.

And what drives the convection currents? The immense amount of heat in the Earth's mantle and core which comes from:

• heat left over from Earth's formation out of hot gas and dust

• gravitation

• friction

• radioactive decay

At any rate, just as hot mash rises from the bottom of a pot of moonshine cooking in a backwoods still, hot rocks inside Earth get less dense and "try" to rise to the surface in a process that shifts the continental plates floating on the mantle. It is this hugely powerful but slow movement that causes collisions between continental plates.

Cold rock -- the kind found in tectonic plates -- cannot deform like hot rock, so the joints between those plates must slip, grind or crunch past each other. This movement occurs at various fault zones between plates. One well-known example is the San Andreas fault in California which causes earthquakes in the Golden State. Another example is the North Anatolian fault, which caused the tragic Turkish quake in August of 1999.

What exactly happens at these plate boundaries?