Issue Date: September 16, 2007

Earthquakes: Why the Earth quakes and moves

The unimaginable forces behind earthquakes and volcanoes are no secret. Yet the notion that these forces cause whole continents to move is barely as old as the Beatles.

In the 1960s, the process known as plate tectonics became clear when researchers were mapping the deep sea in detail. They realized the ocean floors were spreading apart along mid-ocean ridges, where upwelling magma constantly created new material; in other places the continents were slowly colliding. In essence, the map was moving.

Seven major plates and many smaller ones make up the lithosphere, the outer surface of the Earth that's up to 155 miles thick in some parts. Those enormous plates slide over a layer of molten rock, the asthenosphere, at the blistering rate of 0.5 to 4 inches per year.

The San Andreas fault, where the Pacific and North American plates scrape crossways, is a lively example of a "transform boundary." Friction can cause stress to build up until it releases all at once in an earthquake, like the one that hit the San Francisco Bay area in 1989, causing 62 deaths and an estimated $6 billion in damage.

The Himalayas are the most dramatic illustration of a "convergent boundary," a slow-motion pileup of continental and oceanic plates. Incredible forces have lifted the world's tallest mountain range as the Indian plate grinds under the Eurasian plate like a shoe under a rug. Convergence often creates volcanoes such as those in the Andes.

The result is our puzzle-piece map of the world, left over from the splitting up of the supercontinent of Pangea about 200 million years ago.
-- Julian Smith

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