Much about the forces at work within the Earth remains
unknown. But since the 1960s, geologists have increasingly
explained seismic events through the theory of "plate
tectonics." At the core of that theory is a simple
premise--that Earth's seemingly solid surface is actually
fractured and adrift.
Stop the World, I Want to Get Off
Earth's surface consists of a hard shell called the
crust, which is broken into large chunks called plates.
Plates can hold entire continents (as the North American
Plate does) or entire oceans (as the Pacific Plate does).
Often, they hold bits of both.
The crust is actually a hard exterior on top of a much
deeper layer, the mantle. Most of the mantle is made of
thick liquid rock called magma, melted by heat from an even
deeper layer--the Earth's core. The plates that make up the
crust basically "float" on the mantle's magma.
That means the plates and everything on them, including
whole continents and oceans, are in continual motion. Their
movement is slow, ranging from less than an inch a year to a
top speed of about 6 inches (15 cm) a year, but it's real.
Just what drives the movement isn't clear. It could be
gravity. It could be convection currents within the mantle.
It could be both.
Setting Boundaries
At certain places--called "divergent plate
boundaries"--two plates move away from each other, and magma
bubbles up from below the surface and hardens into new
crust. When such divergence occurs under an ocean (as it
usually does), it's called "seafloor spreading." When it
occurs on land, it produces a "rift valley."
At other places--called "convergent plate
boundaries"--two plates crash into each other. When two
continental plates collide, the crunch they produce can
slowly build entire mountain ranges. The tallest mountains
in the world, the Himalayas, just keep on rising, as the
Indian and Eurasian plates continually push against each
other.
When two oceanic plates converge, one generally dives
deep beneath the other, creating an underwater trench. The
deepest part of the Earth, the Marianas Trench, sinks almost
7 miles (11 km) below the ocean surface where the Pacific
and Philippine plates meet.
Finally, seismologists are interested in "transform plate
boundaries," places on the surface where two plates slide
alongside each other. The most widely known--the San Andreas
Fault--runs through California, where the Pacific and North
American plates have been sliding past each other for the
past 10 million years.
Ready to Rumble
In some places, plates creep beneath, above, or beside
each other in a slow and steady fashion. Such movement
creates continual seismic activity, but it's often so faint
that we can't even feel it. Yet in other areas, friction
between two plates can cause them to lock together. When
that happens, pressure builds until the friction is
eventually overcome. In a sudden lurch, the two plates break
free, releasing vast amounts of energy and causing a major
earthquake.
The magnitude of an earthquake depends mainly on the
amount of energy released, which is partly an effect of how
long the two plates have been locked together. Durations can
vary from mere minutes to several centuries. The buildup
helps seismologists predict the timing and intensity of
earthquakes, occasionally leading to warnings that a "big
one" is due at any time. For now, that's about as accurate
as earthquake prediction gets.
Christopher Call
December 29, 2004
Want
to learn more?
See what
happens when you move tectonic
plates
http://www.pbs.org/wgbh/aso/tryit/tectonics/
