|
One of the best known faults is the San Andreas
fault in California. The fault, which marks the plate boundary
between the Pacific oceanic plate and the North American continental
plate, extends over 650 miles (1,050 km) of land.
photo by usgs
|
The biggest scientific breakthrough in the history of seismology --
the study of earthquakes -- came in the middle of the 20th century, with
the development of the theory of plate tectonics. Scientists proposed
the idea of plate tectonics to explain a number of peculiar phenomenon
on earth, such as the apparent movement of continents over time, the
clustering of volcanic activity in certain areas and the presence of
huge ridges at the bottom of the ocean.
The basic theory is that the surface layer of the earth -- the
lithosphere -- is comprised of many plates that slide over the
lubricating asthenosphere layer. At the boundaries between these huge
plates of soil and rock, three different things can happen:
- Plates can move apart - If two plates are moving apart from each
other, hot, molten rock flows up from the layers of mantle below the
lithosphere. This magma comes out on the surface (mostly at the bottom
of the ocean), where it is called lava. As the lava cools, it hardens
to form new lithosphere material, filling in the gap. This is called a
divergent plate boundary.
- Plates can push together - If the two plates are moving toward
each other, one plate typically pushes under the other one. This
subducting plate sinks into the lower mantle layers, where it melts.
At some boundaries where two plates meet, neither plate is in a
position to subduct under the other, so they both push against each
other to form mountains. The lines where plates push toward each other
are called convergent plate boundaries.
- Plates slide against each other - At other boundaries, plates
simply slide by each other -- one moves north and one moves south, for
example. While the plates don't drift directly into each other at
these transform boundaries, they are pushed tightly together. A great
deal of tension builds at the boundary.
Where these plates meet, you'll find faults -- breaks in the earth's
crust where the blocks of rock on each side are moving in different
directions. Earthquakes are much more common along fault lines than they
are anywhere else on the planet.
In the next section, we'll look at some different types of faults and
see how their movement creates earthquakes.