|
Tectonic
Plates
The theory of plate
tectonics proposes that the Earth's crust is broken up into eight major and and
about twenty smaller tectonic plates that move over the surface of the
Earth. These tectonic plates are massive, irregularly shaped slabs of
solid rock. The smaller plates are only a few hundred km in diameter,
while the major plates are thousands of km in diameter. Plate thickness is
also highly variable, ranging from less than 15 km at the oceanic ridges to over
200 km beneath the Himalayan Mountains. With the exception of the
dominantly oceanic Pacific Plate and some of the smaller plates, these plates
consist of both oceanic and continental crust and upper mantle material.
For example, the North American plate consists of the North American continental
mass and the oceanic crust extending out form the eastern edge of the North
American continent to the spreading ridge of the Mid-Atlantic ocean
ridge.
According to plate tectonic
theory, the outer 400 km of the Earth is is broken into two zones, the upper,
thinner, rigidly deforming lithosphere 100 km or less in thickness and the lower
ductilely deforming asthenosphere. The tectonic plates are part of the
upper lithospheric layer that rigidly deforms. Each of these lithospheric
plate acts as a coherent unit that moves relative to the other tectonic plates.
The lithospheric plates
"float" on the lower ductile hotter asthenosphere. This hot
plastic asthenosphere convects in the same way water in beaker over a Bunsen
burner convects. Hot, partially molten peridotite (a dense rock composed
of 60% or more olivine plus pyroxene and plagioclase, spinel, or garnet,
depending upon pressure) upwells at the oceanic ridge and continental rift
zones. Cold, dense oceanic slabs sink back down into the mantle in
subduction zones. The forces generated by these gravitational movement are
great enough to move continental masses a 2 - 15 cm a year. Over tens and
hundreds of millions of years, large tectonic movements can occur.
There are three types of plate
boundaries: 1) those composed of the tectonic plates that are moving
apart, 2) those composed of tectonic plates that are slipping sideways, one past
the other, and 3) those composed of tectonic plates that are colliding or
being pushed together. Regions where plates are diverging are commonly
called spreading centers, and coincide with large oceanic ridges. Plate
boundaries where crustal plates are slipping past each other horizontally are
marked by transform faults. The San Andreas Fault is a transform
fault. Regions where two plates are converging are called subduction zones
or continent-continent collision zones.
|
