Plate Tectonics
Plate tectonics is a model in which the outer shell of the Earth is divided into
a number of thin, rigid plates that are in relative motion with respect to one
another. The relative velocities of the plates are of the order of a few tens of
millimeters per year. A large fraction of all earthquakes, volcanic eruptions,
and mountain building occurs at plate boundaries.
The plates are made up of relatively cool rocks and have an average
thickness of about 100 km. The plates are being continually created and
consumed. At ocean ridges adjacent plates diverge from each other in a pro-
cess known as seafloor spreading. As the adjacent plates diverge, hot mantle
rock ascends to fill the gap. The hot, solid mantle rock behaves like a fluid
because of solid-state creep processes. As the hot mantle rock cools, it be-
comes rigid and accretes to the plates, creating new plate area. For this
reason ocean ridges are also known as accreting plate boundaries. The ac-
cretionary process is symmetric to a first approximation so that the rates
of plate formation on the two sides of a ridge are approximately equal.
