Environmental Engineering Reference
In-Depth Information
Source: U.S. Geological Survey
Plate Tectonics
The Earth's lithosphere (the crust and upper mantle) is broken into
tectonic plates
(also known as lithospheric
plates). These plates are in constant motion atop the
asthenosphere,
which is the Earth's molten mantle layer
that keeps the continents slowly moving. This movement of the continents is called
continental drift.
The reason that tectonic plates are in constant motion is a process called
seafloor spreading,
which is the
movement of the seafloor at the
mid-ocean ridge.
A mid-ocean ridge is the location from which
magma
(mol-
ten rock within the Earth) rises to the surface from the asthenosphere. It looks like a scar across the ocean floor.
As the magma pushes through the crust and hardens, new seafloor is created. As new magma surfaces, it
pushes away the existing seafloor, causing it to spread and move apart.
