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Plate
Plate
A large segment of the Earth’s crust separated from other segments by deep fractures that extend from the surface to the mantle (see: Lithosphere). These segments are also called tectonic plates and crustal plates. The boundaries of plates are common locations for volcanoes and earthquakes.
The Earth’s crust is quite thin, compared to the size of the planet, and it is made of brittle rocks. Movements in the molten material in the mantle below the crust
are therefore able to split up the crust into the giant segments called plates (see: Asthenosphere).
On average, plates are dragged a few centimeters each year. This may not sound like much, but over millions of years it results in the movement of plates over large distances. The Atlantic Ocean,
for example, has formed by this slow movement over the past
100 million years.
As the plates move, they pull apart in places. These areas are
 Plate—The surface crust is a thin layer that “floats” on the mantle below. Here the edges
of the continental plates have been exaggerated to make them easier to see. Volcanoes occur at most edges of the crustal plates.
called spreading boundaries, and they allow magma to flow easily from the mantle to the surface. The lava flowing from fissures
at spreading boundaries is always basalt.
Most spreading boundaries are under the oceans, where the crust is thin. Eruptions under the oceans are rarely noticed and only become obvious where the boundary is on land, as is the case in Iceland.
As the plates pull apart, lava
flows up the fissures to “seal the wound.” This is why the ocean floor is made of basalt rock.
Large earthquakes do not occur at spreading boundaries.
Where plates collide, one plate
is usually pushed below another.
At the same time, rocks that have formed on the seabed may be pushed up to form mountain ranges.
 Plate—The map shows the main plates and plate boundaries.
Juan de Fuca Plate
Pacific Plate
San Andreas Fault
North American Plate
Caribbean Plate
Nazca Plate
Eurasian Plate
Arabian Plate
Philippine Plate
Pacific Plate
African Plate
South American Plate
Indo-Australian Ocean Plate
Constructive plate margins (oceanic ridges offset by transitional faults) Destructive plate margins (oceanic trenches or continental collision zones)
Fold mountain range Direction of plate movement
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