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Composition of Earth's Plates

Also called tectonic plates, the Earth's plates cover every inch of its surface. Making up the lithosphere, or the top interior layer of the Earth, directly under the crust, the plates float on the liquid part of mantle. Floating is what allows them to move, and the movement of the plates is what causes earthquakes and volcanic activity.

Types of Plates
- Sedimentary rock, like limestone, is formed by the build up of various sediments, usually underwater.
  
  There are two types of plates that make up Earth's tectonic plate system: oceanic and continental plates. Oceanic plates are only located under oceans, and there are only three major oceanic plates. The smallest of the three, the Cocos plate, is located west of central American, whereas the largest, the Pacific, spans most of the Pacific ocean. The third, the Nazca plate, is located west of South America. Although continental plates are located under continents, they also usually extend into the ocean as well. Only small continental plates, like the Turkish-Aegean Plate, do not extend into the ocean.
Materials
- Oceanic plates are made of the same material that makes up the ocean's crust, primarily basaltic igneous rock. Igneous rock is a type of volcanic rock, which is created by cooled magma. Less dense than the oceanic plates, continental plates are made from all three types of rock: sedimentary, igneous and metamorphic. Continental plates are much more complex in their composition and have more variation from plate to plate. They tend to have a high percentage of silica and are often made of rocks like granite.
Size
- There are approximately 12 plates that cover the Earth. Though many are larger than the continents they are named for, some are comparatively tiny. For example, the North American plate measures almost 43.5 million square miles, whereas the Galapagos microplate measures only 7,456 square miles. Additionally, the oceanic plates are considerably thinner than the continental plates. On average, the oceanic plates are about six miles thick. Conversely, the continental plates can be anywhere from 25 to 45 miles thick.
Boundaries and Behavior
- Tectonic activity only occurs at the edges of the plates, where two or more plates meet. There are three different types of boundaries between plates. A divergent boundary is a type of extensional boundary that is created when new crust forms between plates. These usually occur between oceanic plates. Transform boundaries and convergent boundaries both cause earthquakes and volcanic activity. Transform boundaries occur when plates slide against each other, creating seismic waves. Convergent boundaries occur when one plate slides underneath another, which also creates seismic waves.

Rocks & Minerals