The Earth is generally considered to have 15 major tectonic plates, seven or eight of which are primary plates, and the others are smaller, secondary plates. The primary plates are the African Plate, the Antarctic Plate, the Eurasian Plate, the Indo-Australian Plate, the North American Plate, the Pacific Plate and the South American Plate, with the Nazca Plate sometimes considered a primary plate instead of a secondary plate. The secondary plates are the Arabian Plate, the Caribbean Plate, the Cocos Plate, the Indian Plate, the Juan de Fuca Plate, the Philippine Sea Plate, the Scotia Plate and sometimes the Nazca Plate. In addition, there are dozens of minor, tertiary plates. Just as there is no consensus about whether the Nazca Plate is a primary or secondary plate, there is no agreement about exactly how many tertiary plates there are, but scientists have identified about 58 of them.
Theory of Plate Tectonics
Tectonic plates are great slabs of rock that form the Earth's top layer, called the lithosphere. Both the continents and the oceans rest on tectonic plates, which float on the asthenosphere, the superheated molten rock below. Scientists believe that over millions of years, these tectonic plates float around, driven by convection currents in the asthenosphere, gathering into supercontinents and scattering again. This is called the theory of plate tectonics. Tectonic plates move about as fast as a person's fingernails grow — about 2 to 4 inches (5 to 10 cm) per year.
Although primary plates often share names with continents or oceans, their boundaries do not simply match those areas. For example, the North American Plate includes Greenland, which typically is associated with Europe. Each of the primary plates also includes areas under one or more oceans. The largest primary plates are the Antarctic Plate, the Eurasian Plate and the North American Plate. The Pacific Plate includes most of the Pacific Ocean as well as part of Southern California.
Secondary and Tertiary Plates
Some scientists classify primary plates and secondary plates together under the term "major plates." Of the plates typically classified as secondary plates, the Arabian Plate and the Indian Plate include the largest areas of land. Tertiary plates are smaller parts of primary or secondary plates that have broken off and are being broken up over time. These are often grouped with their associated primary or secondary plates.
Movement Along Plate Boundaries
When adjacent plates move, they might move away from each other, toward each other or in parallel directions along their boundary. This movement can cause an earthquake and might create a geological formation. When the plates move away from each other, called divergent movement, they can create rift valleys on land. In the ocean or sea, they can create ridges or volcanic islands, which occur when the spreading sea floor provides an opening for molten lava to rise out of the earth.
Convergent movement is when the plates move toward each other. As the plates collide, one might be forced under the other, or they both might be forced upward. Two plates converging under the ocean can cause islands to be formed. An oceanic plate colliding with a continental plate can form mountains near the coast. Two continental plates colliding can form a mountain chain, such as the Indian Plate and the Eurasian Plate forming the Himayalas.
Adjacent plates sometimes move in parallel directions along their boundary, which is called a strike-slip fault. For example, the Pacific Plate moves northward as the adjacent North American Plate moves southward along the San Andreas fault in California. Although movement along fault lines such as the San Andreas does not create formations such as mountains or valleys, it does cause frequent earthquakes.