Plate tectonics - see below

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The theory of plate tectonics is a model which explains processes taking place beneath the earth. It is an expression of convection forces that are taking place in the mantle and having repercussions on the earth’s surface.
The lithosphere is the outer layer of the earth and it is subdivided into rigid plates which float and move on the asthenosphere which is a more fluid and ductile layer.
The theory of plate tectonics was preceded by Alfred Wegener’s theory of Continental Drift which suggested that the earth’s continents were formed from one big continental block, but gradually moved away to their current locations. Later in 1945, Arthur Holmes proposed the hypotheses of convection forces in the mantel being at the origin of continental movements. In 1962, Harry Hess equally suggested that the earth was like some sort of floating carpet in constant motion. Following ideas from the preceding authors, William Jason Morgan, Dan Mckenzie and Xavier Pichon established the theory of continental drift in 1967. They identified two major kinds of plates; oceanic and continental plates. The following plates were identified:

The last two are purely oceanic plates while the remaining are combinations of both types.
This theory has evolved and more research has led to the identification of other plates to add up to the list as follows: (source of plates below is wikipedia.org)
The following tectonic plates currently exist on the Earth's surface with roughly definable boundaries.

Primary plates

These seven plates comprise the bulk of the seven continent's and the Pacific Ocean.

Secondary plates

These smaller plates are generally shown on major plate maps, but with the exception of the Arabian plate do not comprise significant land area.

Tertiary plates

Tertiary plates are grouped with the major plate that they would otherwise be shown as part of on a major plate map. Mostly these are tiny microplates, although in the case of the Nubian-Somalian and Australian-Capricorn-Indian plates these are major plates that are rifting apart. Some models identify more minor plates within current orogens like the Apulian, Explorer, Gorda, and Philippine Mobile Belt plates. The remainder of the tertiary plates are the dwindling remains of much larger ancient plates. There may or may not be scientific consensus as to whether a tertiary plate is a separate plate yet, is still a separate plate, or should be considered a separate plate, thus new research could change this list.[1][2][3][4]

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