During the 1960s, the concept of seafloor spreading was added to the existing theory of continental drift, which speculated that continents moved on the earth's surface. Plate tectonics is a comprehensive geological theory which has evolved from the two notions to explain the movement of Earth's crust, and centers on the concept that plates are able to move because of the relative density of the Earth's oceanic lithosphere to the comparatively weaker asthenosphere.
The outer layers of earth's interior are divided into the lithosphere, which is cooler (in comparison to other inner layers) and more rigid, and the asthenosphere (which is hotter). The earth's lithosphere, which contains both crust and mantle, is broken into seven major and many minor tectonic plates. These plates ride on the fluid and viscous asthenosphere in relation to one another, developing three distinct types of plate boundaries: convergent (with two plates sliding toward each other), transform (where two plates grind past each other), and divergent (with two plates sliding away from each other). Geological process such as volcanic activity, earthquakes, and the creation of mountain ranges occur at these plate boundaries.
Tectonic plates can include continental crust, oceanic crust, or a combination of both Oceanic crust is denser than continental crust, which results in the sinking of oceanic crust to below sea levels while the continental crust thrusts outward.
The scientific community has generally accepted that dissipated heat emanating from the mantle is the original energy source for plate tectonics; however, research continues to find exact data about the driving forces behind plate motion. Regardless of the source of motion, the lateral movement of plates is roughly about 8 centimeters per year.