What Is a Tectonic Plate?

Earth outer shell is broken into about 15 major and several minor tectonic plates, ranging from the vast Pacific Plate (103 million km2) to smaller ones like the Juan de Fuca Plate. Each plate is 100-250 km thick and consists of the crust (oceanic or continental) plus the rigid upper mantle (together called the lithosphere). Plates float on the asthenosphere, a partially molten, ductile layer that allows slow movement driven by mantle convection, slab pull (sinking dense oceanic plate), and ridge push (gravity at mid-ocean ridges). Where plates diverge, new crust forms at mid-ocean ridges; where they converge, one plate subducts beneath another, creating deep trenches, volcanic arcs, and earthquakes; where they slide past each other (transform boundaries), friction builds up and releases in quakes. Alfred Wegener proposed continental drift in 1912, but plate tectonics was not accepted until seafloor spreading evidence accumulated in the 1960s.