Mountains
Mountains are formed by the folding, faulting, or upwarping of the Earth's surface due to the movement of plates (see plate tectonics) or by the emplacement of volcanic rock onto the surface. For example, the Himalayan Mountains where India meets the Eurasian Plate were formed by a collision between plates that caused extreme compressional folding and the uplifting of large areas. The mountain ranges around the Pacific basin are attributed to the sinking of one plate beneath another. See also plateau. . (Britannica Concise Encyclopedia 2005)

plate tectonics
Theory that the Earth's lithosphere (the crust and upper portion of the mantle) is divided into about 12 large plates and several small ones that float on and travel independently over the asthenosphere.

The theory revolutionized the geological sciences in the 1960s by combining the earlier idea of continental drift and the new concept of seafloor spreading into a coherent whole. Each plate consists of rigid rock created by upwelling magma at oceanic ridges, where plates diverge. Where two plates converge, a subduction zone forms, in which one plate is forced under another and into the Earth's mantle. The majority of the earthquakes and volcanoes on the Earth's surface occur along the margins of tectonic plates. The interior of a plate moves as a rigid body, with only minor flexing, few earthquakes, and relatively little volcanic activity. (Britannica Concise Encyclopedia 2005)