Fold mountains are mountains that form mainly by two or more of tectonic plates when are pushed together Earth's crust. Before either plate tectonic theory developed, or the internal architecture of thrust belts became well understood, the term was used for most mountain belts, such as the Himalayas. The term is still fairly common in physical geography literature but has otherwise generally fallen out of use except as described below. The forces responsible for formation of fold mountains are called orogenic movements. The term orogenic has derived from a Greek word meaning mountain building. These forces act at tangent to the surface of the earth and are primarily a result of plate tectonics.