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A mountain is usually produced by the movement of lithospheric plates, either orogenic movement or epeirogenic movement. The compressional forces, isostatic uplift and intrusion of igneous matter forces surface rock upward, creating a landform higher than the surrounding features. The height of the feature makes it either a hill or, if higher and steeper, a mountain. The absolute heights of features termed mountains and hills vary greatly according to an area's terrain. The major mountains tend to occur in long linear arcs, indicating tectonic plate boundaries and activity. Two types of mountain are formed depending on how the rock reacts to the tectonic forces—block mountains or fold mountains.
Compressional forces in continental collisions may cause the compressed region to thicken, so the upper surface is forced upward. To balance the weight of the earth surface, much of the compressed rock is forced downward, producing deep "mountain roots". Mountains therefore form downward as well as upward (isostasy). However, in some continental collisions part of one continent may simply override part of the others, crumpling in the process.
Some isolated mountains were produced by volcanoes, including many apparently small islands that reach a great height above the ocean floor.
Block mountains are created when large areas are widely broken up by faults creating large vertical displacements. This occurrence is fairly common. The uplifted blocks are block mountains or horsts. The intervening dropped blocks are termed graben: these can be small or form extensive rift valley systems. This form of landscape can be seen in East Africa, the Vosges, the Basin and Range province of Western North America and the Rhine valley. These areas often occur when the regional stress is extensional and the crust is thinned.