A mountain is a landform that extends above the surrounding terrain in a limited area. A mountain is generally much higher and steeper than a hill, but there is considerable overlap, and usage often depends on local custom; see also Dune.
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 upwards, 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 topography. The major mountains tend to occur in long linear arcs, indicating tectonic plate boundaries and activity. Mountain creation tends to occur in discrete periods, referred to as orogenies (orogeny). Two types of mountain are formed depending on how the rock reacts to the tectonic forces - block mountains or fold mountains.
Block mountains are created when large areas are widely broken up by faults creating large vertical displacements. 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.
Where rock does not fault it folds, either symmetrically or asymmetrically. The upfolds are anticlines and the downfolds are synclines, in asymmetric folding there may also be recumbent and overturned folds. The Jura mountains are an example of folding. Over time, erosion can bring about an inversion of relief, the soft upthrust rock is worn away so the anticlines are actually lower than the tougher rock of the synclines.
The height of a mountain is that above mean sea level. Thus a mountain may be higher than another one even if the top is closer to the center of the earth.
The highest mountain on Earth is Mount Everest, set in the world's most significant mountain range, the Himalayas. Another contender for this title is Mauna Loa, which stands taller than Mount Everest when measured from its base on the ocean floor but not in terms of summit altitude above sea level. The summit Mount Chimborazo in Ecuador is 2,150 m further away from the Earth's centre than that of Everest, since the Earth bulges at the equator, but at 6,272 m it is not even the tallest peak in the Andes. The tallest mountain in the solar system is Olympus Mons, located on Mars.
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