A mountain is an elevated portion of the Earth's crust, generally with steep sides that show significant exposed bedrock. It is usually higher than a hill, typically rising at least 300 meters (980 ft) above the surrounding land
. Mountains are formed through tectonic forces, erosion, or volcanism, which act on time scales of up to tens of millions of years. Once mountain building ceases, mountains are slowly leveled through the action of weathering, through slumping and other forms of mass wasting, as well as through erosion by rivers and glaciers
. Mountains can be found all over the world, and they usually have steep, sloping sides and sharp or rounded ridges, and a high point, called a peak or summit
. Most geologists classify a mountain as a landform that rises at least 1,000 feet (300 meters) or more above its surrounding area
