A mountain is a natural elevation of Earth’s surface that rises conspicuously above the surrounding terrain. Mountains commonly have substantial local relief, sloping sides, and a summit or crest, although their shapes range from rounded uplands to sharply pointed peaks. They may stand alone or form connected ranges. Their development reflects interactions between crustal movement, volcanic activity, and processes that wear down and reshape the land. (apps.usgs.gov)
Definition and measurement
There is no universally accepted height threshold separating a mountain from a hill. Naming conventions vary, and the United States Board on Geographic Names does not officially distinguish mountains, hills, and peaks by numerical criteria. A landform’s height relative to its surroundings is therefore important alongside its absolute elevation. (usgs.gov)
Several measurements describe mountain height. Elevation expresses a summit’s position above a reference level, usually mean sea level; base-to-summit height measures the vertical distance from its foot. These measures need not identify the same mountain as the tallest. Mount Everest has the highest summit above sea level, at approximately 8.85 kilometres. Mauna Kea rises roughly 10 kilometres from its submarine base to its summit. Chimborazo’s summit lies farthest from Earth’s centre because Earth bulges around the equator. These distinctions illustrate why claims about the “highest mountain” require a specified reference. (cdn.oceanservice.noaa.gov)
Formation and geological structure
Mountain building, or orogeny, is a major subject of geology. Many mountain belts develop through plate tectonics, which moves and deforms large sections of Earth’s outer shell. Compression can fold rock layers, push blocks over one another along faults, and thicken the continental crust. The Himalayas are a prominent example: their uplift is associated with the continuing collision between the Indian and Eurasian plates. (pubs.usgs.gov)
Mountains also form where crustal extension produces fractures and displacement. Along a fault, one block may rise relative to another, creating a steep mountain front beside a lower basin. The Teton Range in western North America illustrates this fault-block development. Its landscape records both earlier regional mountain building and later movement along the Teton fault, rather than a single formation process. (nps.gov)
A volcano can build a mountain through repeated accumulation of lava and fragmented material around eruptive vents. Volcanic mountains are not necessarily steep cones: some have broad, gently sloping profiles. Mountain classifications describe dominant processes, but individual ranges may combine deformation, volcanism, and extensive erosion over their histories. (usgs.gov)
Erosion and mountain landscapes
Uplift does not determine mountain shape alone. Erosion removes material, while rock strength influences which parts of a landscape remain high. In the Great Smoky Mountains, comparatively resistant rocks form prominent peaks after less resistant material has been worn away. Flowing water transports sediment downslope and cuts valleys, continually modifying the relief created by tectonic processes. (home.nps.gov)
A glacier reshapes mountains by moving over bedrock and transporting debris. Glaciated valleys commonly have broad, U-shaped cross-sections, unlike the narrower V-shaped valleys typically cut by streams. Glacial erosion also produces bowl-shaped cirques, narrow ridges called arêtes, and pointed horns where erosion attacks a peak from several sides. These features may remain long after the ice disappears, preserving evidence of earlier glaciation. (usgs.gov)
Climate and water
Mountains strongly influence climate. Temperatures generally decrease with elevation, shortening growing seasons and increasing the likelihood of snow. Mountain slopes also receive differing amounts of sunlight, producing contrasts in temperature and moisture over relatively short distances. Consequently, neighbouring slopes and valleys can support markedly different environmental conditions. (nps.gov)
When air is forced upward over a ridge, it cools and may form clouds and precipitation. Mountain barriers can leave areas on their sheltered side much drier, creating a rain shadow. The Olympic Mountains, for example, produce a pronounced rainfall contrast across the Olympic Peninsula. Mountains thus affect both their own weather and conditions in surrounding lowlands. (nps.gov)
Mountains play a central role in the water cycle. Snow stores precipitation until melting releases it, often supplying downstream areas during seasons of lower rainfall. Mountain headwaters feed major rivers, and mountain freshwater supports drinking supplies, irrigation, industry, and hydroelectric power. More than half of humanity relies on freshwater originating in mountain regions. Changes in snow and ice storage can therefore affect communities far beyond the mountains themselves. (fao.org)
Ecosystems and human activity
Mountain ecosystems vary with elevation, exposure, soil, and moisture, supporting substantial biodiversity. In many ranges, lower slopes carry forests, while higher elevations support smaller plants adapted to cold conditions and short growing seasons. The treeline is a transition zone marking the upper limit of upright tree growth, not necessarily a sharp boundary. Above it, alpine vegetation can include diverse grasses, herbs, mosses, and lichens. (fao.org)
Mountain livelihoods include agriculture, livestock keeping, forestry, and tourism. Mountains also have cultural and spiritual significance, while their water resources sustain settlements and economic activity downstream. Land degradation, overexploitation, and natural disasters can affect both mountain communities and neighbouring regions. Climate change is altering glacier storage and habitats; species adapted to cold mountain environments may face diminishing suitable areas as conditions warm. These pressures connect mountain conservation with water management and the maintenance of local livelihoods. (fao.org)