Mountain

Mountains are elevated landforms rising prominently above their surroundings, typically exhibiting steep slopes, exposed bedrock, and a restricted summit area. Though definitions vary by region and scientific discipline, mountains generally stand higher and steeper than hills and often form part of extensive mountain ranges shaped by tectonic and volcanic forces. Their great elevations influence climate, ecosystems, and human activities, creating distinctive environments across the globe.

Definitions and Classification

There is no universally accepted definition of a mountain. Common criteria include elevation, relief, steepness, and geological structure. Many sources consider features rising at least 300 metres above adjacent terrain to qualify as mountains, though thresholds vary. In the United Kingdom and the Republic of Ireland, elevations above 600 metres are commonly regarded as mountains, a definition also used for access and mapping purposes. Other classification systems incorporate topographical prominence, requiring a rise of 300 metres or more from the surrounding landscape.
International approaches differ. The United Nations Environment Programme identifies mountainous environments based on elevation bands, slope thresholds, and relative relief. Using such criteria, mountains constitute approximately one quarter of the Earth’s land surface, with especially extensive mountainous areas in western North and South America, Central Asia, and southern Europe.

Geological Origins

Mountains arise through several geological processes involving the movement and deformation of the Earth’s crust. The three principal types are volcanic, fold, and fault-block mountains.
Volcanic mountainsVolcanic activity occurs at subduction zones, mid-ocean ridges, and mantle hotspots. Melting of the mantle or crust generates magma that may rise to the surface and erupt, forming volcanic cones such as shield volcanoes or stratovolcanoes. Examples include Mount Fuji and Mount Pinatubo. Magma that solidifies underground can produce dome mountains without erupting at the surface.
Fold mountainsThese mountains form when tectonic plates collide, compressing rocks and creating large-scale folds. The crust thickens, and buoyant continental lithosphere rises isostatically. The resulting structures display anticlines, synclines, and in some cases overturned or recumbent folds. The Jura Mountains and the Balkan Mountains exemplify this type.
Fault-block mountainsWhere the crust is stretched under extensional forces, blocks of rock may be uplifted or dropped along faults. Uplifted blocks form horsts, while downthrown blocks create grabens. This landscape characterises regions such as the East African Rift, the Basin and Range Province of the western United States, and parts of central Europe.

Erosion and Landscape Evolution

Once formed, mountains are shaped continuously by erosion. Water, wind, ice, and gravity wear down uplifted terrain, producing younger surface features than the underlying rocks. Glaciers carve cirques, arêtes, and pointed summits, while rivers cut deep valleys. Plateau mountains, such as the Catskills, originate from the erosion of uplifted plateaus rather than from direct tectonic uplift.

Climate and Environmental Zones

Mountain climates differ markedly from surrounding lowlands due to decreasing temperature with altitude. This cooling results from radiative heating of the surface and convective uplift of warm air, governed by the adiabatic lapse rate. Dry air cools by roughly 9.8°C per kilometre of ascent, whereas moist air cools more slowly due to latent heat release during condensation.
Local climates depend on altitude, latitude, and proximity to oceans. Higher elevations tend to experience alpine conditions with increased wind, heavier snowfall, and shortened growing seasons. A rise of 1000 metres in elevation may approximate a poleward shift of several degrees in latitude. These environmental gradients create distinct ecological zones, with vegetation and wildlife changing progressively from foothills to high summits.

Distribution and Human Use

Mountains occur on every continent and constitute major physiographic features. The Himalayas contain Earth’s highest summit, Mount Everest, rising 8848.86 metres above sea level. Measured from its submarine base, Mauna Kea in Hawaii exceeds this height, making it the tallest mountain by total vertical extent. Olympus Mons on Mars, a giant shield volcano, is the tallest known mountain in the Solar System.
Mountain terrain has traditionally posed challenges for agriculture, favouring pastoralism, forestry, and resource extraction such as mining. In modern times, mountains have become centres for tourism and recreation, with activities including mountaineering, skiing, and hiking.