A mountain is a large landform that rises prominently above the surrounding area, typically culminating in a peak. Geologists classify mountains based on the geological forces that created them over millions of years. This classification results in four primary types: Fold Mountains, Fault-Block Mountains, Volcanic Mountains, and Dome Mountains/Residual Landforms.
Fold Mountains
Fold mountains are the most common type of mountain range, often representing the highest and most complex systems. They form primarily at convergent plate boundaries, where two or more tectonic plates move toward each other. The intense compression from this collision forces layers of the Earth’s crust to buckle, crumple, and fold upward, similar to pushing a rug against a wall.
This folding process creates characteristic upward arches called anticlines and downward troughs known as synclines. Examples include the Himalayas in Asia and the Alps in Europe. The rock layers involved are often sedimentary rocks that were initially deposited horizontally before being deformed by lateral compression.
Fault-Block Mountains
Fault-block mountains form under conditions of tension, where the stretching or pulling apart of the Earth’s crust occurs, typically at divergent boundaries. This horizontal extension causes the brittle upper crust to fracture into large blocks separated by normal faults. A normal fault occurs when the rock above the fault surface moves downward relative to the rock below it.
As the crust stretches, certain blocks are uplifted, forming mountains called horsts. Concurrently, adjacent blocks drop down, creating valleys or basins known as grabens. This results in a landscape of alternating steep-sided ranges and flat valleys, seen in the Basin and Range Province in the western United States. The Sierra Nevada range in California is a prominent example.
Volcanic Mountains
Volcanic mountains are built through the accumulation of erupted materials like ash and lava around a central vent in the Earth’s crust. They form when magma finds a pathway to the surface, where it cools and solidifies, gradually constructing a cone-shaped landform. These mountains often occur at subduction zones or over stationary mantle plumes called hot spots.
At subduction zones, the melting of the descending plate generates magma that rises, creating steep-sided composite volcanoes, also known as stratovolcanoes, like Mount Fuji in Japan. Over hot spots, less viscous magma erupts repeatedly to form broad, gently sloping shield volcanoes, such as those in Hawaii.
Dome Mountains and Residual Landforms
Dome mountains are created when a large body of magma pushes upward into the crust, causing the overlying rock layers to bulge into a rounded, dome-like shape. This uplift occurs without the magma breaking through the surface, meaning they lack the vents and craters characteristic of volcanic mountains. Over millions of years, erosion strips away the softer overlying rock, exposing the harder, dome-shaped core, as seen in the Black Hills of South Dakota.
Residual mountains, also called erosion mountains, are formed when extensive weathering and erosion wear down an existing elevated landmass, such as a plateau. This process leaves behind isolated, resistant peaks.