Mountains are landforms that rise above their surroundings, with steep sides and a confined summit. A mountain is generally considered higher than a hill, often rising at least 300 meters above the adjacent land. While some exist as isolated peaks, most are found within extensive ranges or chains. These features form over millions of years, shaping Earth’s diverse landscapes.
Fold Mountains
Fold mountains are the most common type of mountain, forming when Earth’s tectonic plates collide. This collision generates compressional forces that cause the crustal rocks to buckle and bend. The bending of these rock layers creates wavelike structures known as folds. Upward-arching folds are called anticlines, while downward-arching folds are known as synclines.
The formation of fold mountains, a process called orogeny, occurs over millions of years as layers of rock are squeezed. The Himalayas, for example, began forming between 40 and 50 million years ago from the collision of the Indian and Eurasian tectonic plates. This convergence continues to uplift the range, making the Himalayas some of the youngest and tallest fold mountains on Earth.
Fault-Block Mountains
Fault-block mountains form from large blocks of Earth’s crust moving along fault lines. They result from tensional forces pulling the crust apart, causing it to stretch and fracture. When the crust is subjected to such pulling forces, sections can break and move vertically relative to each other.
Uplifted blocks are called horsts; down-dropped blocks are grabens. Horsts form mountains or ridges, while grabens create valleys. The Sierra Nevada mountain range in California formed as a large block of Earth’s crust was uplifted and tilted westward along major fault zones. The dramatic eastern escarpment of the Sierra Nevada illustrates the significant vertical displacement from this faulting.
Volcanic Mountains
Volcanic mountains form from molten rock (magma) erupting onto the surface. These mountains grow as lava flows, ash, and other volcanic debris are ejected and solidify around a central vent. The shape and size of a volcanic mountain depend on the type of magma and the nature of its eruptions.
Stratovolcanoes, also known as composite volcanoes, are characterized by their steep, conical shape and are built from alternating layers of viscous lava and ash. Mount Fuji in Japan is an example of a stratovolcano, formed over tens of thousands of years through multiple eruptive phases. In contrast, shield volcanoes have broad, gently sloping profiles, formed by highly fluid lava that flows great distances before solidifying.
Dome Mountains
Dome mountains form when molten magma pushes up Earth’s crust without erupting. This creates a rounded, dome-shaped bulge. The overlying sedimentary rock layers are uplifted and arched over the intrusion.
Over time, the softer outer layers of rock are worn away by erosion, revealing the harder, more resistant igneous or metamorphic rock core. This erosional process exposes the underlying dome structure, giving these mountains their characteristic rounded appearance. The Black Hills of South Dakota are an example, having formed from an uplift event approximately 65-70 million years ago, with their central core of ancient rock now exposed.
Erosion Mountains
Erosion mountains are shaped by weathering and erosion, rather than direct uplift from tectonic activity or volcanic eruptions. They originate from existing plateaus or uplifted areas where softer rock layers are gradually worn away by wind, water, and ice. The more resistant rock remains, forming isolated peaks, ridges, or distinctive flat-topped features.
This process carves out the landscape, leaving behind remnants of the original elevated terrain. Mesas, which are broad, flat-topped mountains with steep sides, and buttes, which are smaller, isolated hills with steep slopes, are examples of landforms shaped by erosion. These features often exhibit a cap of harder rock that protects the softer layers beneath from being eroded as quickly.