Mountains are formed by the movements of the Earth's crust and tectonic plates. Movements deep beneath the Earth's surface cause a variety of reactions, which results in different types of mountains. Mountains can form as a result of volcanic activity, the collision of two tectonic plates or movement along a fault line.
The forces that form a mountain are generated far below the Earth's surface. Two of the most common forms of mountains are formed by interactions between the Earth's tectonic plates. These theoretical plates form the Earth's crust and upper mantel, and they move independently of one another. When the plates collide, one possible outcome is the formation of mountains. In the case of volcanic mountains, the collision of two plates results in melting rock that rises to the surface. This melted rock or magma breaks through to the surface and slowly builds into a mountain. In other instances, it is blocked and bulges up under the surface. Once it cools, the top layer of soil erodes away, leaving a dome-shaped mountain.
Fold mountains are also a result of two tectonic plates colliding. In the case of fold mountains, however, one tectonic plate buckles and folds up, creating large mountain ranges. When two tectonic plates grind against each other at a fault line, the Earth's surface can rise or fall, which creates fault-block mountains. These mountains usually have steep sides and are bordered by valleys.