Mountains are the highest lands on Earth. Mountains are conventionally divided into four categories according to their mode of origin.
The formation of a mountain range takes millions of years. During these years, the internal forces of the earth uplifting the land are fighting against erosion wearing it down. Internal forces must push up the land faster than the external forces constantly eroding it.
Folds are formed in the rock strata by the internal earth movements. Mountain range mainly consisting of uplifted folded sedimentary rocks are called fold mountains. When these rocks are subjected to horizontal compression forces for millions of years, they get bent into up and down folds.
The Himalayas in Asia, the Alps in Europe, the Rockies in North America and the Andes in South America are the most prominent fold mountains of the world. Since these mountain ranges were formed during the most recent mountain building period, they are known as young fold mountains. Some of these mountain ranges, for example, Himalayas, are still rising.
Block mountains are also formed by the internal earth movements. When the forces of tension act on the rocks, they create faults in them. When the land between the two almost parallel faults is raised above the adjoining areas, it forms a block mountain.
The Vosges in France, Black Forest Mountains in Germany and Sierra Nevada in North America are the typical examples of block mountains.
The interior of the earth is extremely hot. Due to high temperature deep inside the earth rocks turn into a molten magma. When this molten rock material is ejected to the earth’s surface during volcanic eruption, it accumulates around the vent and may take the form of a cone. The height of the cone increases with each eruption and it takes the form of a mountain.
Mount Mauna Loa in Hawaii Islands, Mount Popa in Myanmar, Vesuvius in Italy, Cotopaxi in Equador and Fuji Yama in Japan are examples of volcanic mountains.
The weathering and different agents of erosion - rivers, winds, glaciers - are constantly acting on the earth’s crust. As soon as an elevated mountain range appears on the earth’s surface, the agents of gradation begin their work of leveling it down. After thousands of years, soft rocks are worn down into sand and the hard rocks are left standing up in the area that has been reduced in height. These are called residual mountains.
Hills like the Nilgiris, the Parasnath, the Rajmahal and the Aravalis in India are examples of residual mountains.