A mountain is a tract of land rising considerably above the surrounding surface. Mountains are usually found connected in chains or ranges. Geologists have made extensive study of mountain formations. It has been revealed by studies that they are formed as ranges. The ranges of mountains along with many small and big hills extend over long distances. To study their formation in a systematic way, geologists have divided them into four categories. Different types of mountains are formed in different ways. But all the mountains have been formed due to violent changes on the Earth’s surface, millions of years ago. The formation of four types of mountains are as follows — The first type of mountains are the volcanic mountains. These mountains are made of lava and ash. Due to the agitations in the Earth’s interior, the Earth’s surface breaks open at a point from where the internal molten matter comes out as lava. The volcanic mountains are cone-shaped with a large hole or crater at the top. The Vesuvius of Italy, the Fujiyama of Japan, the Hood and the Rainier of the United States of America are famous volcanic mountains. The second type of mountains are the folded mountains. These are made of many layers of rocks. These layers are formed because of extreme contractions and pressures inside the Earth. The Alps mountain ranges and the Ural have been formed in this way. The third type of mountains are dome-shaped and hence they are called dome mountains. When molten lava comes out of the Earth with great pressure, it cools off in the form of a dome and, as a result, dome mountains are formed. The Black Hills of South Dakota are examples of dome mountains. The fourth type of mountains are called block mountains. These are formed due to the occurrence of faults in the layers of the Earth. Sometimes there are such great upheavals inside the Earth that whole blocks of rock from inside the Earth come up and constitute the block mountains. The Sierra Nevada Range of California in the United States of America is an example of the block mountains. One block of Sierra Nevada Range is 640 kilometres long and 128 kilometres wide. Mountains do not come up in a few days or even years; they take a very long time to form. The rate of increase in the height of mountains is as low as a few millimetres per year. Many mountains get destroyed with the passage of time. Rains, storms, and flowing water coming from molten ice constantly erode small parts of the soil and rocks of mountains. As a result of this process even big mountains are converted into small hills or plains, in due course of time.