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 these 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 the violent changes on the Earth’s surface, millions of years ago. The formation of four types of mountains can be understood in the following way.
The first type of mountains is 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 at places from where the internal matter of the Earth 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 Ranier of the United States of America are the famous volcanic mountains of the world.
The second type of mountains is 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 have been formed in this way.
The third type of mountains is the dome-shaped and hence it is 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 the examples of the dome mountains.
The fourth type of mountains is 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 block of rocks 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. Its one block is 640 km long and 128 km wide.
The rate of increase in the height of mountains is as low as a few millimeters per year.
However, with the passage of time, mountains get destroyed too. Rains, storms, and the flowing water coming from molten ice go on eroding 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.
