Mountains were formed when the earth's surface began to cool and harden. However the mountains that period were worn away millions of years ago. The ones we see now were formed much later. Mountains are worn away by the action of wind, rain, frost and other such natural forces. They change shape and are flattened by the onslaught of nature's forces.
New mountains are formed as a result of violent changes in the surface of the earth. Geologists divide the mountains into four categories to the way they were formed. The fold mountains are made up of several layers of different types of rocks that were squeezed by great pressure into folds. This explains why the fossils of marine animals and plants can be sometimes found on the peaks of fold mountains. The Himalayas, Andes, Alps and the Rockies are examples of fold mountains.
Some times molten lava from beneath the earths surface tries to force up the rocks in order to come out. They are unsuccessful, but leave a bulge on the surface. These are known as the dome mountains. The black hills of south Dakota, in the USA are the eroded remains of such a type of mountain formation.
At certain places huge blocks of rock can split and slide along lines of weaknesses or faults, as they are generally called. Great masses of rocks can tilt or be lifted above the level of the rest of the neighboring rocks. Such mountains, known as block mountains rise abruptly from the surrounding plains. The sierra Nevada Range of California in the western part of USA, is an excellent example of this kind.
Due to disturbances in the layers below the surface of the earth, the crust cracks. The lava from the inner pars of the earth comes gushing out of these gaps as a volcano. As the molten lava flows out in subsequent eruptions a volcanic mountain is formed. Mount Fuji in Japan, mount Vesuvius in Italy and mount st.helens in the USA are mountains that are formed by volcanic activity.
There are certain mountain ranges that are formed by a combination of different methods. The Rockies in North America is basically an example of a fold mountain range. Yet there are mountains in this range, which are formed due to doming, faulting and outpourings of lava.
