There are three main types of mountains: fold mountains, block mountains and volcanoes.
Fold mountain chains form regular patterns that stand out on maps. They are formed in long, narrow belts, in areas that have undergone periods of great movement and stress.
For example, the Himalayas, which contain the world's highest mountains, were raised up when a plate, bearing what is now India, broke away from Gondwanaland and moved steadily northwards until it was forced against the Asian continent.
From about 53 million years ago, the sedimentary rocks between the colliding plates were thrown upwards and wrinkled to form the Himalayas.
Eventually, India was welded onto Asia.
The Alps were formed in a similar way about 26 million years ago, when a plate supporting what is now Italy was rammed against Europe.
The crust under mountain chains is much thicker than the crust under continents.
The average thickness of the crust under continents is only 35 to 40 kilometers (22 to 25 miles). Under mountain chains, the crust may be as much as 70 kilometers (43 miles) thick.
Sediment collects by the millions of tons on the continental shelves and in ocean trenches, the deepest parts of the oceans. These ocean trenches form where an ocean plate is forced beneath a continental plate.
While this is happening, volcanic action usually begins. Volcanic rocks are injected into the area. Eventually, the ocean closes, scraping up the sediment, crumpling the rocks and pushing them upwards.
Because this thick mass of sediment is lighter than the denser material below, it floats. Through a process called isostasy, the crumpled, deep mass of rock gradually rises until it reaches a point of equilibrium.
This explains how high mountain chains are formed along the original edges of continents and why fossil remains of sea animals are found near the top of high mountains, such as Mt Everest.
Age of Mountains
While a mountain chain is rising upwards, the forces of erosion, such as weathering, running water and moving ice, are also wearing it down.
Eventually, over millions of years, high, jagged peaks are reduced to flat plains.
The world's highest fold mountains are, therefore, comparatively young.
The Alps and the Himalayas are the youngest. They probably are still rising. In some places, violent earthquakes and volcanic action accompany the movement.
The Rocky Mountains and the Andes in the western part of the Americas are also relatively young and active.
In comparison, the eroded Appalachians in the east have no earthquakes or volcanoes.
The Appalachians are many kilometers from any plate edge.
The Scottish Highlands and the Russian Urals are older fold mountains.
Large tracts of flat land are partly the roots of mountains that were eroded long ago.
These shield or platform areas cover large parts of Canada, Australia and Siberia.
These shields are ancient Pre-Cambrian landmasses, which have been in existence almost since the start of geological time.
They are the complete opposite of young mountain ranges, which have not been in existence long enough to be worn down.
Most continents combine these contrasting types of landscape.
Folds and Faults
Although most of the Earth's rocks were originally laid down in flat layers, few remain that way for long. Rocks can be squeezed and shaped as if they were made of putty.
Some rocks flow when under pressure. For example, salt may flow to the surface in domes. It is sometimes powerful enough to bend the overlying rocks.
Folds can be several kilometers long. The two main parts of a fold are the anticline (the upfold or arch-like part) and the syncline (the downfold). Folds can be extremely complex. They may be refolded many times as a mountain range forms.
Most rocks will take only so much bending before they crack and break. Cracking forms vertical joints in rocks.
When each side of the crack begins to move, a fault is created.
Faults vary in size. Some are only a few meters in length. Others, called geofractures, can extend across a continent.
Often, one side of a fault drops relative to the other. Therefore, some parts of the land are raised up high above other parts. A fault plane separates them.
If two parallel fault planes form, the center may drop down to form a graben, or a long rift valley.
Alternatively, a block may rise up to form a horst, a high, straight-sided ridge, such as the Vosges Mountains in France.
Large horsts, such as the North American Sierra Nevada mountains and the Rwenzori range in East Africa, are called block mountains.
The best-known rift valleys are those of the Rhine and Jordan rivers, and the East African lakes area.
The movement along faults often takes place gradually over millions of years.
However, if there is no lubrication, the rocks may stick together. When this happens, pressure builds up and may finally be released in a sudden, violent jerk, causing an earthquake.