Mountain Ranges

Great mountain ranges such as the Andes in South America usually lie along the edges of continents.

Most mountain ranges are made by the folding of rock layers as tectonic plates move slowly together. High ranges are geologically young because they are soon worn down. The Himalayas are 25 million years old.

Many ranges are still growing. The Himalayas grow a few centimetres each year as the Indian plate pushes into Asia.

Mountain-building is slow because rocks flow like thick treacle. Rock is pushed up like a bow wave in front of a boat as one tectonic plate pushes into another.

Satellite techniques show that the central peaks of the Andes and Himalayas are rising. The outer peaks are sinking as the rock flows slowly away from the "bow wave" .

Mountain-building is very active during the orogenic (mountain-forming) phases that last millions of years. Different orogenic phases occur in different places, for example the Alpine, Caledonian, Hercynian in Europe and the Huronian, Nevadian and Pasadenian in North America. The Caledonian was about 550 million years ago.

Mountain-building makes the Earth's crust especially thick under mountains, giving them very deep "roots" . As mountains are worn down, their weight reduces and the "roots" float upwords. This is called isostasy.