Folds and faults
A cross-section through a rift valley Sliding plates and drifting continents are responsible for some of the Earth’s major landscape features. If a large slab or plate of the Earth’s surface is gradually squeezed, the solid rock slowly wrinkles and crumples. Its layers become wavy folds. When, in other places, rocks are stretched or bent they crack or split along weak points. These cracks are known as faults. They may be straight or zigzag and form narrow slits or wide valleys.
When the layers of rock in the Earth's crust fold, the land’s surface is pushed up as hills or mountains. This process is called orogeny. The wind, rain, sun, ice or snow may wear down the folds as fast as they are pushed up, keeping the surface low and rounded. But if the folds rise more quickly they form high, jagged peaks. The world’s great mountains, including the Himalayas in Asia, Andes in South America, Rockies in North America and Alps in Europe, are all fold mountains.
A fault runs diagonally through volcanic rockTypes of fault
Faults are large cracks in the Earth’s crust where the rocks move against each other. It is not easy to shift massive chunks of crust, so it takes a great amount of pressure to build up before the rocks will move. When they do, the movement is often sudden and violent, sending vibrations through the Earth in all directions. These vibrations are what we call earthquakes.
A block or strip of land sometimes slips down between two faults to make a valley with steep slopes on either side. This is called a rift valley. Rifting can also make mountains, as the rocks on either side move in and squeeze the central block upwards. Raised blocks are called horsts, while those that slip down are called grabens.
Normal, reverse and strike-slip faults
Consultant: Ian Fairchild