Deformation of Rocks
Rocks become deformed when the Earth’s crust is compressed or stretched. The forces needed to do this act over millions of years – deformation is a very slow process!
Compression (squashing) occurs as tectonic plates are pushed together and the crust becomes shorter and thicker, building mountain ranges like the Alps or Himalayas. The rocks are folded and metamorphosed at depth. They may be faulted nearer the surface.
Folded rocks in the Alps.
Fold and Thrust fault, Torquay, Devon.
Tension (stretching) occurs as tectonic plates are pulled apart and the crust becomes thinner. Rocks near to the surface become faulted, sometimes producing rift valleys, such as the East African Rift shown in the photograph below.
The East African Rift : Alexander Deligtisch
Fault at Kilve beach, Somerset.