fold

The folding of rock strata occurs where compression causes them to buckle. Over time, folding can assume highly complicated forms, as can sometimes be seen in the rock layers of cliff faces or deep cuttings in the rock. Folding contributed to the formation of great mountain chains such as the Himalayas.

At Stair Hole in Dorset, England, the sedimentary beds of Portland and Purbeck limestone have been folded into a step- or stair-like shape, by tectonic movements during the Alpine mountain-building period, 30 million years ago. The folding has also fractured the limestone, and the lines of weakness thus created were later exploited by marine erosion, forming small arches in the limestone.

Folded rocks in the Little Karoo, South Africa. Owing to the movements of tectonic plates, horizontal rock layers (strata) are folded or bent. The folded rock may fracture, producing lines of weakness. These weaknesses or faults are exploited by rivers and other agents of erosion, and are more easily broken down than unfractured rock.

Some of the lava from the 1973 eruption of Helgafell, a volcano on the Icelandic island of Heimaey, was distorted as it flowed from the cone. Pressure from movements of the Earth caused it to bend; as the lava cooled and solidified, the curves and folds were preserved.

In geology, a deformation (bend) in beds or layers of rock. Folds are caused by pressures within the Earth's crust resulting from plate-tectonic activity. Rocks are slowly pushed and compressed together, forming folds. Such deformation usually occurs in sedimentary layers that are softer and more flexible. If the force is more sudden, and the rock more brittle, then a fault forms instead of a fold.

Folding can result in gentle slopes or mountain chains such as the Rocky Mountains and the Alps. They can be eroded to form escarpments, giving rise to an undulating topography.

If the bend of the fold is arched up in the middle it is called an anticline; if it sags downwards in the middle it is called a syncline. The line along which a bed of rock folds is called its axis. The axial plane is the plane joining the axes of successive beds.