fault

Faults are caused by the movement of rock layers, producing such features as block mountains and rift valleys. A normal fault is caused by a tension or stretching force acting in the rock layers. A reverse fault is caused by compression forces. Faults can continue to move for thousands or millions of years.

The Caledonian Canal, which joins the lakes along the Great Glen fault in Scotland. Earth movements hundreds of millions of years ago displaced rocks either side of the fault. Weaknesses were created in the neighbouring rock, and these weaknesses have since been exploited by river and glacier activity. Loch Ness occupies part of this fault, and a river and the canal occupy it between the loch and Inverness.

Fracture in the Earth's crust, on either side of which rocks have moved past each other. Faults may occur where rocks are being pushed together (compression) or pulled apart (tension) by plate tectonics, movements of the plates of the Earth's crust. When large forces build up quickly in rocks, they become brittle and break; folds result from a more gradual compression. Faults involve displacements, or offsets, ranging from the microscopic scale to hundreds of kilometres. Large offsets along a fault are the result of the accumulation of many small movements (metres or less) over long periods of time. Large movements cause detectable earthquakes, such as those experienced along the San Andreas Fault in California, USA.

Faults produce lines of weakness on the Earth's surface (along their strike) that are often exploited by processes of weathering and erosion. Coastal caves and geos (narrow inlets) often form along faults and, on a larger scale, rivers may follow the line of a fault. The Great Glen Fault in Scotland is an excellent example of a fault, and Loch Ness on this fault is an example of a fault-line feature.

Faults are planar features. The orientation of a fault is described by the inclination of the fault plane with respect to the horizontal (an angle known as dip) and its direction in the horizontal plane (the strike). Faults at a high angle with respect to the horizontal (in which the fault plane is steep) are classified as either normal faults, where the hanging wall (the body of rock above the fault) has moved down relative to the footwall (the body of rock below the fault), or reverse faults, where the hanging wall has moved up relative to the footwall. Normal faults occur where rocks on either side have moved apart. Reverse faults occur where rocks on either side have been forced together. A reverse fault that forms a low angle with the horizontal plane is called a thrust fault.

A lateral fault, or strike–slip fault, occurs where the relative movement along the fault plane is sideways. A transform fault is a major strike–slip fault along a plate boundary, that joins two other plate boundaries – two spreading centres, two subduction zones, or one spreading centre and one subduction zone. The San Andreas Fault is a transform fault.