Faults

A fault is a rock fracture where blocks of rock have slipped past each other. Faults usually occur in fault zones, which are often along the boundaries between tectonic plates. Faults are tipically caused by earthquakes.

Single earthquakes rarely move block more than a few centimetres. Repeated small earthquakes can shift blocks hundreds of kilometres.

Types of faults:

1. Compression faults are faults caused by rocks being squeezed together, perhaps by converging plates.
2. Tention faults are faults caused by rocks being pulled together, perhaps by diverging plates.
3. Normal, or dip - slip, faults are tension faults where the rock fractures and slips straight down.
4. A wrench, or tear, fault occurs when plates slide past each other and make blocks slip horizontally.

fig. 4 The diagram shows a normal fault

Large wrench faults, such as the San Andreas in California, USA, are called transcurrent faults.

Rift valleys are huge, through - shaped valleys created by faulting, such as Africa's Great Rift Valley. The floor is a trough - down block called a graben. Some geologists think they are caused by tention, other by compression.

Horts block are blocks of rock thrown up between normal faults, often creating a high plateau.