Faults are raptures in the earth’s crust. Faults are created by tensional or compressional forces in the crust which result in rock snapping. These different forces maybe generated along diverging boundaries where tension is experienced or converging boundaries where there’s compression. Besides convergent and divergent forces magma can cause faulting.
When magma fails to break through the surface it may start to swell near the surface exerting tremendous force against the crust which may cause it to snap. The East African Rift is thought to have been formed by this.
Types of Faults
Normal Fault/Dip Slip
A normal fault results when tension rips the crust so that the hanging wall slide along the footwall.
A reverse fault is where the hanging wall (downthrow) slides up the footwall (upthrow) Reverse faults result from compressional forces that squeeze the crust until it snaps causing the downthrow to be move towards the upthrow.
Thrust faults are low angle reverse faults.
Slip faults are a result of lateral movements e.g San Andreas Fault caused by plates sliding past each other.
Rift valleys are blocks (downthrows) which have subsided between two lines of fault or simply downthrows between two upthrows. The two fault lines are V shaped. The upthrows or raised blocks are called hosts while the downthrow is the valley or graben. The Great East African Rift Valley is a good example.
A half graben is a block (downthrow) which have slided down over a curved fault called a listric fault resulting in subsidence over the curved fault and uprising on the other fault called antithetic fault.
Half grabens are called so because they only have one major curved fault (listric) on which they subside unlike on a rift valley where the graben slide down between two parallel faults.
Tilted blocks are formed from tilted fault lines creating lifted blocks and the intervening downthrows as valleys.
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