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Normal faults cause an "extension" in the Earths Crust. However this extension IS NOT caused by "tension".

Rocks in the ground are subject to compressive forces at all times, if this were not the case the Earth would fly apart.

However the direction and relative magnitude of the greatest compressive force can change with time as a result of the movement of the Earths tectonic plates (driven by the heat energy emanating from the Earth's core).

If there were no plate tectonics, the direction of greatest force would always point downwards and would be caused by "gravity".

Hoverer, when plate tectonics drives plates together, the greatest force can become horizontal and increase to the point where it exceeds the strength of the rock and the rock will fracture along a REVERSE fault which shortens and thickens the crust.

Likewise, when plates are moving apart, horizontal compression can decrease (relative to the other compressive forces acting on the rock) to the point where gravity exceeds the strength of the rock and the rock will fracture along a NORMAL fault which extends and thins the crust.

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10y ago

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