normal fault

normal fault, type of fault in which lengthening, or extension, of a portion of Earth’s crust causes a block of rock above the fault (hanging wall) to move downward relative to the block below (footwall). The dip angle of the sliding surface on either side tends to be steep, often between 45° and 90°. Normal faults differ from reverse faults (also called thrust faults), which are generated by compression that forces a hanging wall upward relative to the footwall. They are common, often found on the fringes of many mountain ranges of the world in addition to being primary components of rift valleys along the margins of divergent tectonic plates (see also plate tectonics). Rift valleys are formed by the sliding of the hanging walls downward many thousands of meters, where the hanging walls become valley floors.

Normal faults are sometimes seen in groups and can result in horst and graben topography, which is characterized by a series of raised and lowered blocks. Horsts and grabens are bounded on both sides by steeply dipping normal faults, with those forming horsts generally dipping away from each other and those forming grabens generally dipping toward each other. The Basin and Range Province in the western United States is made up of an extensive network of horsts and grabens. The region features basins 10 to 30 km (roughly 6 to 20 miles) wide and 50 to 200 km (30 to 125 miles) long that are separated by similarly shaped ranges of mountains.

Rift valleys are tectonic valleys that are typically formed between normal faults (see also tectonic landform). The East African Rift System has been forming for some 30 million years, leading to several deep lakes, such as Tanganyika. Volcanism, resulting in peaks such as Kilimanjaro, has also been seen along the length of this rift valley between the African and Somalian plates. The Baikal Rift Valley in Russia, Death Valley in the southwestern United States, the Rhine Rift Valley in Germany, and the rift valleys at the divergent plate boundary between the North American and Eurasian plates in Iceland are other examples. Numerous submarine rift valleys have been discovered along the crests of the large ridges that run throughout Earth’s oceans. These ridges are centers of seafloor spreading—areas where magma from the mantle wells up, cools to form new oceanic crust, and moves away from the crests in either direction (see also spreading centre).