A geological fault is a fracture within the Earth’s crust where significant movement of the rock blocks on either side has taken place. The style of movement classifies these fractures into several distinct types. The normal fault is one classification, characterized by a specific geometry and the type of stress that causes the movement. Understanding this fault provides insight into how the planet’s surface stretches and separates over geological time.
Defining the Normal Fault
A normal fault is distinguished by the relative motion of the rock blocks on either side of the inclined fracture plane. Geologists identify two sections of rock surrounding the fault: the hanging wall and the footwall. The hanging wall is the block positioned above the fault plane, while the footwall is the block situated below it.
In a normal fault, the hanging wall block moves downward relative to the footwall block. This vertical drop along the angled fault plane is known as dip-slip motion. The movement is the direct result of tensional stress, meaning the crust is being pulled apart or stretched. This extensional force effectively lengthens the crustal area where the fault occurs. Normal faults typically have a steep dip, often inclined between 45 and 90 degrees.
The Geological Context of Formation
The tensional stress required to create normal faults is generated in tectonic environments where the Earth’s crust is undergoing extension. This stretching and thinning of the lithosphere occurs most prominently at divergent plate boundaries, where two tectonic plates are actively moving away from each other. As the crust is pulled apart, it fractures, leading to the formation of numerous normal faults.
The continuous movement along these faults results in distinctive topographical features known as horst and graben structures. A graben is a block of crust that has dropped downward between two parallel normal faults, often forming a valley or rift basin. Conversely, a horst is the uplifted block of crust that remains between two adjacent grabens, forming a ridge or mountain range.
The East African Rift System is a prominent, active example of a continental rift zone demonstrating this process on a massive scale. This rift is characterized by a series of down-dropped grabens and uplifted horsts, showing the stretching of the African plate. Another large-scale example is the Basin and Range Province in the western United States, which exhibits alternating valleys and mountain ranges created by extensive normal faulting.
Comparison to Other Major Fault Types
The characteristics of the normal fault are best understood by contrasting them with the two other primary classifications of faults: reverse and strike-slip faults. Normal and reverse faults both involve vertical, or dip-slip, movement, but they respond to different forces. Normal faults are caused by tensional forces that pull the crust apart, while reverse faults are the product of compressional forces that push the crust together.
This difference in stress dictates the relative movement of the rock blocks. In a reverse fault, the hanging wall moves up and over the footwall, resulting in a shortening of the crust. This action is the opposite of the downward movement of the hanging wall seen in a normal fault. Reverse faults with a shallow angle are often termed thrust faults and are common in mountain-building areas like the Himalayas.
The third major type, a strike-slip fault, differs fundamentally because the movement is predominantly horizontal, or lateral, rather than vertical. The blocks slide past each other along the fault plane with little to no up-or-down motion. This horizontal shearing force neither lengthens nor shortens the crust; instead, it causes lateral displacement. Therefore, the defining feature of a normal fault—the downward drop of the hanging wall due to crustal extension—sets it apart from the shortening action of a reverse fault and the horizontal movement of a strike-slip fault.