Earth’s outer crust is a dynamic system, constantly shifting and fracturing due to immense forces. These fractures, known as faults, are breaks in the rock along which movement has occurred. Faults are responsible for earthquakes and the dramatic shaping of the landscape. Understanding the different types of faults is fundamental to grasping the processes that create mountains, valleys, and rift systems worldwide. The normal fault is the most common type of break and shows how the crust reacts when it is pulled apart.
Defining Normal Faults
A normal fault is a specific category of dip-slip fault, meaning the movement of the rock blocks is primarily vertical along the inclined fault plane. Geologists use the terms “hanging wall” and “footwall” to describe the two blocks of rock separated by the fault. The hanging wall sits above the fault plane, while the footwall lies beneath it.
This terminology originated from early English coal miners. If a miner walked along the fault, the footwall would be beneath their feet, and the hanging wall would be the block hanging over their head. In a normal fault, the hanging wall always moves down relative to the footwall. This downward motion is the defining characteristic of a normal fault, resulting in a vertical offset of rock layers.
The Tectonic Mechanism: Extensional Stress
The primary force responsible for normal faults is extensional stress, also called tensional stress. This force pulls the Earth’s crust horizontally apart, stretching and lengthening the rock mass. This process is most frequently associated with divergent plate boundaries, where tectonic plates are actively moving away from one another.
As the crust is pulled in opposite directions, the tension overcomes the strength of the brittle rock, causing it to fracture. Once the rock breaks, gravity assists the downward movement of the unsupported hanging wall block along the inclined fault plane. This action effectively thins the crust, accommodating the pulling force exerted by underlying mantle convection currents. The fault plane in a normal fault is typically steep, often dipping between 45 and 90 degrees.
Where Normal Faults Shape the Landscape
The geological features created by normal faulting often define entire regions of the world. Extensional tectonics frequently produces a landscape characterized by alternating raised and lowered blocks of crust. The uplifted blocks are known as horsts, and the down-dropped blocks are called grabens.
When a graben is large and linear, it forms a rift valley, a key signature of continental rifting. The East African Rift System is a prime example, a massive chain of down-dropped valleys where the continent is being torn apart. Another expression of this faulting style is the Basin and Range Province in the Western United States, covering much of Nevada and Utah. This region displays numerous parallel mountain ranges (horsts) separated by broad valleys (grabens), created by the stretching of the crust.
Distinguishing Normal Faults from Other Types
To appreciate the normal fault, it is helpful to compare it to the two other major categories: reverse faults and strike-slip faults. Reverse faults are the opposite of normal faults, caused by compressional stress that pushes the crust together. In a reverse fault, the hanging wall block moves up relative to the footwall, resulting in a shortening and thickening of the crust.
Strike-slip faults are caused by shear stress, where the rock blocks slide horizontally past one another with minimal vertical movement. The San Andreas Fault in California is a famous example, where the motion is lateral along a nearly vertical fault plane. Normal faults, with their downward-moving hanging wall and extensional origin, are distinct from the compressional uplift of reverse faults and the shearing of strike-slip faults.