Earth’s crust is a dynamic outer layer, constantly shaped by various forces. These forces can cause the crust to fracture, leading to geological faults. A fault represents a break in the rock where the blocks on either side have moved relative to each other. Among the different types of faults, a normal fault is a common occurrence, revealing specific patterns of movement within the Earth.
What Defines a Normal Fault?
A normal fault is characterized by a specific geometry and direction of movement. The fault plane is a tilted surface within the Earth, which often dips steeply. The block of rock situated above this inclined fault plane is termed the “hanging wall,” while the block below it is known as the “footwall.” The defining feature of a normal fault is that the hanging wall moves downward relative to the footwall. This movement is primarily vertical, or “dip-slip.”
The Role of Tensional Stress
Normal faults are primarily caused by a type of geological force called tensional stress. This stress occurs when forces pull rocks apart, stretching and thinning the Earth’s crust. When this pulling force exceeds the inherent strength of the rock, the crust fractures. The resulting movement involves the hanging wall block sliding downward along the fault plane, accommodating the extension. The downward motion of the hanging wall signifies that the crust is undergoing extension and becoming thinner in that area.
Plate Tectonic Environments
Tensional stress predominantly occurs at divergent plate boundaries. These are regions where two tectonic plates are actively moving away from each other. As plates separate, the underlying mantle material rises, and the crust above is subjected to significant pulling forces. Notable examples include mid-ocean ridges, where new oceanic crust is generated, and continental rift zones such as the East African Rift Valley. Normal faults are characteristic features of these extensional environments, accommodating the stretching of the Earth’s lithosphere.
Landform Creation
Cumulative movement along normal faults significantly alters the Earth’s surface, creating distinct geological features. A depressed block known as a graben forms when a block of land drops down between two parallel normal faults that dip towards each other; these often manifest as long, linear valleys, with the classic example being rift valleys. Conversely, an elevated block of land situated between two normal faults that dip away from each other is called a horst. Horsts can form features such as plateaus, mountain ranges, or ridges. The Basin and Range Province in the western United States is a good example of a landscape shaped by numerous horsts and grabens, showcasing the results of crustal extension.