A geological fault is a fracture or zone of fractures that separates two blocks of rock within the Earth’s crust. Significant movement or displacement of the rock blocks occurs along this fracture surface over time. These faults result from the immense forces generated by plate tectonics and stress on the lithosphere. Normal faults are one of the three main classifications, distinguished by how the two rock blocks move relative to each other. Understanding normal fault mechanics provides insight into how the Earth’s surface stretches and thins.
The Stress That Creates Normal Faults
The formation of a normal fault is driven by tensional stress, also known as extensional stress. This stress occurs when forces pull the crust apart in opposite directions, effectively stretching the rock mass. This pulling action is most often found at divergent plate boundaries, where tectonic plates are moving away from one another.
As the crust is pulled, it thins out and elongates horizontally. When the brittle upper layer of the Earth can no longer withstand this stretching force, it fractures. The resulting break is the fault plane, the surface along which movement occurs. This extensional environment is the prerequisite for the distinct movement seen in a normal fault.
Identifying the Fault Structure and Movement
To understand the motion in a normal fault, geologists define the two blocks of rock separated by the inclined fault plane. The block situated above the fault plane is called the hanging wall, while the block below is termed the footwall. This terminology originates from mining, where a miner standing along the fault would have the footwall beneath their feet and the hanging wall above their head.
The characteristic of a normal fault is that the hanging wall moves downward relative to the footwall. This movement is driven by gravity as the crust is pulled apart and thinned by tensional forces. The fault plane is inclined at a steep angle, often between 45 and 60 degrees, providing the slope for the hanging wall to slide down.
The effect of this slip is a lengthening and extension of the crust across the fault zone. Displacement along the fault can range from a few meters to thousands of meters over geological time. As the hanging wall drops, rock layers on either side become offset, providing visual evidence of the separation and vertical displacement. This vertical movement classifies normal faults as a type of dip-slip fault.
Large-Scale Geological Features
Continuous normal faulting over millions of years translates localized vertical movement into massive changes in surface topography. When the crust experiences widespread extension, a system of parallel normal faults develops, creating alternating raised and lowered blocks. These structures are known as horst and graben topography.
The blocks that move downward between two parallel normal faults are called grabens. A graben forms a trough or depression, and when large, it is referred to as a rift valley. The East African Rift Valley, where the African continent is slowly pulling apart, is a prime example of this structure.
Conversely, the blocks of crust that remain uplifted between two adjacent grabens are termed horsts. These blocks form long, elevated features such as plateaus, ridges, or mountain ranges. The characteristic landscape of the Basin and Range Province in the western United States, which includes alternating mountain ranges (horsts) and valleys (grabens), results from this large-scale normal faulting. The formation of these expansive features demonstrates how the simple downward slip of the hanging wall can fundamentally reshape the continental crust.