A geological fault represents a fracture within the Earth’s crust where rock masses on opposing sides have moved relative to one another. This displacement records the immense tectonic forces acting beneath the surface. Understanding the orientation and movement of the rock masses involved is foundational for geologists to classify the type of fault and the forces that created it. The key to this classification lies in identifying the specific names given to the rock blocks situated directly above and below the inclined fracture surface.
Defining the Hanging Wall and Footwall
The two blocks of rock separated by an angled fault plane are known as the hanging wall and the footwall. The hanging wall is the mass of rock that lies structurally above the fault plane, while the footwall is the block positioned beneath it. This terminology originated from historical mining practices.
Miners often excavated along fault zones where mineral deposits accumulated. When following an angled vein of ore, a miner found one wall overhanging their head—the hanging wall, where they might hang a lamp. The lower block, the one they stood upon, was termed the footwall.
Visualizing the Fault Plane and Blocks
To correctly identify the blocks, geologists focus on the angle of the fault, known as its dip. The fault plane is rarely vertical, meaning it typically slopes into the Earth at some angle. This angle is what creates the necessary spatial distinction between the two rock blocks.
Imagine the fault plane as a ramp dividing the crustal rock. The rock mass resting directly on top of that ramp is the hanging wall, while the footwall is the rock mass supporting the ramp from underneath. A classic visual aid involves picturing a person standing on the exposed fault surface. Their feet would be on the footwall block, and the hanging wall block would be directly above their head. This simple spatial relationship applies consistently to all non-vertical faults.
Linking Block Movement to Fault Classification
The distinction between the hanging wall and footwall is crucial because their relative movement determines the fault’s classification. Faults showing movement primarily up or down the dip of the fault plane are called dip-slip faults. These are divided into categories based on the direction the hanging wall moves in relation to the footwall.
Normal faults occur when the Earth’s crust is being stretched (extension). In this environment, the hanging wall moves downward relative to the footwall, sliding down the inclined fault plane due to gravity. This type of faulting causes the affected section of the crust to lengthen.
Reverse faults form under compressional forces, where the crust is being squeezed together. Here, the hanging wall moves upward relative to the footwall, overriding the footwall block. A specific type of reverse fault with a shallow dip angle, typically less than 45 degrees, is called a thrust fault. Both reverse and thrust faults cause a shortening of the crust.
When movement is predominantly horizontal, such as in a strike-slip fault, the fault plane is often nearly vertical. In these cases, the concepts of hanging wall and footwall are less relevant for classification. However, for any fault with a significant vertical component, identifying these blocks is the fundamental first step in assessing the tectonic history.