The rock surface immediately above a fault is known as the hanging wall.
Understanding Fault Planes and Rock Blocks
When geological forces cause the Earth's crust to fracture and move, a fault is formed. A fault is essentially a planar fracture or discontinuity in a volume of rock across which there has been significant displacement. The rocks on either side of this fracture are referred to as rock blocks. Identifying these blocks is fundamental to understanding fault mechanics and classification.
The body of rock situated above the fault plane is referred to as the hanging wall. Conversely, the body of rock below the fault plane is called the footwall. This terminology originated from miners who, when working in ore veins within faults, would often have their lanterns hanging from the rock surface above them (the hanging wall) while standing on the rock surface below their feet (the footwall).
Distinguishing Between Hanging Wall and Footwall
The clear distinction between the hanging wall and footwall is crucial for classifying the type of fault and interpreting the geological stresses that caused the movement.
Term | Description | Significance |
---|---|---|
Hanging Wall | The block of rock located above the fault plane. | In normal faults, the hanging wall moves down relative to the footwall. In reverse faults and thrust faults, the hanging wall moves up relative to the footwall. |
Footwall | The block of rock located below the fault plane. | Serves as the stable reference point against which the movement of the hanging wall is measured. |
Geological Significance
The identification and analysis of hanging walls and footwalls are pivotal in various aspects of geology:
- Fault Classification: The relative movement of the hanging wall with respect to the footwall defines the type of fault.
- Normal Faults: Occur where the crust is being pulled apart (extensional forces), causing the hanging wall to move downwards relative to the footwall.
- Reverse Faults: Form where the crust is being compressed (compressional forces), resulting in the hanging wall moving upwards relative to the footwall. When the fault plane is at a low angle (less than 45 degrees), it is specifically called a thrust fault.
- Strike-Slip Faults: Involve horizontal movement where blocks slide past each other, with less emphasis on distinct hanging and footwall movement in the traditional sense, though the terms can still apply to the adjacent blocks.
- Structural Analysis: Geologists use these concepts to map and interpret subsurface structures, which is vital for understanding tectonic processes and regional geology.
- Resource Exploration: Knowledge of fault types and the displacement of hanging walls and footwalls is critical in the exploration for resources such as oil, natural gas, and mineral deposits, as faults can act as conduits or barriers for fluid migration.
- Seismic Hazard Assessment: Understanding the movement along fault planes helps in assessing seismic risks, as faults are often the source of earthquakes.
To learn more about faults and their mechanics, you can explore resources on structural geology.