Draw a cross-sectional sketch of a normal fault. Label the hanging wall and the footwall. Use your sketch to explain how a normal fault accommodates crustal extension. Sketch a reverse fault and show how it accommodates crustal shortening.
The Correct Answer and Explanation is:
Sketches:
Normal Fault (Extension)
/|
/ |
/ | ← Hanging Wall
/ |
------------ ← Fault Plane
\ |
\ | ← Footwall
\ |
\|
In a normal fault, the hanging wall moves down relative to the footwall.
Reverse Fault (Compression)
/|
/ | ← Hanging Wall
/ |
------------ ← Fault Plane
\ |
\ | ← Footwall
\|
In a reverse fault, the hanging wall moves up relative to the footwall.
Explanation:
Faults are fractures in the Earth’s crust where blocks of rock move relative to each other. They are typically classified by the direction of movement and the type of stress involved. Two primary fault types are normal faults and reverse faults, which form under different tectonic conditions.
A normal fault occurs in regions where the crust is being pulled apart, such as at divergent plate boundaries. In a normal fault, the hanging wall moves downward relative to the footwall. This movement accommodates crustal extension, meaning the Earth’s crust is being stretched and thinned. These faults are common in rift valleys and mid-ocean ridges. The down-dropping of the hanging wall creates space and relieves the tensional stress, helping to accommodate the increasing horizontal distance between two points in the crust.
In contrast, a reverse fault forms in areas of crustal compression, such as at convergent plate boundaries. Here, the hanging wall moves upward relative to the footwall. This motion results in crustal shortening—the Earth’s crust is being compressed and thickened. Reverse faults are common in mountain-building zones, where tectonic plates collide. The upward movement of the hanging wall effectively shortens the horizontal distance and increases crustal thickness, helping to build up mountain ranges.
In summary, normal faults accommodate crustal extension by allowing rock to move downward and apart, while reverse faults accommodate crustal shortening by allowing rock to move upward and compress. These fault movements are critical in shaping Earth’s topography and tectonic evolution.
