Question

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.

Short answer

Normal faults show crustal extension with a downward hanging wall; reverse faults show crustal shortening with an upward hanging wall.

Step-by-step solution

Understanding Faults

In geology, a fault is a fracture or zone of fractures between two blocks of rock, leading to relative movement on either side. A normal fault and a reverse fault are two fundamental types of faults, differentiated by their movement.

Sketching a Normal Fault

In a normal fault, the hanging wall moves downward relative to the footwall due to extensional forces. Draw two blocks: the top block (hanging wall) should be sliding downwards relative to the bottom block (footwall). This mimics the effect of crustal extension where the Earth's crust is being pulled apart.

Labeling the Normal Fault Diagram

Label the top block as 'Hanging Wall' and the bottom block as 'Footwall' on your sketch. The movement of the hanging wall downward relative to the footwall is a defining characteristic of normal faults.

Explaining Crustal Extension

In a normal fault, extensional forces are at work. These forces pull the crust apart, leading to a drop of the hanging wall, which results in crustal thinning and extension.

Sketching a Reverse Fault

In a reverse fault, the hanging wall moves upward relative to the footwall due to compressional forces. Draw the same two blocks, but now the top block (hanging wall) should move upwards relative to the bottom block (footwall). This is indicative of crustal shortening.

Labeling the Reverse Fault Diagram

Label the top block again as 'Hanging Wall' and the bottom block as 'Footwall'. This movement of the hanging wall upwards relative to the footwall signifies the compressional forces in action.

Explaining Crustal Shortening

In a reverse fault, compressional forces cause the Earth's crust to shorten by pushing the hanging wall up and over the footwall. This results in a thicker and shorter crust.

Key concepts

Normal Fault

A normal fault occurs when the crust is subjected to extensional forces, causing it to be pulled apart. In this type of fault, the hanging wall, which is the upper part of the fault, moves downward relative to the footwall, the lower part. This movement results in the extension and thinning of the Earth's crust as it stretches out. Normal faults are typical of regions that are undergoing crustal extension, such as rift zones. Understanding this mechanism helps in recognizing how tension in the crust can lead to significant geological formations.

Reverse Fault

In contrast to a normal fault, a reverse fault is formed through compressional forces, which push the Earth's crust together. This stress causes the hanging wall to move upwards relative to the footwall. As the crust is compressed in a reverse fault, it leads to crustal shortening, making the crust thicker and structurally higher. These types of faults are common at convergent plate boundaries, where tectonic plates push against each other. The resulting uplift often gives rise to mountain ranges and significant geological features.

Crustal Extension

Crustal extension refers to the process where the Earth's crust is stretched, leading to its thinning. This phenomenon is crucial in areas with normal faults, where the outward pull causes the hanging wall to drop and the land to spread apart. Crustal extension often occurs in environments like rift valleys, where tectonic forces stretch the land, creating elongated basins and thinning the crust over time. Understanding crustal extension helps explain the formation of new crust and geological features like mid-ocean ridges.

Crustal Shortening

When the Earth's crust is pushed together, crustal shortening takes place. This process is characterized by compression that results in the thickening of the crust, typically associated with reverse faults. The pushing forces cause the crust to buckle and overlap, leading to uplift and the formation of features such as mountain ranges. Crustal shortening is a common occurrence at convergent plate boundaries and is an essential factor in understanding how Earth's topography changes due to compressional tectonic activity.

Hanging Wall

In fault geology, the hanging wall is the block of rock that lies above the fault plane. Notably, in a normal fault, the hanging wall moves downwards due to extensional forces, while in a reverse fault, it is pushed upwards by compressional forces. The movement and position of the hanging wall provide critical insights into the types of stress affecting a region and the resultant geological features. Recognizing the hanging wall's behavior aids in understanding fault mechanics and the resultant geological structures.

Footwall

The footwall is the block of rock located below the fault plane. In both normal and reverse faults, the footwall serves as the baseline against which the movement of the hanging wall is compared. While the footwall remains relatively stable, the hanging wall's movement—whether ascending in a reverse fault or descending in a normal fault—determines the type of faulting and the associated geological implications. Identifying the footwall is crucial in mapping out faults and understanding the stress dynamics within the Earth's crust.