The Earth’s outer layer, the lithosphere, is fractured into massive tectonic plates that are in constant motion. These plates interact along boundaries, and the immense forces generated can cause the brittle rock of the crust to fracture. A geological fault is a break in the crust where the blocks of rock on either side have moved relative to one another. The type of fault that forms is directly linked to how the tectonic plates are interacting at that location.
Understanding Faults and Stress
Faults are the result of stress, which is the force applied over a specific area of rock. Geologists recognize three primary types of stress acting upon the Earth’s crust: compression, tension, and shear. Compression is a squeezing stress that pushes rock masses together, effectively shortening the crust. Tension is a pulling force that stretches the rock and causes the crust to lengthen. Shear stress involves forces that cause the rock masses to slide past one another horizontally.
To describe the movement along a fault plane, which is the surface of the fracture, geologists use the terms “hanging wall” and “footwall.” The hanging wall is the block of rock that sits above the inclined fault plane. Conversely, the footwall is the block of rock located beneath the fault plane. The relative movement of the hanging wall and footwall is what defines the type of fault.
The Reverse Fault and Convergent Boundaries
The type of plate boundary that creates a reverse fault is the convergent boundary. At a convergent boundary, two tectonic plates are moving toward one another, generating the necessary compressional stress. A reverse fault is defined by the hanging wall block moving upward relative to the footwall block. This upward movement occurs under the intense squeezing forces of compression.
The collision or subduction at a convergent boundary provides the powerful horizontal compression that pushes one block of crust up and over the other. This process results in crustal shortening and thickening. Reverse faults are commonly associated with the formation of major mountain ranges, such as the Himalayas. A reverse fault typically has a fault plane with a steep dip, often greater than 30 degrees.
A variation of the reverse fault is the thrust fault, which is a reverse fault with a shallow dip, generally less than 45 degrees. These low-angle thrust faults are especially effective at accommodating the massive crustal shortening seen in continental collision zones. In subduction zones, which are a type of convergent boundary where one plate sinks beneath another, the largest reverse faults are megathrust faults. Megathrust faults are known for generating the most powerful earthquakes on Earth.
Other Major Fault Types and Their Tectonic Settings
In contrast to the reverse fault, a normal fault is created by tensional stress, which pulls the crust apart. This fault type is characterized by the hanging wall moving downward relative to the footwall. Normal faults are typically found at divergent plate boundaries, where plates are moving away from each other, such as along the Mid-Atlantic Ridge.
The third main fault classification is the strike-slip fault, which is caused by shear stress. In this scenario, the two blocks of rock slide past each other horizontally, with very little vertical motion. Strike-slip faults are the characteristic feature of transform plate boundaries, where two plates grind past one another. The San Andreas Fault in California is a famous example of a transform boundary exhibiting this type of horizontal, lateral movement.