A geological fault is a planar fracture or discontinuity within the Earth’s crust where the rocks on either side have moved significantly relative to one another. These structures, which can range in size from a few centimeters to thousands of kilometers, are fundamental features that shape the planet’s surface and are the source of most earthquakes. Faults represent a permanent break in the rock material, accommodating the intense forces that continually deform the lithosphere. Understanding when and why these breaks occur requires examining the forces that build up within the rock and the physical conditions that determine how the rock responds.
The Forces That Deform Rock
The causes of faulting are the mechanical forces imposed on the Earth’s crust, defined as stress. Stress is the amount of force applied over a specific area, and it is the direct action that causes a rock body to change its shape or volume. The resulting change is known as strain, which represents the deformation a rock experiences in response to the applied stress.
Three primary types of directed stress cause rock deformation. Compression is a squeezing stress that pushes rock masses together, causing the crust to shorten or thicken. Tension is a pulling-apart stress that stretches the rock and causes the crust to lengthen. The third type, shear stress, involves parallel forces moving in opposite directions, causing one part of the rock body to slide past another.
Initially, rocks may undergo elastic strain, returning to their original shape once the stress is removed. If the stress continues to increase beyond the rock’s strength limit, the strain becomes permanent, leading to either folding or a complete fracture that forms a fault.
Physical Conditions That Lead to Breaking
For a fault to form, the rock must undergo brittle deformation, which is the process of fracturing or breaking. This type of breaking happens when the applied stress rapidly exceeds the rock’s internal strength. The main alternative is ductile deformation, where the rock flows or bends permanently without fracturing, typically resulting in geological folds.
Physical conditions within the Earth determine which type of deformation occurs. Rocks near the surface, where temperatures and confining pressures are relatively low, tend to behave in a brittle manner, favoring fracturing. Conversely, rocks deep in the crust, subjected to high temperatures and immense confining pressures, are more likely to deform in a ductile, flowing manner.
The rate at which stress is applied, known as the strain rate, also dictates the rock’s response. If the stress is applied very quickly, the rock is more likely to break suddenly in a brittle fashion. If the stress is applied slowly over millions of years, the rock has time to accommodate the deformation by bending or flowing. Therefore, faults are primarily a shallow-crust feature that occurs when rock is cool and the stress is applied abruptly.
Categorizing Fault Movement
Faults are categorized by the relative motion of the two blocks separated by the fault plane. To describe this movement, geologists use the terms hanging wall and footwall, which originated in mining. The hanging wall is the block of rock positioned above the inclined fault plane, while the footwall is the block below it.
Normal Faults
Normal faults result from tensional stress that pulls the crust apart. The hanging wall block moves downward relative to the footwall block, which effectively lengthens and thins the crust. This downward movement is consistent with the pull of gravity on a stretched section of the crust.
Reverse Faults
Reverse faults, and their shallow-angle variants called thrust faults, form in response to compressional stress that pushes the crust together. The hanging wall block moves upward relative to the footwall block, causing the crust to shorten and thicken. This geometry is commonly associated with mountain-building processes.
Strike-Slip Faults
Strike-slip faults are caused by shear stress and involve movement that is predominantly horizontal, or side-to-side. Along a strike-slip fault, the rock blocks slide past each other with very little vertical motion. The San Andreas Fault is an example of this type, where movement is lateral along a nearly vertical fault plane.
Tectonic Settings Where Faults Form
The large-scale movement of the Earth’s lithospheric plates provides the engine for the forces that create faults. Plate tectonics dictates the major stress regime—compression, tension, or shear—in any given region, which in turn determines the type of fault that forms. Faults are therefore most common and largest at the boundaries between these massive plates.
Divergent Boundaries
Divergent boundaries, where two plates are pulling away from each other, create a tensional stress regime. This stretching naturally leads to the formation of normal faults, such as those seen along the Mid-Atlantic Ridge or in continental rift valleys. Here, the crust is pulled apart, and blocks drop down to fill the void.
Convergent Boundaries
Convergent boundaries, where plates collide, impose compressional stress on the crust. This squeezing results in the formation of reverse faults and low-angle thrust faults, which push rock masses upward to form mountain ranges. The subduction zones where one plate slides beneath another are home to megathrust faults.
Transform Boundaries
Transform boundaries are areas where two plates slide horizontally past one another, generating shear stress. This side-by-side motion is the direct cause of strike-slip faults, which accommodates the lateral movement of the plates. While faults can also occur within the interior of plates due to localized forces, the majority of significant faulting is concentrated in these three major tectonic settings.