A geological fault is a fracture or a zone of fractures within the Earth’s crust where the rocks on either side have moved relative to one another. This displacement can range from a few centimeters to hundreds of kilometers over geological time. The movement along a fault’s surface, known as the fault plane, is systematically classified based on the direction of slip. Scientists categorize these movements into three primary types, which offer direct insight into the immense tectonic forces shaping the planet.
Defining Fault Movement and Stress
Understanding how faults are classified requires defining the two blocks of rock involved in the movement, particularly for inclined faults. The hanging wall is the rock mass that lies above the inclined fault plane. The second block, situated below the fault plane, is called the footwall. The relative movement between the hanging wall and the footwall dictates the type of fault.
This movement is driven by three fundamental types of stress: tensional, compressional, and shear. Tensional stress pulls rocks apart, compressional stress pushes them together, and shear stress causes them to slide past one another horizontally.
Normal Faults
Normal faults are the geological expression of tensional stress, which acts to stretch and pull apart a section of the Earth’s crust. This type of faulting is characterized by the hanging wall block moving downward relative to the footwall block. The movement effectively lengthens the crust, a process known as crustal extension.
These faults are most commonly associated with divergent plate boundaries, where tectonic plates are actively moving away from each other. Normal faults typically form at steep angles, often ranging between 45 and 90 degrees. A network of normal faults can create distinctive topographical features called horsts (uplifted blocks) and grabens (down-dropped blocks), which often form rift valleys. The Basin and Range Province in the western United States is a prominent example of this extensive faulting.
Reverse and Thrust Faults
Reverse and thrust faults both form in response to compressional stress, where rocks are squeezed and pushed together. This stress regimen results in the hanging wall block moving upward relative to the footwall block. This upward movement causes the crust to shorten horizontally, resulting in a net crustal thickening.
The distinction between a reverse fault and a thrust fault is based on the angle at which the fault plane dips, or its inclination measured from the horizontal. A standard reverse fault is characterized by a relatively steep dip angle, generally greater than 45 degrees. Conversely, a thrust fault is a subtype of reverse fault defined by a much shallower dip, typically less than 30 degrees.
This shallow angle allows for extensive horizontal displacement, often placing older rock units on top of younger ones. Both types are characteristic of convergent plate boundaries, where plates collide, leading to large-scale mountain-building events known as orogenesis. Major mountain chains, such as the Rocky Mountains and the Himalayas, are linked to the crustal shortening accommodated by these faults.
Strike-Slip Faults
Strike-slip faults are the result of shear stress, which involves forces acting parallel to each other but in opposite horizontal directions. The defining characteristic of a strike-slip fault is that the movement of the rock blocks is almost entirely horizontal, with little to no vertical displacement. The fault plane itself is usually near-vertical.
These faults are categorized into two types based on the relative direction of movement. A right-lateral (or dextral) strike-slip fault is one where an observer standing on one side sees the opposite block move to the right. Conversely, a left-lateral (or sinistral) strike-slip fault is one where the opposite block appears to move to the left. Strike-slip faults are common along transform plate boundaries, where two plates grind past each other horizontally. The San Andreas Fault in California is a well-known example of a right-lateral strike-slip fault.