The Earth’s crust, or lithosphere, is a dynamic shell of rock constantly subjected to immense internal forces. It is not a single, solid piece, but a mosaic of tectonic plates that interact at their boundaries. These interactions generate stress, which can ultimately cause the crust to break. The most significant type of break or crack in the Earth’s crust is called a fault.
Defining the Fault: Displacement and Movement
A fault is defined as a planar fracture or zone of fractures within rock where the blocks on either side have moved relative to one another. This defining characteristic is called displacement, or slip. Without measurable movement along the fracture surface, the break is not classified as a fault.
Faults can range from a few millimeters of movement to thousands of kilometers in length, representing repeated displacements over geologic time. Geologists categorize the two blocks separated by an inclined fault plane as the hanging wall and the footwall. The hanging wall is the block that lies above the fault plane, while the footwall is the block that lies beneath it. This terminology helps describe the relative vertical movement between the two sides. This movement may happen rapidly, resulting in an earthquake, or slowly, a process known as aseismic creep.
Understanding Fractures and Joints
Not every crack in the Earth’s crust is a fault; many are simply fractures or joints. A fracture is a general term for any break in a rock body that causes a loss of cohesion. These breaks occur when applied stress exceeds the rock’s internal strength, but they do not involve shifting of the rock masses.
The term joint is used for a fracture where there has been no observable movement of the rock parallel to the break. Joints are extremely common features in rock formations and often form under tensional stress, such as when rock cools and contracts. They can also form when overlying rock is removed by erosion, causing the rock beneath to expand due to pressure release.
The Tectonic Forces That Cause Cracks
The underlying cause for all major cracks in the crust is stress, defined as the force applied to a rock per unit area. Tectonic plate motion generates three primary types of stress that lead to the deformation and breaking of the lithosphere. The accumulation of this stress eventually exceeds the rock’s brittle strength, leading to a sudden break and the release of stored energy.
Compressional Stress
Compressional stress involves forces pushing rocks together, causing them to shorten or fold. This stress is typical at convergent plate boundaries where plates collide.
Tensional Stress
Tensional stress involves forces pulling rocks apart, causing them to lengthen and break. This is the dominant stress at divergent plate boundaries, such as mid-ocean ridges.
Shear Stress
Shear stress occurs when forces act parallel to each other but in opposite horizontal directions, causing rock layers to slide past one another. This stress is characteristic of transform plate boundaries, where plates grind along each other.
Major Classifications of Faults
Faults are classified based on the direction of the relative movement, or slip, between the hanging wall and the footwall block. These classifications correspond directly to the three main types of tectonic stress. The most common types are dip-slip faults (primarily vertical movement) and strike-slip faults (primarily horizontal movement).
Normal faults are a type of dip-slip fault caused by tensional stress. The hanging wall block moves downward relative to the footwall block, accommodating the extension of the crust. They are commonly found in regions like the Basin and Range Province in the Western United States.
Reverse faults are a dip-slip type caused by compressional stress, resulting in crustal shortening. The hanging wall block moves upward relative to the footwall block, forcing older rock layers over younger ones. A specific reverse fault with a shallow angle (less than 45 degrees) is known as a thrust fault, which is common in mountain-building areas like the Himalayas.
Strike-slip faults are the result of shear stress, where the two blocks slide horizontally past one another with little vertical motion. The San Andreas Fault in California is a well-known example. These faults are further defined as right-lateral or left-lateral based on the direction the opposite block moves when viewed across the fault line.