Geological stress represents the force applied to a rock body, acting as the primary engine for the deformation that shapes the Earth’s crust. This force, measured as force per unit area, is constantly at work due to the movements of tectonic plates and the weight of overlying material. While a uniform stress from the weight of rock exists everywhere, it is the non-uniform or directed stresses that cause geological change. These directed forces are categorized into three main types, each defining a distinct way that rock material is pushed, pulled, or sheared, leading to the creation of mountains, valleys, and fault lines.
Defining Compressive Stress
Compressive stress involves forces directed inward, pushing two masses of rock toward each other in a squeezing motion. It is a common force in areas where tectonic plates are moving together, such as at convergent boundaries. The force vector is oriented perpendicular to the plane of the rock and pushes inward from opposite sides.
For the rock material to accommodate this inward push, it must either decrease in volume or change its shape by folding or fracturing. Deep within the crust, where temperatures and confining pressures are high, the rock tends to deform plastically. Near the surface, however, the rock is more likely to break abruptly under the immense pressure of the compression. This squeezing action is the driving force behind the creation of some of the planet’s largest mountain ranges.
Defining Tensional Stress
Tensional stress, often called extensional stress, occurs when forces pull a rock body apart. These forces are directed away from each other, resulting in the thinning of the crustal section over which it acts. This process is typically found at divergent plate boundaries, where tectonic plates are moving away from one another.
The force vector in tensional stress is also perpendicular to the rock’s surface, but the vectors point outward, causing the rock to neck down. As the rock is pulled, it undergoes elastic deformation until the stress exceeds the rock’s strength, causing it to fracture. This pulling apart of the crust creates space that can be filled by new material or result in the sinking of large blocks of land. The effect of this stretching is a widening and lowering of the affected area over geological time.
Defining Shear Stress
Shear stress involves forces that are applied parallel to a rock body’s surface rather than perpendicular to it. This force causes one part of the rock to move in one direction while an adjacent part moves in the opposite direction. It is a tearing or sliding motion, similar to the action of a pair of scissors.
The forces are offset, resulting in a lateral displacement along the plane where the stress is most concentrated. This non-uniform application of force seeks to change the shape of the rock body without necessarily changing its volume. Shear stress is most characteristic of transform plate boundaries, where plates slide horizontally past one another. This type of stress is responsible for the movement along major, large-scale fault systems worldwide.
Stress, Strain, and Resulting Structures
The three types of stress each produce a specific type of strain and a corresponding geological structure. The resulting strain represents the change in a rock’s shape or volume due to the applied force.
Compressive stress leads to a strain known as shortening, which manifests in two primary structures: folds and reverse faults. Folds are bends in layered rock that occur when the rock deforms plastically deep underground, while reverse faults form when one block of rock is pushed up and over another in a brittle environment.
Conversely, tensional stress creates a lengthening strain, which is expressed as normal faults. In a normal fault, the hanging wall block moves down relative to the footwall block, allowing the crust to stretch and thin, often leading to the formation of rift valleys. Shear stress results in a lateral displacement strain, which produces strike-slip faults where the blocks of crust slide past each other horizontally.