The Earth’s crust is a dynamic system where immense forces constantly act upon rock structures. These forces, stemming primarily from the movement of tectonic plates, create internal pressure known as geological stress. This stress is the fundamental mechanism that drives nearly all large-scale geological phenomena, including mountain building, faulting, and the creation of ocean basins. The way a rock body responds to this stress determines the shape of the features we see on the surface.
Defining Geological Stress
Geological stress is defined as the force applied over a specific area of rock material, which leads to deformation. It is important to distinguish stress from strain, which is the resulting change in the rock’s shape or volume caused by the application of stress. A primary type of pressure is confining stress, or lithostatic pressure, caused by the weight of overlying rock that pushes equally on the buried rock from all directions. This uniform pressure does not typically cause a change in shape, but it does make the rock stronger and less likely to fracture.
Differential stress is the unequal application of force from different directions, and this stress causes rocks to deform. Differential stress is categorized into the three principal types of stress—compression, tension, and shear—each creating distinct geological structures. The rock’s response to these stresses, whether it bends plastically or breaks in a brittle fashion, depends heavily on the pressure, temperature, and composition of the rock body. Rocks near the surface, which are cooler, tend to fracture, while deeply buried rocks, which are hotter and under greater pressure, tend to bend or flow.
The Action of Compression
Compressional stress involves forces directed toward one another, effectively squeezing a rock body from opposing sides. This action shortens the rock mass horizontally and often causes it to thicken vertically. Compression is the most common type of stress found at convergent plate boundaries, where tectonic plates collide.
A primary result of compressional stress is the formation of folds, where rock layers crumple and bend without breaking, such as the anticlines and synclines found in mountain ranges. If the stress exceeds the rock’s strength, it can cause brittle failure, resulting in the creation of reverse faults or thrust faults. In a thrust fault, the hanging wall block moves up and over the footwall block, leading to significant crustal shortening. The massive scale of mountain belts, like the Himalayas, is a direct result of sustained compressional forces.
The Action of Tension
Tensional stress, sometimes called extensional stress, involves forces pulling a rock body apart in opposite directions. This action causes the rock mass to lengthen horizontally and thin vertically. Tensional stress is the primary force operating at divergent plate boundaries, where tectonic plates are moving away from each other.
The rock’s response to this pulling action is often fracturing, which leads to the formation of normal faults. In a normal fault, the hanging wall block moves down relative to the footwall block, resulting in the extension of the crust. This process creates large-scale features like rift valleys, where a block of crust drops down between two parallel normal faults. The East African Rift Valley system represents an active example of crust being stretched and thinned by persistent tensional forces.
The Action of Shear
Shear stress occurs when forces are parallel to each other but are acting in opposite horizontal directions. Instead of squeezing or pulling, this stress causes parts of the rock body to slide past one another. The effect is a lateral displacement where rock layers are smeared or cut.
This type of stress is most characteristic of transform plate boundaries, where two plates slide horizontally past one another, such as the San Andreas Fault in California. The resulting deformation is typically a strike-slip fault, which involves horizontal movement of rock blocks along the fault plane. Shear stress can also cause localized zones of deformation called shear zones, where the rocks are stretched and rotated into complex fabrics. The grinding motion of shear stress can produce significant seismic activity as strain energy is suddenly released.