Each type of geologic fault is produced by a specific type of stress acting on rock. Tensional stress creates normal faults, compressional stress creates reverse faults, and shear stress creates strike-slip faults. These three stress types correspond to rocks being pulled apart, pushed together, or forced to slide past each other, and the fault that forms depends entirely on which force dominates.
Stress and Strain in Rock
Stress is simply force applied per unit area to rock. When that force exceeds the rock’s internal strength, the rock deforms, a response geologists call strain. Sometimes strain bends rock gradually into folds. Other times it snaps the rock along a fracture, creating a fault with blocks that move relative to each other.
Three types of stress act on Earth’s crust:
- Tensional stress: forces pulling in opposite directions, stretching and thinning the rock.
- Compressional stress: forces pushing together, shortening and thickening the rock.
- Shear stress: forces acting parallel but in opposite directions, tearing the rock laterally.
Tensional Stress Creates Normal Faults
When rock is pulled apart by tensional (extensional) stress, the result is a normal fault. The rock fractures along an inclined plane, and the block sitting above that plane, called the hanging wall, drops down relative to the block below it, called the footwall. Gravity assists the motion once the fracture forms, which is why normal faults are sometimes called gravity faults.
Normal faults are the signature feature of divergent plate boundaries, where tectonic plates move away from each other. The Mid-Atlantic Ridge is a classic example: seafloor spreading pulls the crust apart, generating tension cracks and normal faults along the rift. On land, the Basin and Range Province in the western United States shows the same process. Tensional forces have stretched the crust there, producing a landscape of alternating mountain ranges and valleys bounded by normal faults. The Sierra Nevada range along Owens Valley is another well-known example.
Compressional Stress Creates Reverse Faults
Compressional stress, where forces push rock together, produces reverse faults. In a reverse fault the hanging wall moves up relative to the footwall, essentially stacking rock on top of itself. This is the opposite motion of a normal fault.
Reverse faults are subdivided by the angle of the fracture plane. A fault with a steep dip (greater than 30 degrees) is classified as a reverse fault. When the angle is shallower, it’s called a thrust fault. Thrust faults often develop staircase-shaped paths through rock layers, with flat segments connected by steeper ramps. Both types form in regions where the crust is being compressed.
Convergent plate boundaries are where you’ll find reverse and thrust faults most often. When two continental plates collide head-on, neither sinks beneath the other because continental rock is relatively buoyant. Instead the crust buckles, folds, and pushes upward along reverse faults. The Himalayas formed this way, through the ongoing collision of the Indian and Eurasian plates.
Shear Stress Creates Strike-Slip Faults
Shear stress acts when two masses of rock slide horizontally past each other in opposite directions. The resulting fault is called a strike-slip fault because the movement runs along the “strike,” or horizontal direction, of the fault plane. There is little to no vertical displacement.
Strike-slip faults are further classified by the direction the far side moves when you stand on one side and look across. If the opposite block moves to the right, it’s a right-lateral (dextral) fault. If it moves to the left, it’s a left-lateral (sinistral) fault. The San Andreas Fault in California is a right-lateral strike-slip fault running roughly 1,300 kilometers and extending at least 25 kilometers deep. It marks the boundary where the Pacific Plate and North American Plate grind past each other.
Strike-slip faults define transform plate boundaries, where crust is neither created nor destroyed. Most transform faults actually sit on the ocean floor, offsetting mid-ocean ridges in a zig-zag pattern. They tend to produce shallow earthquakes.
Combined Stress Creates Oblique-Slip Faults
Not every fault fits neatly into one category. When a rock mass experiences a combination of stress types at the same time, the resulting fault can show both vertical and horizontal displacement. These are called oblique-slip faults. A combination of shearing with either tension or compression produces this mixed motion. You can think of an oblique-slip fault as a hybrid: part normal or reverse, part strike-slip. They’re common in regions where tectonic forces don’t act in a single clean direction.
Quick Reference: Stress and Fault Types
- Tensional stress → normal fault (hanging wall drops down) → divergent boundaries
- Compressional stress → reverse or thrust fault (hanging wall pushes up) → convergent boundaries
- Shear stress → strike-slip fault (blocks slide horizontally) → transform boundaries
- Combined stress → oblique-slip fault (both vertical and horizontal movement)