Each type of geological fault is produced by a specific type of stress acting on rock in Earth’s crust. Tensional stress creates normal faults, compressional stress creates reverse faults, and shear stress creates strike-slip faults. Understanding which stress produces which fault comes down to the direction forces are pushing, pulling, or sliding the rock.
Tensional Stress and Normal Faults
Tensional stress (also called extensional stress) occurs when forces pull rock apart in opposite directions. When the rock can no longer stretch, it fractures along an angled plane, and the block of rock above that plane drops downward relative to the block below. This is a normal fault.
Think of it as pulling a rigid block until it cracks diagonally: the upper piece slides down along the break. The result is that the land surface stretches out and drops in elevation. When multiple normal faults form parallel to each other, the dropped-down blocks between them create valleys called grabens, while the elevated blocks between them are called horsts. The Basin and Range Province across Nevada, Utah, and surrounding states is one of the best examples of this pattern, where the crust has been stretching for millions of years, producing dozens of alternating mountain ranges and flat valleys. Normal faults also form along oceanic ridge systems, where tectonic plates are pulling apart underwater.
Compressional Stress and Reverse Faults
Compressional stress is the opposite: forces push rock together from both sides. When rock fails under compression, it fractures along an angled plane and the block above the fault is forced upward relative to the block below. This is a reverse fault, and it results in crustal shortening, meaning the total horizontal distance across the area gets smaller as rock stacks up on itself.
When the angle of the fault plane is especially shallow (closer to horizontal than vertical), the reverse fault is called a thrust fault. Thrust faults can push enormous slabs of rock for miles over the surface. Both reverse and thrust faults are common at convergent plate boundaries, where two plates collide. The Pacific Ring of Fire is a major convergent zone where the edges of colliding plates buckle upward into mountain ranges or one plate bends beneath the other into deep ocean trenches. The Himalayan mountain range was built largely through reverse and thrust faulting as the Indian plate collided with the Eurasian plate.
Shear Stress and Strike-Slip Faults
Shear stress occurs when forces push two blocks of rock in opposite horizontal directions, sliding them past each other. The resulting fracture is a strike-slip fault, where the movement is almost entirely side to side rather than up or down. If you stood on one side of the fault and looked across, the other side would appear to have shifted left or right.
Strike-slip faults form at transform plate boundaries, where two tectonic plates grind laterally past one another. The San Andreas Fault in California is the most famous example. It marks the boundary where the Pacific plate slides northwest relative to the North American plate. The rocks along these boundaries get pulverized over time, creating a linear fault valley. The 1906 San Francisco earthquake provided key evidence for how strike-slip faults work: geologist Henry Fielding Reid observed that a fence crossing the fault had gradually been bent into an S-shape by the slow plate movement, then snapped straight during the earthquake with an offset where the fault had ruptured. This observation led to the elastic rebound theory, which explains that the crust stores elastic energy like a stretched rubber band and releases it suddenly when the rock breaks or slips.
How Stress Builds Until Rock Breaks
Rock doesn’t fracture the instant stress is applied. Instead, the crust gradually accumulates strain over years, decades, or centuries. The rock bends and deforms slightly while storing elastic energy, much like bending a stick. When the stress exceeds the rock’s strength, the stored energy is released suddenly as the rock snaps along a fault plane, producing an earthquake. This cycle of slow buildup and sudden release repeats over and over along active faults.
Whether stress produces a clean fracture (a fault) or a slow bend (a fold) depends largely on conditions inside the Earth. At shallow depths where pressure and temperature are relatively low, rock is brittle: it breaks sharply when stress is high enough, forming a fault. At greater depths, higher pressure and temperature make the same rock behave in a more ductile way, allowing it to deform and flow without snapping. The depth at which rock transitions from brittle to ductile behavior varies depending on rock type, temperature, and the type of stress involved. Research has shown that this transition happens at much higher pressures under extensional stress than under compressional stress, meaning rock can tolerate more pulling force before it shifts to ductile behavior than it can tolerate squeezing force.
Matching Fault Types to Plate Boundaries
The type of stress acting on rock is directly tied to what the tectonic plates are doing at a given boundary:
- Divergent boundaries produce tensional stress as plates pull apart. Normal faults dominate. The Mid-Atlantic Ridge, where the North American and Eurasian plates are separating, is a classic example.
- Convergent boundaries produce compressional stress as plates collide. Reverse and thrust faults dominate. The subduction zones ringing the Pacific Ocean and the collision zone forming the Himalayas are prime examples.
- Transform boundaries produce shear stress as plates slide past each other. Strike-slip faults dominate. The San Andreas Fault zone extends both on land and underwater along the California coast.
Oblique-Slip Faults and Combined Stress
Not every fault fits neatly into one category. When two types of stress act on rock at the same time, the resulting fault shows a combination of movements. An oblique-slip fault has both vertical displacement (like a normal or reverse fault) and horizontal displacement (like a strike-slip fault). These form where tectonic forces aren’t perfectly aligned with one stress direction, which is common at plate boundaries that are slightly angled rather than running perfectly parallel or perpendicular to the direction of plate motion. In practice, many real-world faults have at least some oblique component, even if they’re classified primarily as one type.