Normal faults are caused by tensional (extensional) stress, the force that pulls Earth’s crust apart. When tectonic plates or sections of crust move away from each other, the rock stretches until it fractures along an inclined plane, and one block drops down relative to the other. This stretching and thinning of the crust is the defining mechanism behind every normal fault on the planet.
How Tensional Stress Creates a Normal Fault
Think of tensional stress as a tug-of-war acting on rock. Two forces pull in opposite directions, stretching the crust horizontally. Rock is strong under compression but relatively weak under tension, so once the stress exceeds the rock’s strength, it breaks along an angled surface called the fault plane.
When the fracture forms, the block of rock sitting above the angled fault plane (called the hanging wall) slides downward relative to the block below it (the footwall). This downward slip is what makes the fault “normal,” a term that dates back to early mining, where miners noticed the hanging wall slipped down along the slope of the fault under its own weight. Interestingly, during earthquakes on normal faults, the footwall doesn’t just stay put. It actually lifts upward slightly, so the two blocks move apart both horizontally and vertically at the same time.
Where Tensional Stress Occurs
Tensional stress is associated with divergent plate boundaries, places where two tectonic plates move away from each other. Mid-ocean ridges are the most widespread example: as plates separate along the seafloor, magma rises to fill the gap, and normal faults line both sides of the ridge. On land, the same process shows up wherever continental crust is being pulled apart.
The East African Rift is one of the most dramatic active examples. There, the African continent is slowly splitting into two plates, the Nubian and the Somalian. Extensional stress has generated a network of steeply dipping normal faults that created a series of elongated lowland valleys flanked by highlands. Flood basalts erupted through fissures, and the classic pattern of raised and dropped blocks took shape across the landscape. If rifting continues, the valley will eventually sink low enough for ocean water to flood in, potentially forming a narrow ocean basin with its own mid-ocean ridge.
The Basin and Range Province of the western United States is another textbook case. Stretching across parts of eight states, this region is considered one of the finest modern examples of large-scale continental extension. The landscape consists of remarkably evenly spaced, roughly north-south mountain ranges separated by flat desert basins, all formed by high-angle normal faulting. Geologists estimate the crust here has been stretched by 100 to 300 kilometers over the course of the Cenozoic era, a significant increase in width driven entirely by tensional forces.
Typical Geometry of Normal Faults
Normal faults don’t break vertically. They form at an angle, and that angle (called the dip) has a characteristic range. A global analysis of earthquake data published in Geophysical Journal International found that normal faults most commonly dip at 40 to 50 degrees from horizontal. Classical theory predicted steeper angles of around 58 to 68 degrees, but real-world measurements consistently show somewhat shallower dips. Among all fault types, normal faults actually show the narrowest spread of dip angles, clustering tightly around that 40-to-50-degree peak regardless of whether they’re in the Basin and Range, the East African Rift, the Apennines, or oceanic settings.
Horsts and Grabens
When tensional stress produces not just one normal fault but a series of parallel faults, the landscape develops a distinctive pattern of raised and lowered blocks. A graben is a block of crust that has dropped down between two normal faults, forming a valley or basin. A horst is the opposite: a block that remains elevated between two faults, forming a ridge or plateau. The Basin and Range Province is essentially a repeating sequence of horsts and grabens stretching across the American West, and the rift valleys of East Africa follow the same structural blueprint.
How Normal Faults Differ From Other Fault Types
The three main types of tectonic stress each produce a different kind of fault. Tensional stress, which pulls crust apart, produces normal faults. Compressional stress, which pushes crust together, produces reverse (or thrust) faults where the hanging wall is shoved upward over the footwall. Shear stress, where two blocks slide horizontally past each other, produces strike-slip faults. These three stress types correspond to the three kinds of plate boundaries: divergent, convergent, and transform.
The simplest way to remember the difference between a normal fault and a reverse fault is the direction the hanging wall moves. In a normal fault, it drops down. In a reverse fault, it rides up. Both involve movement along an inclined fault plane, but the forces driving them are opposites: tension versus compression. Normal faults stretch and thin the crust, while reverse faults shorten and thicken it. Strike-slip faults, by contrast, involve no vertical movement at all, just horizontal sliding.
Normal fault earthquakes occur along oceanic ridge systems and in regions of active crustal extension. They tend to be shallower than subduction-zone earthquakes, since they occur in crust that is being stretched and thinned rather than compressed and thickened. The U.S. Geological Survey notes that normal faulting is commonly observed in the Basin and Range Province and along oceanic ridges, both settings where the crust is under persistent tension.