Earth’s outer shell, known as the crust, is a dynamic and ever-changing landscape. This rigid layer is not a single, solid piece but is broken into numerous segments called tectonic plates. The boundaries where these plates meet are often marked by fractures in the rock, known as faults. Movement along these faults, driven by immense forces within the Earth, constantly shapes the planet’s surface, leading to the formation of diverse geological features and events.
What Normal Faults Are
A normal fault is a specific type of fracture where rocks on either side move relative to each other due to extensional forces. This movement is characterized by the block of rock above the fault plane, termed the “hanging wall,” sliding downward relative to the block below, known as the “footwall”. The terms “hanging wall” and “footwall” originated from mining, where miners would stand on the footwall and hang their lanterns from the hanging wall.
Normal faults form in response to tensional stress, which is a pulling-apart force that stretches and thins the Earth’s crust. This contrasts with compressional stress, which pushes rocks together and forms different fault types. The sliding surface of a normal fault often has a steep angle, typically between 45° and 90°, allowing for significant vertical displacement.
These faults are a type of dip-slip fault, meaning the primary movement of the rock blocks occurs vertically along the incline of the fault plane. This vertical displacement is a direct result of the crust lengthening or extending. Normal faults are common and frequently observed in regions where the Earth’s crust is actively being pulled apart.
Where Normal Faults Form
Normal faults predominantly occur in geological environments undergoing significant crustal extension, particularly at divergent plate boundaries where tectonic plates move away from each other. This pulling apart creates the tensional stress necessary for these faults to develop.
One prominent setting is rift zones, both on continents and in oceans. Continental rifts, such as the East African Rift Valley, are areas where a continent is actively splitting apart. Here, the stretching of the brittle upper crust leads to the formation of numerous normal faults, creating characteristic elongated valleys known as rift valleys, often flanked by uplifted blocks. Similarly, mid-ocean ridges, like the Mid-Atlantic Ridge, are underwater mountain ranges where new oceanic crust is generated as plates diverge. Normal faults are common along these ridges, accommodating the extension and contributing to the rugged seafloor topography.
Another extensive region characterized by normal faulting is Basin and Range Provinces, exemplified by the western United States. This vast area experiences broad crustal stretching, which has resulted in a distinctive topography of alternating parallel mountain ranges (horsts) and valleys (grabens). These features are bounded by numerous normal faults, with the valleys representing down-dropped blocks and the mountains being uplifted blocks.
Normal faults can also be found in more subtle extensional settings. Passive continental margins, which are transitions between continental and oceanic crust not currently active plate boundaries, can exhibit normal faulting. These margins form from ancient rifting events, and while generally stable, they can still experience normal faulting due to processes like sediment loading or residual extension from their formation. Finally, gravitational collapse can induce normal faulting in some large, elevated regions, such as certain mountain ranges. The immense weight of these elevated landmasses can cause the crust to spread outwards, leading to tensional stress and the formation of normal faults.
Impact of Normal Faults
Movement along normal faults can generate earthquakes, as accumulated stress is suddenly released during a slip event. These fault movements shape the Earth’s surface significantly, creating distinct topographic features. Rift valleys, formed by the downward movement of crustal blocks, are prominent examples. The horst and graben topography, seen in regions like the Basin and Range Province, with its alternating uplifted mountain blocks and subsided valleys, is a direct result of extensive normal faulting.
Normal faults also play a role in the accumulation of natural resources. They can create structural traps for oil and gas by offsetting rock layers and forming impermeable barriers. In some areas, these faults provide pathways for hot fluids, influencing the development of geothermal energy resources.