Fault (geology) facts for kids

A large normal fault in Triassic to Lower Jurassic Blue Anchor Formation sediments near Blue Anchor, Somerset
Red rock = Triassic mudstone
Blue-grey rock = latest Triassic to earliest Jurassic
The line of the fault runs along the beach, shown by the rock / sand divide

Cross-sectional illustration of normal and reverse dip-slip faults

A fault in the Grands Causses as seen from Bédarieux, France. The left side moves down while the right side moves up.

Schematic illustration of the two strike-slip fault types.

The San Andreas Fault, a right-lateral strike-slip fault caused the massive 1906 San Francisco earthquake

A fault is a fracture, or break, in the Earth's crust (lithosphere). Some faults are active. Here, sections of rock move past each other. This sometimes makes earthquakes.

Faulting occurs when shear stress on a rock overcomes the forces which hold it together. The fracture itself is called a fault plane. When it is exposed at the Earth's surface, it may form a cliff or steep slope called a fault scarp.

The angle between the fault plane and an imaginary horizontal plane is called the dip angle of the fault. Faults may dip shallowly or steeply.

Faults are categorized into three types:

1. A normal fault is one where the fault dips toward the downthrown block. Normal faults occur in rifted terranes, such as Mid-Ocean Ridges, the African Rift, and the Basin and Range Province of western North America. It is shown on geologic maps as a black line with either a block pattern on the downthrown side, or the letters U/D showing the upthrown and downthrown sides.
2. A reverse fault (if steeply dipping) or thrust fault (if shallowly dipping) is a fault where the fault plane dips toward the upthrown block. It is shown on the geologic map with triangular teeth pointing toward the upthrown side of the fault. Reverse and thrust faults shorten (horizontally) and thicken the crust. They are characteristic of collision origins.
3. A transform (oceanic) or strike-slip (continental) fault is one where the relative motion is horizontal. The fault plane is usually vertical and can be horizontal. Famous examples of these include the San Andreas Fault of California, the Alpine Fault of New Zealand's south island, and the Anatolian Fault in Turkey.

Images for kids

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  Satellite image of a fault in the Taklamakan Desert. The two colorful ridges (at bottom left and top right) used to form a single continuous line, but have been split apart by movement along the fault.

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  Normal fault in La Herradura Formation, Morro Solar, Peru. The light layer of rock shows the displacement. A second normal fault is at the right.

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  A fault in Morocco. The fault plane is the steeply leftward-dipping line in the centre of the photo, which is the plane along which the rock layers to the left have slipped downwards, relative to the layers to the right of the fault.

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  Normal fault and drag folds (eastern flanks of the Bighorn Mountains, Wyoming, USA)

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  Microfault showing a piercing point (the coin's diameter is 18 mm (0.71 in))

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  Normal faults in Spain, between which rock layers have slipped downwards (at photo's centre)

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  Listric fault (red line)

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  Salmon-colored fault gouge and associated fault separates two different rock types on the left (dark gray) and right (light gray). From the Gobi of Mongolia.

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  Inactive fault from Sudbury to Sault Ste. Marie, Northern Ontario, Canada

See also

In Spanish: Falla para niños