Fault (geology)

In geology, a fault is a planar fracture or break in layers of rock. The rocks on either side of the fault have moved past each other. This happens because of forces deep inside the Earth. The biggest faults are where Earth’s giant plates meet. When these faults shift suddenly, they can cause powerful earthquakes.

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.

A fault plane is the flat surface where the rock breaks. You can often see where a fault reaches the surface as a line called a fault trace or fault line. Geologists use these lines on maps to show where faults are located.

Sometimes many faults run close together in an area called a fault zone. This zone is filled with crushed rock. Over time, the rocks can get broken into smaller pieces as the faults keep moving.

Mechanisms of faulting

Slip, heave, throw

Slip is the way rocks move on either side of a fault. It tells us how the rocks move relative to each other. The throw of a fault is how far the rocks move up or down, and the heave is how far they move sideways. We can see this movement by looking at how rock layers bend near the fault, called drag folding. To measure the exact throw and heave, we look for points that line up on both sides of the fault, known as a piercing point. Usually, we can only tell the direction of slip and estimate the throw and heave.

Hanging wall and footwall

When rocks split apart, the two sides are called the hanging wall and the footwall. The hanging wall is the side above the split, and the footwall is the side below it. These names come from old mining times.

This helps us tell two main types of faults apart. In a reverse fault, the hanging wall moves up. In a normal fault, the hanging wall moves down. This helps us learn how the rocks moved.

Fault types

Schematic illustration of the two strike-slip fault types, as seen from above

Faults are grouped by the angle they make with the Earth's surface and how they move. They can be:

Vertical cross-sectional view, along a plane perpendicular to the fault plane, illustrating normal and reverse dip-slip faults

Strike-slip faults, where the ground moves sideways past the fault.
Dip-slip faults, where the ground moves up or down.
Oblique-slip faults, which have both side-to-side and up-and-down movement.

Normal fault diagram

Strike-slip faults have a near-vertical surface, and the ground moves left or right with little up-and-down motion. A special type is the transform fault, which forms the boundary between moving plates, such as the Alpine Fault in New Zealand.

Dip-slip faults can be normal (where the ground moves down) or reverse (where the ground moves up). Normal faults can form landscapes with valleys and ridges, called basin and range topography. Reverse faults, including thrust faults, show the ground being pushed up, often forming folds in the Earth's surface.

Oblique-slip faults have both side-to-side and up-and-down movement.

Other types include ring faults, which form circular patterns around collapsed volcanic areas.

Fault rock

Structure of a fault

Faults have a layer of changed rock. This rock looks different depending on where the fault is, what kind of rock is there, and if any special liquids are present. Fault rocks are grouped by how they look and how they formed.

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.

The main kinds of fault rock are:

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

Cataclasite – a type of fault rock that stays together but looks rough, or it may break apart easily. It has sharp pieces of rock in a finer mix.
- Tectonic or fault breccia – a coarser type of cataclasite with many visible pieces.
- Fault gouge – a fine, clay-like type of cataclasite with very small pieces and sometimes a flat structure. Rock pieces may be present.
  - Clay smear – a clay-rich type of fault gouge formed in layers with lots of clay.
Mylonite – a fault rock that stays together well and has a smooth structure from the grains being crushed down. It often has rounded pieces and rock bits similar to the surrounding minerals.
Pseudotachylyte – a very fine, glassy-looking material that appears black and sharp. It fills thin spaces in the rock and forms during earthquakes.

Impacts on structures and people

Faults can affect buildings, tunnels, and slopes. When building important places like schools or power plants, it’s important to know where faults are because they can change how the ground moves.

In places like California, rules say you can’t build new homes or tanks right on top of faults that have moved in the last 11,700 years. Scientists study the ground to learn how old a fault is and how often it moves, helping to keep people safe.

Faults and ore deposits

Many valuable minerals are found near faults. This is because the broken rock in fault zones can let melted rock from deep underground rise or allow fluids with minerals to flow through. Big mineral deposits are often found where faults cross each other.

One example is in northern Chile, where the Domeyko Fault has important copper mines like Chuquicamata, Collahuasi, El Abra, El Salvador, La Escondida, and Potrerillos. Further south in Chile, mines such as Los Bronces and El Teniente are where two fault systems meet.

Sometimes, deep faults can trap melted rock. When conditions are right, this rock can change and then burst upward to form mineral deposits closer to the Earth's surface. porphyry copper deposits igneous differentiation

Groundwater

Faults are places where rocks are weaker. This lets water flow through them more easily. Water can mix with the rocks and change them over time. Faults can hold and move groundwater, like natural underground rivers.