Glossary of Terms


Select the first letter of the word from the list above to jump to appropriate section of the glossary. If the term you are looking for starts with a digit or symbol, choose the '#' link.

- A -

alluvial fan
A fan-shaped mass of stream-deposited sediments (grain-sized to boulder-sized) that is deposited where a stream flows from a narrow canyon onto a plain or valley floor

A fold that has its limbs pointing down and hinge-line pointing up.

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- B -


The broad slope formed by coalescing alluvial fans, often continuous with the pediment.


Refers to materials that fracture when subjected to stress that exceeds their yield strength.

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- C -


The circulation of a fluid or gas due to contrasts in its density.


Slow, nearly continuous downslope movement that is induced by either freeze/thaw cycles or wet/dry cycles

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- D -

Debris flows

Dense, fluid mixtures of rock, sand, mud, and water that can move rapidly (10s of miles per hour). They have the consistency of concrete and can be highly destructive


Refers to materials that respond to stress by deforming plastically, i.e. the material changes shape permanently without fracturing.

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- E -


Energy released during a sudden brittle rupture of the earth's crust or mantle

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- F -

Foot wall

Rocks that are located below the surface of a fault make up the foot wall of a fault.

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- G -

Geothermal gradient

The change in temperature with depth beneath the earth's surface

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- H -

Hanging wall

Rocks located above a fault surface rest on the foot wall of the fault.


The art of measuring the height of points on the earth's surface
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- I -


An isolated peak jutting out from its own pediment and talus
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- L -


sudden downslope movements of rock or debris


magma that reaches the earth's surface


describes the sense of slip along a fault. Here, crust on one side of the fault moves to the left with respect to an observer who is located on crust on the other side of the fault. Another word for this type of slip is sinistral. See also right-lateral.


the layer of the earth that responds to stress through an elastic/brittle response

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- M -


molten rock, usually rich in silica and containing dissolved gases

Mass wasting

Downhill movement of soil and rock under the direct influence of gravity


Any natural feature that rises significantly above the surrounding landscape

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- N -

Normal fault

A fault that accommodates downward motion of the hanging wall with respect to the foot wall.

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- O -


The process of mountain building. Distinct phases of mountain building are frequently assigned names such as the Taconic orogeny.

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- P -


A gently inclined erosional surface carved into bedrock at the base of a mountain range


An internally rigid part of the earth's lithosphere surrounded by active plate boundaries

Plate boundaries

Fault zones that accommodate the motion between plates. Plate boundaries are most often narrow (a few kilometers to a few tens of kilometers across), but are sometimes as wide as 1000-2000 kilometers in continental lithosphere.


A desert basin that serves as a temporary lake bottom for precipitation that falls on the surrounding watersheds. Playas have no outlet to the sea, meaning that any water that flows into them can leave only through evaporation or percolation into the soil and rocks beneath.


Fragment of rock ejected during a volcanic eruption

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- R -

Reverse fault

A fault that accommodates upward motion of the hanging wall with respect to the foot wall.


Describes the sense of slip across a fault. Right-lateral slip (also called "dextral slip") refers to a fault along which crust on one side of the fault moves to the right with respect to an observer on crust on the other side of the fault. See also left-lateral.

Recumbent fold

A fold in which the limbs are largely horizontal

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- S -


A volcano that has subsided beneath the ocean surface and is no longer active. The vast majority of seamounts appear to have formed above mantle hotspots, which "burn" holes in a plate that drifts over them much like a candle burns a series of holes through a slab of wax that is dragged slowly over its flame.


See talus for definition


Stress is defined as force per unit area. For example, if a force like gravity is pulling downward on a cube, the stress that it exerts on the cube is equal to the total gravitional force acting on the object divided by the surface area of the bottom of the cube.


Motion along a fault that parallels the strike of the fault. This implies that no vertical component to the fault slip.

Subduction zone

Plate boundary where a plate (or fragment thereof) plunges into the mantle. Subduction zones are convergent plate boundaries.


A fold that has its limbs pointing up and hinge-line pointing down.

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- T -


Loose angular rock fragments that spill down a steep slope



See definition for "strike-slip"
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- V -


resistance to flow. A fluid with high viscosity is a fluid that does not flow easily. For example, molasses is highly viscous. Water has low viscosity - it flows readily as long as the temperature exceeds freezing.

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- W -

Wadati-Benioff zone

The inclined zone of earthquakes that define a slab that is subducting into the mantle. Named in honor of its co-discoverers.


A destructive process by which rocks and minerals are broken down by exposure to atmospheric agents such as water and wind.

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