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Extinct volcano
Extinct volcano
A volcano that has shown no signs of activity in historic times (see: Volcanic activity). (Compare with: Dormant volcano.)
Extrusive rock
An igneous rock that has solidified on the surface of the Earth. Lava is the extrusive rock that flows from an eruption. (See also: Intrusive rock.)
F
Fault
A deep fracture or zone of fractures in rock where one plate has moved relative to another (see: Fracture zone). It represents a weak place in the crust.
There are several different types of fault. Transcurrent faults, also known as strike-slip faults and transverse faults, are vertical fractures where the slabs have mostly moved horizontally, like pushing two books next to one another on a table. Dip-slip faults are sloping fractures where the slabs have mostly moved vertically. If the uppermost block moves down, it produces a normal fault, while if the top slides up, it produces a reversed fault. Whenever the fault moves, it produces earthquakes.
(See also: Active fault; Blind fault; Creep; Fault scarp; Fault zone; Locked fault; Thrust fault.)
Reversed fault
 Fault scarp—The steep face, or scarp, is the result of a fault and has been little altered by erosion. This is part of the Transverse Ranges fault system, California.
Transcurrent fault
 Fault—The San Andreas Fault (a transcurrent fault) in California.
 Fault—A transcurrent or wrench fault (top) like the San Andreas Fault; normal fault (middle); and reversed fault (bottom).
Normal fault
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