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Fumarole
Flood basalt
An eruption of basalt that occurs very occasionally, when a large fissure opens in the Earth’s crust. Vast volumes of extremely runny lava flood over the landscape, often extending to cover millions of square kilometers (see: Supervolcano). The largest of them makes up much of the Deccan region of India, but large flood basalts also occur in the Columbia-Snake River basin of the northwestern United States. (See also: Fissure eruption.)
Focal depth
The depth of an earthquake focus below the surface. (See also: Epicenter.)
Focus
The origin of an earthquake, directly below the epicenter. Another word for hypocenter. (See also: Focal depth and Shadow zone.)
Foreshocks
Relatively smaller earthquakes that occur before the main shock. Not all main shocks have foreshocks, and so foreshocks can only sometimes be used to give warning of a major event.
Fracture
A substantial break across a rock. Rock that is fractured is broken extensively by Earth movements; and although the rock itself may be hard, the fractures represent lines of weakness that may make it easier for the rock to be eroded. (See also: Fault and Fracture zone.)
Fracture zone
A region of rock in which fractures are common. Fracture zones are particularly common in folded rock and near faults. Fracture zones are places where
rocks have been broken and are more liable to erosion. They are often connected with bands of
low land. River valleys sometimes follow fracture zones. (See also: Oceanic spreading ridge.)
Fumarole
A small opening or vent from which hot gases puff their way to the surface. Fumaroles may occur in places where there are active volcanoes, or they may occur where there is no volcanic activity. However, even if there is no active volcano present, they represent gases reaching the surface from
a magma chamber far below. Fumaroles are often found in
areas where there are geysers
and mud pools. Examples, include Bumpass Hell in Lassen National Park, California, and Yellowstone National Park, Wyoming.
 Fumarole—Fumaroles are mainly steam with sulfur gases.
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