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brittle and is the focus of most shallow earthquakes.
The crust is broken up into a number of huge slabs, each of which contains both continents and ocean floors (see: Lithosphere). These pieces are called plates
or tectonic plates. The plates
are moved around by the slow flow of the upper mantle (see: Asthenosphere). This process is called plate tectonics.
Volcanoes and earthquakes occur where plates collide. Volcanoes also occur where plates pull apart, and earthquakes can occur where plates slide past one another.
The magma that feeds volcanoes originally begins at the top of the mantle. However, most continental volcanoes are fed from magma chambers that are near the top of the crust.
(See also: Feldspars and Mohorovicic discontinuity.)
Crystal
A mineral that has a regular geometric shape and is bounded by smooth, flat faces. (See also: Phenocryst.)
D
Destructive plate boundary
A zone where plates collide, and one plate is forced down below another. (See also: Benioff zone and Plate tectonics.)
Dike
A wall-like sheet of intrusive igneous rock (typically diorite) that cuts across other rocks. A dike is formed entirely underground.
There are several causes for dike formation. One form of dike occurs at the end of a fissure
eruption. When the eruption stops, the magma in the fissure will cool and turn to rock. This wall of rock makes a dike.
Dikes may also occur in areas surrounding the magma chamber that are not connected with the main vent. They occur when the rock enclosing the magma chamber fractures, allowing magma to flow into the surrounding rock.
Most dikes are between 2m and 6m thick.
(See also: Erosion and Sill.)
Dike swarm
A collection of hundreds or thousands of parallel dikes.
The largest number of dikes
in the world occur under the oceans and are the result of fissure eruptions. They make up most of the ocean crust.
Dikes also occur on land close to active volcanoes. Continental areas containing dike swarms include northwestern Scotland and Northern Ireland.
 Dike—A dike is a band of igneous rock that cuts across other rocks. This wall of rock is a dike.
Diorite
An igneous rock that forms underground. It has properties in between gabbro and granite; it has the same composition as the lava called andesite. Diorite is the typical rock of dikes and sills. Diorite is a medium- to coarse- grained igneous rock that contains about two-thirds feldspar and one-third dark-colored ferromagnesian minerals.
Dormant volcano
A volcano that shows no signs
of volcanic activity today but that has been active in the recent past. It is hard to give an exact status to any volcano because it may erupt only once in several hundred years. Geologists call a volcano dormant when it has not erupted in the
past 100 or more years, but the
ash and lava of its cone suggest (geologically) recent activity.
A dormant volcano may become active again with little warning.
Dike
Dormant volcano
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