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 Basalt—Basalt is the most common volcanic rock. Most basalt is erupted on the ocean floors. However, much basalt is also found in places where this runny lava once flooded over the land. Here it forms thick layers
of rock that, when exposed, show distinctive columns. Hells Canyon, Idaho, is one place
that has these columns.
Basalt
Ash flow
A dense, hot avalanche of rock fragments, gas, and ash that travel rapidly down the sides of a volcano (it is the same as pyroclastic flow).
Asthenosphere
The weak part of the upper mantle below the lithosphere. It is the place in which it is thought that molten materials move slowly. It is the source of much of the world’s magma and the “engine” for the movement of the world’s tectonic plates.
Augite
A dark-green silicate mineral containing calcium, sodium, iron, aluminum, and magnesium. Commonly found in dark-colored igneous rocks such as gabbro. (See also: Ferromagnesian minerals.)
B
Basalt
A black, basic, fine-grained, igneous volcanic rock. Basaltic lava often contains vesicles. (See also: Aa lava and Pahoehoe lava.)
Basalt is the most common rock on the Earth’s surface, covering
all of the world’s ocean floors. It
is produced at the boundaries of
the world’s great tectonic plates and pours out onto the seabed as the plates pull apart. Hawaii and Iceland are volcanic islands that are made entirely from basalt. Basalt cools to form very distinctive hexagonal columns. Although basalt columns are found mainly close to the fissures of ocean floors, they are also found on
land. Iceland and Hawaii are two places that frequently experience eruptions of basalt.
Historically, basalt has been ejected from the mantle to form enormous floods of lava called flood basalts, which have
consolidated into vast black sheets (see: Supervolcano). Flood basalts are also called basalt plateaus or traps. The name trap comes from the world’s largest region of flood basalts, the Deccan Traps, India. Large areas of flood basalts also occur in the Columbia-Snake River region of the northwestern United States and in the Paraná Basin of South America.
Because basalt is a runny form of lava, the eruptions that it produces are not explosive or violent; and since they mainly occur on the ocean bed, many of them go unnoticed.
Basalts are black because they are made of minerals that contain iron known as ferromagnesian minerals. The crystals in basalt are usually too small to be seen without a magnifying glass or
a hand lens. As a result basalt appears to be a uniformly black rock. Occasionally, basalt contains larger crystals that can be seen
as small, shiny minerals. (See
also: Hornblende.)
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