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Basalt Rocks
By Contributor
Basalt,
a volcanic igneous rock, occurs worldwide, but especially in India, Scotland,
Greenland, Iceland, Canada and the northwestern United States.
Most
basalt occurs as lava flows, in narrow dykes or sills.
It's
the basalt from lava flows that make up the enormous sheets of a lava plateau.
This rock is easy to identify.
Note the rock's color. Basalt appears black or
grayish-black, sometimes with a greenish or reddish crust.
Feel its texture. Basalt consists of a fine and even-grain.
The dense rock has no crystals or minerals discernible to the naked eye.
When
freshly broken, basalt has a dull surface.
Determine
its structure with your naked eye or a microscope. Often vesicular or
amygdaloidal, basalt has columnar jointing.
Examine your rock's composition with a microscope. Basalt
occurs more often as pyroxene (shiny, black) and as plagioclase (tabular,
white-gray).
The
presence of olivine gives basalt a green, glassy appearance and is called
olivine basalt.
Also
present may be iron ore (ilmenite and/or magnetite) or bronze-colored biottie.
Tip
Varieties of basalt include olivine basalt and quartz
basalt, which contains a minuscule amount of quartz.
Basalt
is used as a source of iron ore, roadstone aggregate, sapphires or native
copper.
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