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How is gabbro formed?

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Quick Answer

Gabbro, a type of intrusive igneous rock, is formed when magma cools and solidifies inside the crust of the Earth. Because it cools slowly, it is coarsely grained.

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Full Answer

Gabbro is the intrusive equivalent to basalt, an extrusive rock formed by molten magma that cools on the Earth's surface. However, unlike extrusive rocks, which cool rapidly when exposed to the atmosphere, intrusive rocks, surrounded by pre-existing rock, push up from within the Earth and cool in a process that can take millions of years. Large deposits of gabbro lie beneath the oceanic crust, where magma cools extremely slowly to form large-grained holocrystalline masses. Gabbro is found in many places on Earth. It is also present on the moon.

The German geologist Christian Leopold von Buch, who studied the volcanic origins of minerals in Italy, named gabbro after a town in Tuscany. Gabbro is dense and dark-colored. Unlike granite, it contains no quartz and little silica. It contains pyroxene, plagioclase, amphibole and olivine. It often contains large but low-grade amounts of gold, silver, platinum, chromium, nickel and cobalt. Its dark appearance precludes its widespread use in construction, but it is often used for paving stones, headstones for graves and kitchen countertops, in which case it is referred to erroneously as "black granite."

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