Igneous Rocks

Igneous rocks are one of three main types of rocks (along with sedimentary and metamorphic), and also they include both intrusive and extrusive rocks.

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Devils Tower Rock

Devil"s Tower national Monument in Wyoming is an instance of one igneous rock.

Photograph by Koumlhn/ullstein bild courtesy of Getty Images


Igneous rocks form when magma (molten rock) cools and crystallizes, either at volcanoes top top the surface ar of the planet or when the melted rock is still within the crust. Every magma establishes underground, in the reduced crust or upper mantle, since of the intense warmth there.

Igneous rocks have the right to have numerous different compositions, relying on the magma castle cool from. They can likewise look different based upon their cooling conditions. Because that example, 2 rocks from identical magma can come to be either rhyolite or granite, relying on whether castle cool conveniently or slowly.

The two main categories the igneous rocks are extrusive and also intrusive. Extrusive rocks are created on the surface of the planet from lava, i beg your pardon is magma the has arised from underground. Intrusive rocks are created from magma the cools and solidifies in ~ the crust of the planet.

When lava comes out of a volcano and solidifies into extrusive igneous rock, likewise called volcanic, the absent cools very quickly. Crystals within solid volcanic rocks are little because they do not have much time to type until the absent cools every the way, which stop the crystal growth. This fine-grained rocks are well-known as aphanitic—from a Greek word definition “invisible.” castle are given this name due to the fact that the crystals that type within them are so little that they can be seen only with a microscope. If lava cools nearly instantly, the rocks that kind are glassy with no separation, personal, instance crystals, prefer obsidian. Over there are many other type of extrusive igneous rocks. Because that example, Pele’s hair is long, incredibly thin strands of volcanic glass, when pahoehoe is smooth lava that develops shiny, rounded piles.

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Intrusive rocks, also called plutonic rocks, cool progressively without ever before reaching the surface. Castle have big crystals that room usually visible without a microscope. This surface is recognized as a phaneritic texture. Possibly the best-known phaneritic absent is granite. One extreme kind of phaneritic rock is called pegmatite, uncovered often in the U.S. State the Maine. Pegmatite can have a huge range of crystal shapes and also sizes, including some bigger than a person hand.