Igneous Rocks

Igneous rocks are among three main species of rocks (along through sedimentary and metamorphic), and also they encompass both intrusive and also extrusive rocks.

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Devils Tower Rock

Devil"s Tower nationwide Monument in Wyoming is an example of an igneous rock.

Photograph by Koumlhn/ullstein bild courtesy the Getty Images


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Igneous rocks form when magma (molten rock) cools and crystallizes, one of two people at volcanoes top top the surface ar of the earth or while the melted rock is still inside the crust. All magma creates underground, in the reduced crust or upper mantle, because of the intense warm there.

Igneous rocks deserve to have countless different compositions, depending on the magma they cool from. They can also look different based on their cooling conditions. For example, 2 rocks from identical magma can come to be either rhyolite or granite, depending upon whether lock cool quickly or slowly.

The two main categories of igneous rocks space extrusive and intrusive. Extrusive rocks are formed on the surface of the planet from lava, i m sorry is magma that has emerged from underground. Intrusive rocks are created from magma that cools and also solidifies in ~ the tardy of the planet.

When lava comes out of a volcano and solidifies into extrusive igneous rock, additionally called volcanic, the absent cools really quickly. Crystals inside solid volcano rocks are tiny because they execute not have actually much time to type until the absent cools all the way, which stops the decision growth. This fine-grained rocks are well-known as aphanitic—from a Greek word an interpretation “invisible.” castle are offered this name due to the fact that the crystals that kind within them space so tiny that they can be seen just with a microscope. If lava cools almost instantly, the rocks that kind are glassy through no separation, personal, instance crystals, favor obsidian. Over there are numerous other type of extrusive igneous rocks. Because that example, Pele’s hair is long, very thin strands of volcanic glass, when pahoehoe is smooth lava that develops shiny, rounded piles.

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Intrusive rocks, likewise called plutonic rocks, cool gradually without ever reaching the surface. Lock have large crystals that space usually visible there is no a microscope. This surface ar is known as a phaneritic texture. Maybe the best-known phaneritic rock is granite. One extreme form of phaneritic absent is referred to as pegmatite, uncovered often in the U.S. State of Maine. Pegmatite deserve to have a huge variety of crystal shapes and sizes, consisting of some bigger than a human hand.