Igneous rocks are among three main types of rocks (along v sedimentary and also metamorphic), and they incorporate both intrusive and extrusive rocks.
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Devils Tower Rock
Devil"s Tower national Monument in Wyoming is an example of an igneous rock.
Photograph by Koumlhn/ullstein bild courtesy the Getty Images
Igneous rocks form when magma (molten rock) cools and also crystallizes, either at volcanoes ~ above the surface ar of the planet or if the melted rock is still inside the crust. All magma develops underground, in the lower crust or top mantle, since of the intense warmth there.
Igneous rocks have the right to have countless different compositions, relying on the magma castle cool from. They can likewise look different based upon their cooling conditions. For example, 2 rocks from similar magma can end up being either rhyolite or granite, depending upon whether castle cool quickly or slowly.
The two key categories the igneous rocks are extrusive and intrusive. Extrusive rocks are developed on the surface ar of the planet from lava, i m sorry is magma the has emerged from underground. Intrusive rocks are developed from magma the 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 rock cools really quickly. Crystals within solid volcano rocks are little because they do not have actually much time to form until the absent cools every the way, which stops the crystal growth. These fine-grained rocks are recognized as aphanitic—from a Greek word definition “invisible.” castle are provided this name due to the fact that the crystals that kind within them are so little that they can be seen just with a microscope. If lava cools practically instantly, the rocks that type are glassy through no separation, personal, instance crystals, like obsidian. Over there are many other kinds of extrusive igneous rocks. For example, Pele’s hair is long, incredibly thin strands of volcano glass, when pahoehoe is smooth lava that forms shiny, rounded piles.
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Intrusive rocks, additionally called plutonic rocks, cool slowly without ever reaching the surface. They have big crystals that room usually visible there is no a microscope. This surface is recognized as a phaneritic texture. Possibly the best-known phaneritic rock is granite. One extreme form of phaneritic rock is referred to as pegmatite, discovered often in the U.S. State that Maine. Pegmatite have the right to have a huge range of crystal shapes and sizes, including some bigger than a human being hand.