Igneous rocks are one of three main varieties of rocks (along through sedimentary and also metamorphic), and 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 one igneous rock.
Photograph by Koumlhn/ullstein bild courtesy that Getty Images
Igneous rocks form when magma (molten rock) cools and crystallizes, one of two people at volcanoes top top the surface of the planet or while 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 countless different compositions, relying on the magma lock cool from. Lock can also look different based on their cooling conditions. Because that example, 2 rocks from similar magma can become either rhyolite or granite, depending on whether castle cool quickly or slowly.
The two key categories the igneous rocks room extrusive and intrusive. Extrusive rocks are developed on the surface ar of the earth from lava, i m sorry is magma the has emerged from underground. Intrusive rocks are formed from magma that cools and also solidifies in ~ the late of the planet.
When lava comes out of a volcano and also solidifies right into extrusive igneous rock, likewise called volcanic, the rock cools an extremely quickly. Crystals inside solid volcano rocks are tiny because they execute not have much time to type until the absent cools every the way, which stops the decision growth. These fine-grained rocks are well-known as aphanitic—from a Greek word definition “invisible.” castle are given this name because the crystals that type within them room so small that they have the right to be seen just with a microscope. If lava cools nearly instantly, the rocks that kind are glassy through no individual crystals, like obsidian. Over there are numerous other type of extrusive igneous rocks. For example, Pele’s hair is long, extremely thin strands of volcano glass, if pahoehoe is smooth lava that forms shiny, rounded piles.
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Intrusive rocks, also called plutonic rocks, cool gradually without ever reaching the surface. Castle have big crystals that space usually visible there is no a microscope. This surface is well-known as a phaneritic texture. Probably the best-known phaneritic rock is granite. One extreme type of phaneritic rock is called pegmatite, found often in the U.S. State that Maine. Pegmatite can have a huge selection of crystal shapes and sizes, including some bigger than a human being hand.