Igneous rocks form when (molten rock) cools and crystallizes, either at volcanoes on the surface of the Earth or while the melted rock is still inside the crust. All magma develops underground, in the lower crust or upper, because of the intense heat there.Igneous rocks can have many different compositions, depending on the magma they cool from. They can also look different based on their cooling conditions. For example, two rocks from identical magma can become either rhyolite or, depending on whether they cool quickly or slowly.The two main categories of igneous rocks are extrusive and intrusive. Extrusive rocks are formed on the surface of the Earth from, which is magma that has emerged from underground. Intrusive rocks are formed from magma that cools and solidifies within the crust of the planet.When lava comes out of a volcano and solidifies into extrusive igneous rock, also called volcanic, the rock cools very quickly.

Crystals inside solid volcanic rocks are small because they do not have much time to form until the rock cools all the way, which stops the crystal growth. These fine-grained rocks are known as —from a Greek word meaning “invisible.” They are given this name because the crystals that form within them are so small that they can be seen only with a microscope. If lava cools almost instantly, the rocks that form are glassy with no individual crystals, like obsidian. There are many other kinds of extrusive igneous rocks. For example, Pele’s hair is long, extremely thin strands of volcanic glass, while is smooth lava that forms shiny, rounded piles.Intrusive rocks, also called rocks, cool slowly without ever reaching the surface. They have large crystals that are usually visible without a microscope. This surface is known as a texture.

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Perhaps the best-known phaneritic rock is granite. One extreme type of phaneritic rock is called, found often in the U.S. State of Maine. Pegmatite can have a huge variety of crystal shapes and sizes, including some larger than a human hand. Media CreditsThe audio, illustrations, photos, and videos are credited beneath the media asset, except for promotional images, which generally link to another page that contains the media credit. The Rights Holder for media is the person or group credited.DirectorTyson Brown, National Geographic SocietyAuthorNational Geographic SocietyProduction ManagersGina Borgia, National Geographic SocietyJeanna Sullivan, National Geographic SocietyProgram SpecialistsSarah Appleton, National Geographic SocietyMargot Willis, National Geographic SocietyProducerClint ParksLast UpdatedOct. For information on user permissions, please read our.

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Igneous rocks can be divided into four categories based on their chemical composition: felsic, intermediate, mafic, and ultramafic. The diagram of Bowen's reaction.