Igneous rock (from a latin word meaning fire) Igneous rock is formed in the mantle by magma or on the crust by lava (molten rock) being cooled and becoming solid. They may form with or without crystals either below the surface as Intrusive rocks or on the surface as extrusive or volcanic rocks. Intrusive rocks are formed inside the Earth. These rocks cook in the magma longer and cool much slower, as a result the crystals are bigger. Extrusive rocks form on the Earth's surface. They EXit the Earth, usually through a volcano. These rocks have smaller crystal grains because they cool faster. They don't cook as long. Sometimes these rocks cool so fast that they do not form crystals at all. Igneous rocks like Obsidian have a glassy look or Pumice have a rough texture and holes where crystals would have formed but gas took the place of the crystal.
By using the ESRT and following they graph downward and across, you can identify an Igneous rock.
Here is a podcast of an upstate teacher who can help you to review, as he explains rocks. The podcast explains all rocks, although we are just focusing on Igneous rocks right now.
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