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- Igneous Rocks form from magma found deep under Earth’s surface. - Igneous means “fire formed” Igneous means fire formed! Video Intro to Igneous Rocks Magma reaching the surface flows from a volcano as lava. Magma trapped below the surface forms large-grained intrusive igneous rock when it cools. (It takes longer for magma to cool beneath Earth’s surface, thus the mineral grains are large). Magma cooling at or near Earth’s surface forms small-grained extrusive igneous rock. (This magma cools quicker and thus the mineral grains are smaller) Pumice, obsidian, and scoria are examples of volcanic glass. These rocks cooled so quickly that few or no mineral grains formed. Sometimes gases are trapped in the molten material as it cools. The gases eventually escape and leave holes in the rock. (Pumis) -Igneous rocks are classified in two ways. 1. Where they formed – intrusive (under the Earth’s surface) – Extrusive ( on the surface) – Crystal size, large or small, can help identify an igneous rock as intrusive or extrusive. Magma type: basaltic, granitic, or andesitic -#2 Three main types of igneous rock Basaltic igneous rocks are dark-colored and dense. Contain iron and magnesium but very little silica Basaltic lava flows freely from a volcano. How does this explain the black sand beaches in Hawaii? Granitic igneous rocks are lower density and lighter color – Contain more silica and less iron and magnesium – Granitic magma is thick and stiff and contains lots of silica but lesser amounts of iron and magnesium. Andesitic rocks have a mineral composition between those of basaltic and granitic rocks Basaltic, Intrusive: Gabbro Andesitic, Intrusive: Diorite Granitic, Intrusive: Granite Basaltic, Extrusive: Basalt, Andesitic, Extrusive: Andesite Granitic, Extrusive: Rhyolite, Pumice, Obsidian Igneous rocks can be identified by: 1. texture= mineral crystal size (coarse, fine, glassy) 2. mineral content = color dark = lots of Mg and Fe light or pale = lots of quartz and feldspar (silicates) Igneous rock uses: all hard: can take lots of abuse obsidian: arrowheads, knife blades and scalpels, jewelry pumice: in toothpaste, ‘lava’ soap granite: buildings (Washington Monumument), tombstones