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Igneous Rocks Mt. Rushmore Igneous Rocks Mt. Rushmore Half Dome, Yosemite, CA Ansel Adams IGNEOUS ROCKS Some igneous rocks form from cooled lava that has exited Earth’s crust Hawaii Some Igneous rocks cool from magma inside Earth and later become exposed through pressure, uplift, and erosion!!! Half Dome, Yosemite, California Igneous . Think of “ignite” .To light a fire . Igneous rock comes from the heat within the Earth. Earth: Geothermal How are Igneous Rocks Classified? Igneous rocks are classified by: .1. Mineral Composition .2. Crystal Size (Texture) Igneous Rock Formation Most Igneous rocks form directly from Magma which is hot, molten rock . Granite is the most common igneous rock, it cooled from magma Igneous rocks can also form from lava 3. First Classification of Magma: Felsic .Is rich in Silica (SiO2) .Small amounts of Ca, Fe, Mg (Calcium, Iron, & Magnesium) .Slow-moving, thick= HIGH VISCOSITY (resistance to flowing) 3. First Classification of Magma: .Crystallizes to light-colored minerals .Resulting rocks have relatively low density .Felsic rocks make up the continents Examples of FELSIC igneous rocks Rhyolite Pumice 4. Second Classification of Magma: Mafic .Low amounts of Silica (SiO2) .Fe and Mg rich (Iron and Magnesium) .Fast-moving, thinner= LOW Viscosity: less resistance to flowing, more fluid 4. Second Classification of Magma: .Crystallize to dark-colored minerals .Resulting rocks have relatively high density .Mafic rocks make up the ocean floors Mafic Igneous Rock examples: Basalt Gabbro (ocean floor) 5. Crystal Size . It takes time to grow crystals . As magma cools, it crystallizes . If magma cools slowly, what size crystals will form? Large Crystals 5. Crystal Size If Magma cools quickly, small crystals will form because they have little time to grow. Igneous Rock Obsidian cools so fast that no crystals have time to form A.K.A. Volcanic glass Obsidian is dark in color due to iron oxides. 6. Igneous Rocks Intrusive Granite . Igneous rock that crystallizes below Earth’s Large Crystal size surface, from magma . Intrusive= Inside Earth Notice how Granite has large crystals, it cooled slowly from magma deep beneath Earth’s surface Igneous Rocks Extrusive Basalt . Igneous rock that crystallizes or forms at Earth’s surface, from lava Small Crystal size . Extrusive= exited Earth Notice how basalt has small crystals, it cooled quickly at Earth’s surface Lava is magma that has reached the Earth’s surface. Igneous Rocks EXT-rusive . EXITED Earth IN-trusive . Formed IN-side Earth Crystal Size Intrusive Rock (Large Crystal Size) . Because it is inside the Earth, it cools very slowly, giving crystals time to grow Extrusive Rock (Small crystal size) . Cools very quickly so crystals do not have time to form Composition (what it’s made of) of Granite Granite Rock contains the following minerals: .Quartz .Feldspar .Mica Igneous Rock Felsic Mafic (Light Color) (Dark Color) Continents Ocean Floor Intrusive Large crystals Large crystals Cools slowly (SiO2) rich Fe, Mg rich From Magma Extrusive Small or no Small or no crystals crystals Cools quickly (SiO ) rich Fe, Mg rich From Lava 2 IGNEOUS . INTRUSIVE . “in” . Under the surface. Magma cools inside the Earth . Cools and crystalizes slowly . Large crystals IGNEOUS . EXTRUSIVE . “exit” . Magma exits the Earth before cooling . Cools and crystalizes quickly . Small or no crystals.
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