Igneous • Igneous (means fire) • Rock formed from molten magma that cools, then hardens. • Molten rock under the surface is called magma • Molten rock on or very near the surface is called lava. • Rock formed at volcanoes and at rifts/ridges.
Igneous Rock Classification • Igneous rocks are classified two different ways: • What they are made from (mineral composition) • Where they were formed- determines texture (crystal size)
Igneous Rocks What determines the type of minerals? • The type of minerals that develop depend on the type of magma. • Basaltic (Mafic=magnesium and iron) • Granitic (Felsic=feldspar and silica) • Andesitic 25-45 % dark silicate minerals
Granitic (Felsic) Magma • High viscosity (like oatmeal) • high % of silica (SiO2) • light colored minerals crystallize, producing felsic (light) colored rocks. • These minerals have a relatively low density. • Most of the rocks that form the continents are made from this type of magma.
Granitic (Felsic ) Rocks Rhyolite Pumice Granite http://www.odplegacy.org/operations/labs/thin_section/example1.html
Basaltic (Mafic) Magma • Low viscosity (fast moving-like hot syrup), high iron and magnesium , low silica content. • When it cools, dark colored minerals crystallize, producing basaltic (mafic) dark colored rocks. • These minerals have a relatively high density. • Most of the rocks that form the ocean floor are made of this type of magma.
Basaltic (Mafic) Rocks Basalt scoria http://www.odplegacy.org/operations/labs/thin_section/example1.html
Andesitic Rocks • Andesitic igneous rocks have mineral compositions between those of basaltic and granitic rocks. • Rocks made from andesite tend to be fine-grained.
Intrusive= (interior) Cooled slowly deep within the earth. Intrusive rocks have large crystals. Granite and gabbro are intrusive rocks. Extrusive =(exterior) Cooled quickly on/near surface of earth. Extrusive rocks have small crystals or no crystals. Basalt, rhyolite, scoria, obsidian, and pumice are all extrusive rocks. What determines the size of crystals in igneous rocks? Molten rock can cool in two ways, deep within the Earth or on/near the surface.
Intrusive Intrusive igneous rocks have larger crystals and a coarse grained texture. Extrusive Extrusive igneous rocks have either: small crystals and a fine-grained texture or if they cooled too quickly for crystals to form they have a glassy texture. Rocks with a glassy texture will either look truly glassy (obsidian), or full of air hole (such as pumice or scoria). Pumice is actually so full of air hole it will float in water Texture The size of crystal is referred to as the texture of the rock. Obsidian pumice
How did this lava cool? Intrusive extrusive
Fine-Grained Texture(individual crystals are hard to see)EXTRUSIVE
I N T R U S I V E Coarse-Grained Texture(mineral grains are large & distinct)
Granitecoarse-grained texture Intrusive or extrusive? Intrusive
Basalt fine-grained texture Intrusive or extrusive? Extrusive
Igneous Rocks Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form. Is it glassy or coarse-grained? Is this rock Intrusive or Extrusive? glassy, extrusive
Scoria Intrusive or extrusive? Extrusive
Batholith • The largest type of intrusion • Magma cools before reaching the surface. • Sometimes they are exposed due to erosion.
Laccolith • They are flat at the bottom and domed at the top.
Dike • Magma is squeezed into vertical cracks that cut across the rock layers.
Sill • Magma is squeezed into horizontal cracks that cut across the rock layers.