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Chapter 6 Rocks 6.2 Igneous Rocks Igneous Rock Formation • Form from the cooling of molten rock – Extrusive (volcanic)form at or near the surface from lava – Intrusive (plutonic) – form under the surface from magma Igneous Rock Formation • Igneous rocks are classified by mineral composition (mafic or felsic) and texture • Texture – is the size, shape and arrangement of grains (crystals) Texture • Grain or crystal size depends on: – Rate of cooling • Fast = small • Slow = large – Dissolved gasses • More = larger – Order of solidification The Starting Material • Magma is Classified as Felsic, Mafic or and intermediate • Felsic – Light colored – High silica magma with abundance of quartz and feldspar. – Thick slow flowing in molten state. – Most intrusive rocks. – Acidic The Starting Material • Mafic – Dark colored, – Low silica ferromagnesian magma – More fluid than felsic magma. – Most extrusive igneous rocks. Underground Magma • Magma can cool slow or fast depending on where cooling occurs – Intrusive rocks cool very slowly underground – Result of slow cooling • Coarse (large) grains INTRUSIVE – large grains GRANITE GABBRO At the Surface • Magma pouring onto the surface is called lava – Extrusive igneous rocks cool rapidly – Microscopic grains or none at all EXTRUSIVE – fine grains BASALT OBSIDIAN At the Surface • Porphyry – Rocks that cool both underground and at the surface – Results in large and small grains PORPHYRY – large grains embedded in fine grains RHYOLITE PORPHYRY Igneous families • Grouped according to mineral composition • Texture is determined where it cools – intrusive or extrusive – Each family can have coarse grained, fine grained or glassy textures – Also can have vesicles • Vesicles are gas cavities within the rock Igneous Families • Granite family – Felsic magma – Light colored – Quartz and Feldspar • Coarse – Granite • Fine – Rhyolite • Glassy – Pumice and Obsidian • Gabbro Family – Mafic magma – Dark colored – Pyroxene and Olivine • Coarse – Gabbro • Fine – Basalt • Vesicular – Scoria Igneous Families • Diorite family – Intermediate • Coarse – Diorite • Fine – Andesite IGNEOUS INTRUSIONS • PLUTONS – A ROCK MASS THAT FORMS WHEN MAGMA COOLS UNDER GROUND – DIKES – SILLS – LACOLITHS – VOLCANIC NECKS – BATHOLITHS – LARGEST OF ALL PLUTONS, FORM THE CORES OF MOST MOUNTAINS Volcanic neck Laccolith Stock Dike Sill Batholith DIKE SILL LACOLITH VOLCANIC NECK BATHOLITH
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