GEOL 101 1nd Edition Lecture 6 Outline of Last Lecture I Classification of Minerals II Classification of Silicate Minerals III Classification of Non silicates IV Mineral Resources Outline of Current Lecture I Characteristics of Magma II Igneous Texures III Igneous Compositions Current Lecture Characteristics of Magma Igneous rocks forms as molten or melted rocks cool down and become a solid Molten rock below the surface is called magma and forms deep within the crust It is more buoyant than the solid crust so eventually it flows up Once it has risen up it is known as lava It cools and crystalizes mineral crystals begin to form If magma cools and crystalizes within the earth the rocks are termed intrusive or plutonic Ex Granite If lava cools and crystalizes above the surface the rocks are termed extrusive or volcanic rocks Ex Basalt Magma consists of 3 components Melt a liquid portion is composed of mobile ions Solids if any are silicate minerals that came from the melt Volatile gasses dissolved in melt like water vapor carbon dioxide and sulfur dioxide Crystallization of magma Meaning how the environment where the magma cools will affect the rate of the cooling The cooling rate will determine the texture These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Igneous rocks are usually classified by their texture and composition Two rocks may have the same texture Ex Granite and Gabbro are both coarse grained but they are composed of very different minerals Igneous Textures Texture Shows the overall appearance based on size shape and arrangement of minerals Factors that affect the texture Rate of cooling Amount of Silica SiO2 Amount of dissolved gasses practically dissolved H2O Rate of cooling The slower the cooling the longer minerals grow and will make large crystals The faster the cooling the less time minerals have to grow and have microscopic crystals REALLY fast cooling form glass no crystals Amount of silica SiO2 present More silica in the melt has higher viscosity gooier and flows like honey More silica will have a lower crystallization temperature Amount of dissolved gasses More dissolved gasses has higher viscosity More dissolved gasses form holes vesicles in the rock Phaneritic coarse grained textures Slow cooling in the crust Crust insolates the magma allowing for lots of time for crystals to grow Do not need microscope for crystals Aphanitic fine grained texture Rapid rate of cooling lava magma at or near surface Need microscope to see crystals MAY contain holes MAY contain glassy fragments Porphyritic texture Minerals form at different temperatures and rates Indicates more than one rate of cooling Large crystals Phenocrysts are embedded in a matrix of smaller crystals Groundmass Glassy textures Very rapid cooling Resulting rock is called Obsidean Vesicular texture Results from rapid cooling of molten rock that has a high volatile content Gas bubbles escape while rock is cooling and makes tiny holes Resulting rock is called Pumice Pyroclastic texture Fragments of rock and ash ejected during a violent volcanic eruption that welded together by hot gas and ash Textures often appear to more similar to sedimentary rocks Resulting rock is called Tuf and Breccia Pegmatitic texture Exceptionally coarse grained Form in late stages of crystallization of granite magmas Many gemstones are mined here Igneous Compositions Igneous rocks are composed primarily of silicate minerals So they are chemically classified by the amount of silica present Minerals with high silica Light silicates Felsic o Quartz o Muscovite Mica o K rich feldspar o Na rich feldspar Minerals with low silica Dark silicates Mafic o Olivine o Pyroxene o Amphibole o Biotite Mica o Ca rich feldspar Felsic Rocks Minerals are generally light colored o White o Light gray o Pink o Clear o Black Minerals included have high Silicon and Aluminum and low Iron and Magnesium o Quartz o K feldspar o Na plagioclase o Muscovite Mica o Amphibole o Biotite Mica Majority of the Continental crust Intermediate Rocks Minerals range from light to dark in color o White o Light gray o Dark gray o Black Minerals have moderate amounts of silicon aluminum iron and magnesium o Na Ca Plagioclase o Amphiboles o Pyroxenes Mafic Rocks Minerals are dark colored o Dark gray o Black o Green Minerals included have low Silicon and Aluminum and high Iron and Magnesium o Ca Plagioclase o Amphiboles o Pyroxenes o Olivine Majority of the Oceanic crust Ultramafic Rocks Minerals are green and black Minerals included have VERY low Silicon and Aluminum and VERY high Iron and Magnesium o Olivine o Pyroxene o Ca Plagioclase