Transcript Document
The Rock CycleMinerals form rocks All rocks can be transformed into other rock types Rocks are divided into 3 categories Igneous- crystalline- forms as liquid cools Metamorphic- crystalline-forms as rocks are heated and squeezed Sedimentary- non-crystalline- smaller pieces or chemicals from other rocks Erosion QuickTime™ and a decompressor are needed to see this picture. Eruption or near surface cooling & crystallization (Heat & Pressure) Igneous formed from Magma and Lava Magma • molten rock below Earth's surface. L ava • magma on the Earth's surface. Pyroclastic material • (pyro = fire, clastic = debris) • Airborne lava — cools as it falls Composition of the magma • Analogous to what makes up the “stew" • What chemical elements are present • What material has the magma moved through Temperature of the melt • Not only how hot, but how long it stays that hot • also relates to pressure of the molten rock Cooling environment • fast vs slow • Internal vs External Water content Bowen's Reaction Series- Important ! Plagioclase (Ca-feldspar) Olivine Pyroxene Amphibole Biotite Plagioclase (Na-feldspar) Orthoclase (K-feldspar) Muscovite Quartz What things might you describe when looking at an igneous rock? Composition of Igneous rocks Felsic or Sialic magma • Si-rich (> 65%) • rich in K, and Al • little Ca, Fe, and Mg. Intermediate magma • between the two extremes in Si content and other atoms. Mafic magma • Si - poor (< 35%) • richer in Ca, Fe, and Mg. Crystal Settling QuickTime™ and a Photo - JPEG decompressor are needed to see this picture. Magma Mixing QuickTime™ and a Photo - JPEG decompressor are needed to see this picture. QuickTime™ and a Photo - JPEG decompressor are needed to see this picture. magma Assimilation Ways of Changing Magma Composition Viscosity of Magma/ Lava Viscosity- important for volcanic activity • the resistance of a liquid to flow — high viscosity = thick and stiff — low viscosity = thin and "runny". Related to: • amount of water (H2O) in magma • amount of silica (Si) in magma • Mafic — thin, low viscosity • Felsic — thick, high viscosity Igneous Rock TexturesPhaneritic Texture how big are the minerals? • Visible Mineral Grains distinguishable as different colored interlocking shapes Aphanitic Texture • Mineral grains too small to be seen — microscopic Porphyritic Texture • Two distinct sizes of mineral grains • Large and Small — Large = Phenocrysts — Small = Groundmass or Matrix • Groundmass — Grains may be either visible or not Porphyritic texture Large white crystals are phenocryts QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Aphanitic basalt (mafic composition) constitutes the groundmass Cooling Histories Minerals need time and space to grow • More time = Bigger crystals — visible mineral grains P & T control cooling rates of magma • Temp — Earth is a good insulator » holds in heat » keeps out cool — Atmosphere is a relatively bad insulator » transfers heat easily • Earth has pressure — Weight of overlying rocks — Magma trying to push up (density) — water vapor (steam), wants to expand Categories of Igneous EOF Granite Intrusive rocks • cool beneath Earth's surface • cool very slowly • higher P & T — Phaneritic textures Extrusive rocks Rhyolite • cool on the Earth's Surface • cool relatively fast • lower T & P — Aphanitic textures Basalt — Pyroclastic textures porphyry Complex • Partially cools below and above — Porphyritic textures Igneous rock names • determined by texture — size and arrangement of mineral grains • AND by mineral composition — minerals affect rock color and indicate temperature of creation QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. QuickTime™ and a TIFF (Uncompressed) decompressor are needed to see this picture. Pegmatite A Pegmatite is a very coarse-grained igneous rock. Crystals are >2 cm, often larger. Most are granitic, although mafic pegmatites can form. Biotite mineral grain Feldspar mineral grain Quartz mineral grain Granite & Rhyolite Phaneritic Texture Felsic magma Granite What are the textures in these two rocks ? Aphanitic Texture Felsic magma Rhyolite Andesite & Diorite Aphanitic texture forms from intermediate composition magma Phaneritic texture -forms from intermediate composition magma What are the textures in these two rocks ? Basalt & Gabbro aphanitic texture forms from mafic magma phaneritic texture forms from mafic magma Basalt Gabbro » Peridotite- the abundance (more than 40%) of Olivine crystals makes the rock take on a green appearance Obsidian- conchoidal fracture And association with volcanic rocks Volcanic Glass Tuffs &Tuffs Breccias - look like sedimentary rock & Breccias But they are not….it is volcanic ash that is lithified Tuffs - may be welded or not Always consists of fragments smaller than 2mm in diameter Welded means they were still hot enough that they partially recrystallized they were emplaced and consolidated. If they are not welded, it means they were not hot enough to partially recrystallize when they settled on the landscape Breccias - have a tuffaceous matrix but have a large percentage of coarse-grained angular fragment. They may be welded or not too. How do we tell where the igneous rocks formed? What can we derive from the rocks about the conditions of formation? Pyroclastics Lava flow Volcanic neck Igneous dike Igneous Sill Batholith Sierra Nevada Batholith- Home to Yosemite National Park and a lot of Granites and Grano-diorites Sierra Nevada Batholith Areal extent of the Idaho Batholith- a huge (15,400 square miles) intrusive body of primarily felsic composition (granites) igneous rocks An igneous dikeDiscordant with surrounding rock It cuts across other rocks An igneous sillConcordant with surrounding rock It runs parallel to other rocks Volcanic Necks & Igneous Dikes Neck Dike igneous Igneous sill sill Lava flows Flood basalts create lava cliffs on the Columbia Plateau