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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 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 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 Igneous Rock Textures: • Phaneritic texture • Mineral grains are visible • Intrusive • Aphanitic texture • Grains too small to see • Extrusive • Porphyritic texture • Larger grains in small grained matrix • Cools above and below 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 What are the textures in these two rocks ? Rhyolite Andesite & Diorite What are the textures in these two rocks ? Basalt & Gabbro aphanitic texture forms from mafic magma Basalt » phaneritic texture forms from mafic magma Gabbro Peridotite- the abundance of Olivine crystals (more than 40%) makes the rock take on a green appearance Obsidian- a popular rock among paleopeople for tool making Volcanic Glass Tuffs & Breccias Tuffs, Pumice & Breccias- look like sedimentary rock But they are not….it is volcanic ash that is lithified How do we tell where the igneous rocks formed and what can we derive from the rocks about the conditions of formation? Pyroclastics Lava flow Volcanic neck Igneous dike Igneous Sill Batholith Enchanted Rock is called an “Exfoliation Dome” because of the layers of rock that are exfoliating from the surface 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 Necks & Ig Dikes Neck Dike igneous Igneous sill sill
