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The Rock Cycle Rocks: a solid collection of minerals or mineral materials. Rock Types Igneous Sedimentary Metamorphic http://www.childrensmuseum.org/geomysteries/faq1.html Rock Cycle Process of events in which rocks can be weathered, melted, altered, and formed over millions of yrs. http://www.ltcconline.net/julian/california/cycle.gif Rock Cycle Processes SEDIMENTARY ROCKS: on surface Weathering / Deposition of Sediments / Cementation (lithification) Sedimentary Rock IGNEOUS ROCKS: On surface (extrusive) or interior (intrusive) Melting / Cooling & Solidifying Igneous Rock Metamorphic Rock: Usually inside Earth (@ depths associated w. higher pressures & temps.) Heat / Pressure Metamorphic Rock Weathering: wind & water breaks rocks down into fragments (Makes sedimentary) Cementation: Water “glues” sediment together to make rock Igneous Rocks • From latin “ignis” = fire • Formed when molten rock cools and solidifies. • The slower rock cools more crystals Composition: Magma = Molten rock and trapped gases • mostly elements Si, O (Al, Fe, Ca, Na, K, Mg) + gases like water vapor • Gases – kept in magma by pressure of surrounding rock (want to escape) • Magma rises b/c its less dense than surrounding rock. (It’s warmer & has gases) • As magma cools, elements combine to form (mostly silicates) minerals Extrusive vs. Intrusive Intrusive – magma cools while trapped beneath E’s surfaces • uplift; weathering & erosion expose buried intrusive rocks over time • Cooling takes millions of yrs.; Bigger crystals Characteristics: Bigger/more crystals = Coarse grained (longer to cool) • Ex: Granite Extrusive vs. Intrusive Extrusive – magma cools & crystalizes on E’s surface Characteristics: air pockets/vesicles; glassy; few or small crystals • Ex: Basalt, Obsidian, Rhyolite, Scoria Basalt http://www.geol.umd.edu/~tholtz/G104/lectures/104rox.html Granite Characteristics of Igneous Rocks 1. Coarse Grained Texture Slow cooling Large crystals “chunky”crystals (intrusive) Ex: Granites Characteristics of Igneous Rocks 2. Fine-Grained Texture • Cool quickly (no time for big crystal growth) • Small, interconnected mineral grains Characteristics of Igneous Rocks 3. Glassy Cool too quickly for minerals to organize and accumulate Ex: obsidian, pumice, scoria Characteristics of Igneous Rocks 4. Porphyritic Texture • Mix of large crystals (phenocrysts) surrounded by finegrained minerals. • Minerals crystallize @ diff. rates • Cool deep w/in E. Igneous Rock Textures http://geology.csupomona.edu/drjessey/class/Gsc101/Igneous.html Igneous Rock Types: Composition Granitic: ~70% silica • Main minerals = Light colored silicate minerals (Quartz + feldspars) • Less than 30% dark minerals (biotite mica: amphibole) • Common in continental crust • Ex: Granite Minerals of Granites In thin section under a microscope Igneous Rock Types: Composition Andesitic: • Mostly plagioclase feldspar(light colored; gray/beige) • ~ 25-45% dark silicate minerals (biotite mica, amphibole, pyroxene) • Form @ volcanoes near colliding plate boundaries (ocean plate subducted under continent; Andes in S. America) • Ex: Andesite Igneous Rock Types: Composition Basaltic: • 45 – 85% dark minerals magnesium and iron (Less silica) • Dense & dark • Common in ocean crust • Ex: Basalt & Gabbro Igneous Rock Types: Composition Ultramafic: • 85% dark silicate minerals (least silica) • Dense & dark (green) • Common in upper mantle • Ex: peridotite = olivine + pyroxene (green sand of Hawaii) Classification of Igneous Rocks Classification of Igneous Rocks Basalt Granite Palisades Sill, NJ (basaltic) Giant’s Causeway, Ireland Metamorphic Rock • Formed when HEAT, PRESSURE, or Hydrothermal solutions w/in E cause changes in the texture and mineral content of rocks. • Rock undergoes a change w/out melting over millions of yrs. • Minerals migrate, react with other minerals, realign/rearrange • Usually form at bases of Mts.; near magma, hot springs, faults, or plate boundaries. Examples: • Limestone + heat + pressure Marble • Mudstone/Shale + heat + pressure Slate 2 Types of Metamorphism 1. Contact Metamorphism Rocks near magma intrusions; minerals migrate and rearrange; “Low Grade” 2 Types of Metamorphism 2. Regional Metamorphism Happens over large area; usually near colliding plate boundaries; at great depths; Mt. Building areas; “High Grade”; minerals recrystallize into banding patterns Classification of Metamorphic Rocks 1. Foliated: •Platy minerals realign and b/cm more compact and dense (slates) *Platy minerals = flat = clays, micas •Minerals rearrange into bands or stripes(= Foliations) •Ex: Shale (sedimentary mud stone) slate Granites (igneous) schists & gneisses. Rocks viewed with a microscope Microscope View of GRANITE (IGNEOUS): Minerals are randomly oriented Microscope View of GNEISS (METAMORPHIC): Minerals are NOT randomly arranged. (Green/Blue minerals are lined up in parallel layers or foliations) Metamorphism Green, platy, sparkly minerals Classification of Metamorphic Rocks 1. Non-Foliated: No banded texture; contain only one mineral Ex: limestone (calcite) marble Forming Sedimentary Rocks: • Formed when pieces of rock or sediment get squeezed and cemented together (= lithified) Sediments: Fragments made by weathering & erosion of existing rocks Erosion = breaking up rock fragments & transporting them. Agents of Erosion (What erodes rock?) Water; Wind; Ice; Gravity Deposition, Compaction & Cementation • Deposition: sediments settle in new locations (river beds, ocean bottoms, new land). – Smaller sediments travel farther than larger ones. • Compaction: buried sediments get squeezed by overlying sediments; water b/w fragments is forced out • Cementation = minerals in solution precipitate in pore space b/w fragments = glue Rocks viewed with a microscope Sandstones (Sedimentary): Grains of sand cemented together with pore space between. Features of Sedimentary Rocks • Visible clasts (pebbles, sand grains, shells) • Fossils = imprints or remains of organisms • Ripple marks • Mudcracks Features of Sedimentary Rocks Conglomerate – made of pebble clasts Sandstone – made of sand clasts Ripple marks – sand deposited by stream Features of Sedimentary Rocks Mud cracks: seasonal lake/desert enviro. oolites – tidal enviro. Stromatolites – fossils of bacterial organisms common in shallow waters Fossils in Sedimentary Rocks Fossils in sedimentary help to 1. Date/determine age or rocks. Age or rock correlates to time when those organisms lived. 2. Provide info on depositional enviro (shallow sea? Tidal area? Swamp? Desert?) Ex: Rocks w/ trilobite fossils are from Precambrian time (540 mya or older); Crinoid fossils = Paleozoic (540 – 254 ma); Ammonite fossils = Mesozoic (64 -254 ma) Types of Sedimentary Rocks A. Clastic: form fr. broken fragments of other rocks (held together by cement) Ex.: Conglomorate, sandstone, mudstone B. Chemical: form fr. minerals that precipitate out of solution. Ex.: gypsum, rock salts, limestone C. Organic(biochemical): form as a result of organic (living) processes Ex. :Fossiliferous limestone made from shells of organisms or rocks w/ fossils in them or Anthracite (coal) made from plant matter that did not get decomposed. Conglomerate: Clastic Tufas (salt towers): Chemical Limestone Cliffs of Dover: Organic Sandstone: Clastic Sedimentary Rock Classification Where does each rock form? Igneous: surface (extrusive) or deep in earth (intrusive); Heat from Earth’s interior creates igneous rocks Metamorphic: usually deep in the crust (heat from Earth’s interior creates) Sedimentary: on surface; Sun (drives weather) and gravity provide energy for sedimentary processes. Rock ID Check Up Answer these questions on lined paper in complete sentences. • • • • • • • • • • Which rocks were igneous? List # and name. Which of igneous rocks would be considered EXTRUSIVE? Which rock was an intrusive igneous? What characteristic can you observe to confirm that it is intrusive? Did granite form quickly or slowly? How do you know? Which rocks were sedimentary? List # and name. Which sedimentary rocks were clastic? Which sedimentary rock was organic or chemical? Which rocks were metamorphic? List # & name. Shale can turn into slate. What conditions would change shale into slate? Which rock (or rocks) could you find fossils in? Support your answer.