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Geology Part II • The Rock Cycle • Rocks: – Igneous – Sedimentary – Metamorphic • Geologic Profiles • Geologic Time – Relative – Absolute – Radioactive Dating Rocks are minerals? • an indefinite mixture of naturally occurring substances, mainly minerals. • various combinations of minerals and organic substances • • range in size from tiny http://en.wikipedia.org/wiki/Sedimentary_rock pebbles to mountains. • • make up the earth’s crust. Rocks are not always solid • As discussed in plate tectonics magma is liquid rock • While the composition of this basalt flow have the same composition, • the smooth pahoehoe lava was hotter than the jagged aa lava. • Soil and clay are other examples of nonsolid rock Igneous Rocks • Fire Formed • Hardens as magma cools • “original rock” • Lava is on the Earth 2 types of Igneous Rocks 1. Intrusive cools inside the Earth Slow cooling= big crystals • Ex.granite 2. Extrusive cools outside the Earth Fast cooling = tiny crystals • Ex. obsidian Types of Intrusive rocks • Granite is the most common and is referred to as “mountain guts” • Diorite – iron and magnesium • Gabbro – dark, large crystals • Peridotite – green, most of upper crust Yosemite Rock formation http://www.nps.gov/yose/planyourvisit/formations.htm • “El Capitan is the largest monolith of granite in the world.” • Formed deep underground and then uplifted by plate tectonics and shaped by erosion • Age can be determines by radioactive dating Extrusive Igneous Rocks • Basalt – common, hard, dark • Andesite – began cooling in crust finished on surface large/small crystals • Rhyolite – cooled slowly then fast • Pumice – light, spongy look • Obsidian – volcanic glass, cools fast Metamorphic Rock • Created by 1 of 2 processes • Regional process – most common, heat and pressure over large area • Contact – heat and pressure next to magma chambers • 2 types: Foliated, Nonfoliated • Formed from preexisting rock that has been exposed to high heat and/or pressure • Enough to deform but not melt • Cannot determine age by relative or absolute (radioactive dating) Foliated • “layered” rocks • Slate – made from shale • Gneiss – made from granite Nonfoliated • No ‘layers’ • Marble – made from limestone • Quartzite – made from quartz Sedimentary Rock • Formed at the bottom of lakes and oceans by the deposition of sediment (small dust, sand, organic material) that was eroded by wind, water, ice from a preexisting rock. • Only 5% of Earth’s crust • Time and pressure cement particles to form a new rock Sedimentary rock characteristics Breccia: clastic • • • • Often have layers Often contain fossils Not very strong Can determine relative age by location – Top layers are younger • Absolute age cannot be determined 3 types: Clastic, Chemical, Organic • Clastic: Most common • Made of other rocks • Particles accumulate and harden in 2 ways – Weight of upper layers squeeze out liquids – Minerals dissolved in ground water act as cement • Weathering is an active process • Sandstone – sandsized quartz, variety of colors • Shale – most common, mud, clay, silts Chemical • Limestone – 50% calcite from seashells • Coquina – large pieces of shell • Chalk – microscopic pieces of shell • Halite – hard, created by evaporation • Gypsum – soft, created by evaporation • Chert – hard, created by heat Organic • Coal – black, coarse, decayed plant life Special Features of Sedimentary Rocks • Layering (bedding) • Fossils • Geode (minerals in a hole or cavity) • Concretions (minerals form around a grain or fossil) Rock Cycle Interactive Review • http://www.learner.org/interactives/rockcycl e/diagram.html Picture Credits • "Copyright 2009 by Andrew Alden, geology.about.com, reproduced under educational fair use.“ • http://www.learner.org/interactives/rockcycl e/diagram.html • DeJuana Aldrich