73Intro to Rocks with Focus on Igneous RocksI. RocksA. Defined - aggregates of one or more mineralsB. Three Rock Types (based on their mode of origin)1. Igneous: rocks crystallize from molten magmaa. Heating and melting of rocks for form magmab. cooling of magma to form igneous rocks2. Sedimentary: rocks form at near earth's surfacea. Weathering of pre-existing rocks at surfaceb. Erosion and transport of sediment(1) agents of transport(a) wind(b) water(c) ice(d) gravityc. Deposition and lithification of sediment3. Metamorphic: rocks formed by alteration of pre-existing rocksa. agents of metamorphism(1) heat(2) pressure(3) chemically active fluidsC. Rock Cycle1. Full cycle: magma-----cooling/crystallization---igneous rocks---weathering----sediment----lithification/compaction----sedimentary rocks---pressure andtemperature----metamorphism----metamorphic rocks----remelting-----igneous rocksII. Focus on Igneous RocksA. Igneous Rocks- a rock (or agglomeration of one or more minerals) that results from thecooling of magma, or molten rock. 1. As the magma cools, minerals crystallize from the molten rock. a. Magma - molten or hot liquid rock, originates beneath the earth's surface (upto 120 miles beneath), composed of elements found in silicate minerals,water vapor, and gases.b. Lava - magma that is extruded onto the earth's surface via volcanic eruptions(hot magma is confined at depth beneath surface, relatively lighter thanconfining rock, rises upward, may eventually erupt onto earth surface).Simplified Diagram of Rock CycleRock Cycle - Expanded Version with ProcessInteractions Listed74c. Extrusive Igneous Rocks or Volcanic Ig. Rocks - rocks which solidify fromlava (or were extruded onto earth's surface)d. Intrusive Igneous Rocks or Plutonic Ig. Rocks - rocks which solidify frommagma beneath the earth's surface.2. Magma Crystallization Processa. Hot, molten magma: ions of elements are moving freely in a fluid, unorderedstate, as magma cools, the ions slow and begin to form atomic bonds,arranging themselves in orderly patterns --- known as process ofcrystallization.b. Rates of cooling strongly influence size of mineral crystals that develop frommagma/lava.(1) Slow Cooling - few centers of crystal growth develop, ions allowed tomigrate over larger distances - results in rather large mineral crystals .(2) Fast Cooling - many centers of crystal growth, ions readily bondtogether, results in smaller mineral crystals.(3) Very rapid cooling - if magma is quenched instantly, not sufficienttime for ions to bond, results in randomly distributed ions frozen -referred to as glass similar to manmade glass.(4) Multi-phase cooling: slow partial magma cooling, followed by fast lavacooling(a) Porphyritic Rocks- mixed small and large mineral crystals3. Naming Igneous Rocks - Based on compostion and texture of igneous rock. a. Mafic Rocks (from Mag and Fe) - generally darker colored rocks relativelyhigh in iron, magnesium, calcium and low in silicon. Associated with hightemperature end of bowens Reaction Series.(1) Gabbro = plutonic = phaneritic = mafic composition(2) Basalt = volcanic = aphanitic = mafic composition (Ca-rich plag., andPyroxene).b. Felsic (from feldspar and silica) Rocks - generallylighter in color, high insilica, Na, Potassium - consist mainly of quartz, K-feldspar, and Plagioclase(1) Granite = plutonic = phaneritic = felsic composition(2) Rhyolite = volcanic = aphanitic = felsic composition75c. Intermediate - admixtures of both felsic and mafic, dominated by amphibole,intermediate plagioclase feldspar, biotite.(1) Diorite = plutonic = phaneritic = intermediate composition(2) Andesite = volcanic = aphanitc = intermediated comp.d. Ultramafic = very rich in iron and mag., olivine and pyroxene, Ca-richplagioclase(1) intrusive variety only = Peridotite - common in upper mantleB. Naming of Igneous Rocks1. Classification Basisa. Mineral Composition(1) felsic minerals(a) quartz(b) feldspar(c) plagioclase(2) mafic minerals(a) amphibole (hornblende)(b) biotite(c) pyroxeneb. Rock Texture(1) aphanitic = microcrystalline(2) phaneritic = crystals visible with eye(3) porphyritic = mixed fine and coarse mineral crystalsBasic Igneous Rock Scheme____________________________________________________________Texture Mineralogy Name Interpretation____________________________________________________________aphanitic felsic rhyolite volcanic rockphaneritic felsic granite plutonic rockaphanitic intermediate andesite volcanic rockphaneritic intermediate diorite plutonic rockaphanitic mafic basalt volcanic rockphaneritic mafic gabbro plutonic
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