9 igneous rocks

Lynn S Fichter Department of Geology/Environmental Science J ames Madison University Fall, 2005 Co lo r Dis s o lv e d g as s e s Vis c o s ity Po ly m e riz atio n S ilic a Co nte nt S p e c ific Grav ity Te m p e rature Ig ne o us Ro c ks The Two Mo s t Abundant Ro c ks o n Earth Bas alt Dark colored and fine Light colored and coarse Granite Two Ideal Magmas Mafic Mag ma Example Color Position in Reaction Series Basalt Granite Dark (gray/black) Light (white/pink) Top of Reaction Series Pyroxene, Ca Plagioclase; Olivine (trace) Grain Size Fine grained, rapid Cooling cooling at Earth’s History surface Location Ocean Basins Spec Grav High Mineral Composit Lava Char Fe ls ic to Inte rme diate Mag ma Very hot, low viscosity Bottom of Reaction Series Na Plagioclase/ Orthoclase Quartz; trace mafics Large grained, slow cooling deep under ground Continents Low Cool, sticky Igneous Rock Textures Phaneritic Coarse grained Aphanitic Fine grained Glassy Cellular Porphoritic Aphanitic Phe nocrys t Phaneritic 1000’s of years Days to months/years PHANERITIC APHANGLAS S Y ITIC o r to Po rp hy ritic CELLULAR GABBR BAS ALT O Hours to days S CORIA DIORITE ANDES ITE OBS IDIA GRANITE RHYON LITE PUMICE Ig ne o us Ro c k Clas s ific atio n By Te xture and Co lo r Is convenient And a place to start But, it is not a very good classification Primarily because the color of a rock by itself has no meaning There is no theory of rock color, nothing in chemistry or physics that explains why color should change systematically from the top to bottom of Bowen’s Reaction Series Diorite: - 5% quartz, Na plagioclase and amphibole in about equal amounts Diorite: 0% quartz, Na plagioclase and amphibole in about equal amounts; this is a darker version but light and dark colors are still about equal Gabbro: No quartz, Ca/Na to Ca plagioclase and pyroxene in about equal amounts Gabbro: No quartz, Ca/Na to Ca plagioclase and pyroxene in about equal amounts Rhyolite: Minerals cannot be identified in hand specimen Andesite porphyry Minerals cannot be identified in fine grained groundmass, but phenocrysts of amphibole are visible Rhyolite porphyry Minerals cannot be identified in fine grained groundmass, but phenocrysts of orthoclase are visible Basalt Minerals cannot be identified in hand specimen, but come from the top of Bowen’s Reaction Series Basalt porphyry Phenocrysts are a Ca rich plagioclase Granodiorite: Half way between a granite (>20% quartz) and a diorite (no quartz) Quartz here is over 20% but mafics and plagioclase are about equal, a diorite feature MINERAL COMPOSITION KEY FOR PHANERITIC IGNEOUS ROCKS Whitish, light gray, PLAGIOGRANITE Salt & Pepper white>black GRANODIORITE Quartz 0-5% pinkish SYENITE Quartz 5-20% QTZ SYENITE Quartz 0-5% white-ish MONZONITE Quartz 5-20% QTZ MONZONITE Plagio >90% mid to dark gray Pyroxene 20% Light-coloredrocks P LAGIOCLAS E Mafic QUARTZ PLAGIOCLASE Felsic Mo dal Clas s ific atio n Of Ig ne o us Ro c ks O RTHOCLAS E Obsidian: (volcanic glass) No minerals are present since this cooled too fast for them to form Syenite: Orthoclase rich, mafic poor, quartz absent or only a trace Dunite: Olivine >90% As a mineral this would be identified as an olivine [...]... Alkaligranite 97 +3=100% 79+ 18 =97 % 9% 27+52= 79% 91 % 25+2=27% 18+7=25% 18% PERIDOTITE S YENITE ALKALI- PLAGIO- DIORITE GABBRO GRANITE GRANITE DUNITE Ultram afic ANDE- BAS ALT RHYOLITE m e ans the rock S ITE (p o rp hy ry ) PUMICE OBS IDIAN (p o rp hy ry ) (p o rp hy ry ) S CORIA has no fe lds par Ig ne o us Ro c k Clas s ific atio n In Bo we n’s Re ac tio n S e rie s Feldspar Absent Rocks Feldspar Dominated Rocks. .. rie s Feldspar Absent Rocks Feldspar Dominated Rocks Identifying Igneous Rocks – Practically MINERAL COMPOSITION KEY FOR PHANERITIC IGNEOUS ROCKS Whitish, light gray, PLAGIOGRANITE Salt & Pepper white>black GRANODIORITE Quartz 0-5% pinkish SYENITE Quartz 5-20% QTZ SYENITE Quartz 0-5% white-ish MONZONITE Quartz 5-20% QTZ MONZONITE Plagio >90 % mid to dark gray Pyroxene 90 % Felsic MAFICS 10-25%...Ig ne o us Ro c k Clas s ific atio n Mo dal Clas s ific atio n By Texture and percent mineral content Bowen'sReactionSeriesAndIgneousRockClassification Basalt Color does not work for obsidian, which is felsic in composition but light in color OliveGreen Calcic Plagioclase Olivine Pyroxene Amphibole Sodic Biotite Plagioclase Pink... to dark green P ERIDOTITE O LIVINE Pale-green glassy grains Black Chromite crystals typ DUNITE 60-100% 40 - 90 % > 90 % Felsic MAFICS 10-25% (T ONOLITE) Mafic >ORTHOCLASE Inte rmed > 20% Light-coloredrocks P LAGIOCLAS E Mafic QUARTZ PLAGIOCLASE Felsic Mo dal Clas s ific atio n Of Ig ne o us Ro c ks O RTHOCLAS E Mo dal Clas s ific atio n Of Ig ne ... Felsic LightColor Dull, Black Mafic DarkColor DarkGray Quartz Diorite Alkaligranite 97 +3=100% 79+ 18 =97 % 9% 27+52= 79% 91 % 25+2=27% 18+7=25% 18% PERIDOTITE S YENITE ALKALI- PLAGIO- DIORITE GABBRO GRANITE... Re ac tio n S e rie s Feldspar Absent Rocks Feldspar Dominated Rocks Identifying Igneous Rocks – Practically MINERAL COMPOSITION KEY FOR PHANERITIC IGNEOUS ROCKS Whitish, light gray, PLAGIOGRANITE... grains Black Chromite crystals typ DUNITE 60-100% 40 - 90 % > 90 % Felsic MAFICS 10-25% (T ONOLITE) Mafic >ORTHOCLASE Inte rmed > 20% Light-coloredrocks P LAGIOCLAS E Mafic QUARTZ