SPRING 2014GEOL 285:INTRODUCTORY PETROLOGYDr. Helen LangDept. of Geology & GeographyWest Virginia UniversityGeol 285Introductory PetrologyThe study of rocks:Greek: petra = rocklogos = discourse or explanation (study)Petrology is central to Geology,and is based on MineralogyA rock is a naturally-occurring aggregate of minerals or mineraloids.What’s the difference between a rock and a mineral?Rocks are conveniently divided into 3 categories• Igneous rocks - rocks that solidified from molten or partially molten material (magma)• Sedimentary rocks - rocks resulting from the consolidation of loose sediment or chemical precipitation from solution at Earth’s surface• Metamorphic rocks - rocks formed from pre-existing rocks by mineralogical, chemical and textural changes in response to change in conditionsThe Rock Cycle shows how rocks form and change from one type into another (there are many different versions of the rock cycle)Why study petrology?• to Learn about early history of Earth• to Learn about the interior of the Earth– Only the upper crust is exposed or accessible to drilling• Crust is less than 1% of Earth’s volume• 66% of crust is sedimentary • the other 34% is mostly igneous– The Mantle is metamorphic– The Core is liquid and solid metal Fe(Ni)Thickness of Crust is just 1-2% of Earth’s DiameterCrust is to scale hereCrustal thickness is really exaggerated hereWhy study Petrology (continued)• We can learn about the whole Earth only by studying exposed rocks, drill cores and geophysics• Distribution of rock types at Earth’s surface led to Plate Tectonic Theory• We must compare modern processes with the ancient rock record to infer processes that formed the rocks we see at the surfaceWe’ll start with Igneous Rocks:• They were once liquid. • How can we tell?– Properties/behavior of liquids?• What are some likely or expected textures and structures of igneous rocks?– Volcanic– PlutonicIgneous Rocks:Outcrop Characteristics (Table I-1)• Volcanoes and related lava flows• Cross-cutting relations to surrounding rocks (dikes, veins, stocks and batholiths)• Thermal effects on adjacent rocks• Chilled (fine-grained) borders against adjacent rocks• Lack of fossils and stratification• Generally structureless (massive) and composed of interlocking grainsTypical Igneous Textures• Porphyritic• Glassy• Vesicular• Pyroclastic• Interlocking crystallineTypical Igneous Minerals• Plagioclase Feldspars (Ca,Na)(Al,Si)4O8• Alkali Feldspars (Na,K)AlSi3O8•Quartz SiO2• Olivine (Mg,Fe)2SiO4• Pyroxene (Ca,Mg,Fe)2Si2O6• Hornblende• Micas (biotite and muscovite)• Glass (not a mineral, but a mineraloid)FelsicMaficSimple Classification is based on two important characteristics:• Percent mafic and felsic mineral content (dark vs. light)– reflects chemistry of magma– more FeO, MgO, more mafic minerals, darker color–more SiO2, Na2O, K2O, more felsic minerals, lighter color•Grain size– relates to cooling rate– fine-grained or glassy → cooled rapidly– coarse-grained → cooled slowlyIntrusive(Plutonic)coarse-grainedExtrusive(Volcanic)fine-grainedPyroclastic(Volcanic,fragmental)Propertiesof theMagmaMafic (hi CI*)(CI > 65)IntermediateFelsic (low CI*)(CI < 35)C.I. = color index = % mafic minerals = mafic/(mafic+felsic) * 100Gabbro BasaltAndesiteRhyoliteGraniteDioriteScoria,CinderPumice,Tuff, AshHigh density,Low ViscosityLow density,High ViscosityA very General Igneous Rock ClassificationThe amount of SiO2in igneous magmas is quite variable and significant•SiO2in magmas varies between 30 and 80%• Magmas with enough SiO2to crystallize quartz (pure, free SiO2) are said to be Oversaturated with SiO2• Magmas with so little SiO2that they crystallize minerals that are incompatible with quartz are said to be Undersaturatedwith SiO2Intrusive(Plutonic)coarse-grainedExtrusive(Volcanic)fine-grainedPyroclastic(Volcanic,fragmental)Propertiesof theMagmaMafic (hi CI*)IntermediateFelsic (low CI*)C.I. = color index = % mafic minerals = mafic/(mafic+felsic) * 100Gabbro BasaltAndesiteRhyoliteGraniteDioriteScoria,CinderPumice,Tuff, AshHigh density,Low ViscosityLow density,High ViscosityGeneral Igneous Rock ClassificationThickness of Crust is 1-2% of Earth’s DiameterCrust is to scale hereCrustal thickness is really exaggerated