Geol 285, Petrology, Dr. Helen M. Lang - West Virginia University, Spring 2010 Geol 285 - Introduction to Petrology The study of rocks: Greek: petra = rock logos = discourse or explanation (study) Petrology is central to Geology, and is based on Mineralogy A rock is a naturally-occurring aggregate of minerals or mineraloids What is 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 conditions • The Rock Cycle shows how rocks form and change from one type into anotherWhy study petrology? • Learn about early history of Earth • Learn about the interior of the Earth • Only a small part of the crust is exposed or accessible to drilling o Crust is less than 1% of Earth’s volume o 66% of crust is sedimentary o other 34% is mostly igneous • Mantle is metamorphic • Core is liquid and solid metal Fe(Ni) Thickness of Crust is 1-2% of Earth’s Diameter Why 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 • Compare modern processes with ancient rock record; infer processes and explain differences We’ll start with Igneous Rocks: Outcrop Characteristics (see Table I-1 in textbook)• 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 fossils and stratification • Generally structureless (massive) and composed of interlocking grains Typical Igneous Textures • Porphyritic • Glassy • Vesicular • Pyroclastic • Interlocking crystalline Typical 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)Initial Classification based on: • Percent mafic and felsic mineral content • 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 slowly See handout for General Classification of Igneous Rocks and their properties The amount of SiO2 in igneous magmas is quite variable and significant • Magmas with enough SiO2 to crystallize quartz (pure, free SiO2) are said to be Oversaturated with SiO2 • Magmas with so little SiO2 that they crystallize minerals that are incompatible with quartz are said to be Undersaturated with
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