GEOL 101 1nd Edition Lecture 7 Outline of Last Lecture I Characteristics of Magma II Igneous Texures III Igneous Compositions Outline of Current Lecture I Naming Igneous Rocks a Felsic b Intermediate c Mafic d Ultramafic II Origin of Magma Current Lecture Naming Igneous Rocks Chart These notes represent a detailed interpretation of the professor s lecture GradeBuddy is best used as a supplement to your own notes not as a substitute Example If mineral is Vesicular and Felsic the name is Pumice Felsic Coarse Grained o Granite Can be labeled intrusive plutonic or phaneritic About 25 Quartz and 65 feldspar Very abundant Associated with mountains Fine Grained o Rhyolite Can be labeled extrusive aphanitic or volcanic Equal to Granite chemically Less abundant and common than Granite Associated with volcanoes Porphyritic o Porphyritic Granite o Porphyritic Rhyolite Larger crystals called phenocrysts Background material called groundmass Rock named by groundmass Indicates two rates of cooling Glassy o Obsedian Dark colored black or reddish in color High in silica despite dark color Dark color is result of metallic ions Vesicular o Pumice Light colored Glassy with holes Formed because gasses were giving off the cooling lava Associated with volcanic eruptions ROCK THAT FLOATS Pyroclastic fragmental o Tuff ash sized o Breccia ash including large material Forms as hot material from eruption welds together Intermediate Coarse Grained o Diorite Can be labeled intrusive plutonic or phaneritic Composed of plagioclase and amphibole Light and dark minerals look like salt and pepper Fine Grained o Andesite Can be labeled volcanic extrusive and aphanitic Medium gray color Has glassy fragments or holes Most common with explosive volcanism Name from the Andes Porphyritic o Porphyritic Andesite Plagioclase phenocrystals in an andesitic groundmass Vesicular o Scoria Dark red or black because of Iron Magnesium materials Lava Rock Glassy o Obsedian Pyroclastic fragmental o Tuff o Breccia Mafic Coarse Grained o Gabbro Plutonic intrusive and phaneritic Dark gray and black with green sometimes Is large percentage of oceanic crust Fine Grained o Basalt Volcanic extrusive aphanitic VERY dark gray to black Most common extrusive rock Is large portion of oceanic crust Makes islands Iceland and Hawaii Porphyritic o Porphyritic Basalt Feldspar phenocrysts with basalt groundmass Glassy o Basaltic glass Forms as tiny crust around lava that is extruded under water Vesicular o Scoria Fragmental o Tuff o Breccia Ultramafic Coarse Grained o Periodotite Green with black flakes Composed of olivine pyroxene and some amounts of feldspar It is assumed the mineral is exposed when tectonics push up to the surface from mantle Fine Grained o Komatite Very rare and restricted to the Archean era o Kimerlite Volcanic pipers fed from deep mantle sources Mechanism that brings diamonds up to the surface Why do we name by texture and composition Some rocks have either the same texture but NOT the same composition or different texture but the SAME composition Origins of Magma Crust and mantle are solid rock but partial melting of rocks are caused by 1 Heating of rocks Most intuitive melt when heated up o Heated because temperature goes up as you go deep into earth s crust o Increase is called Geothermal Gradient Rocks in lower crust or upper mantel are already in melting point 2 Pressure changes In high pressure rocks are solid in high temperatures When pressure is reduced the rock will melt Decompression Melting Causes melting at seafloor spreading center mid ocean ridge o Melting at seafloor causes thinning of the crust in the center 3 Addition to volatiles Water in particular Volatiles lower melting point wet rocks melt at lower temperatures than dry rocks IMPORTANT FACTOR for convergent boundaries
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