Office Hours Tuesday Thursday 11 00am 12 00pm or by appointment GEOL 1001 3 Instructor Dr Tiffany Roberts Room Howe Russell 130 Office E209 Howe Russell Office Phone 225 578 2801 Email tiffanyroberts lsu edu TA Abah Omale Email aomale1 lsu edu Office location 205 Howe Russell Office Hours M W 1 00 2 00pm Exam 2 Chapters 4 7 Date 3 12 14 Location Himes Testing Center 2 11 14 Chapter 4 Magma Igneous Rocks Magma Important concepts cid 127 Magma Formation Composition Igneous Process Composition Texture Nomenclature Igneous geologic features Magma Completely or partially molten rocks Buoyant material At earth s surface lava Components cid 127 Melt liquid Crystals solid Volatiles gas Crystalization cid 127 Magma cools to form intrusive igneous rocks plutonic Lava cools to form extrusive igneous rocks volcanic Origin Earth s crust and mantle solid not molten rock Then where does magma come from cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 Geothermal gradient cid 127 With increasing depth Increasing pressure Increasing temperature cid 127 Magma generation 1 Decrease pressure Divergent boundaries cid 127 Mantle plumes Decompression melting 3 Add water volatiles Subduction zones Flux melting 5 Increase temp cid 127 Magma heat transfer Independent of plate boundary Composition Silicates cid 127 Mafic dark silicates cid 127 Magnesium and ferrum iron Felsic light silicates Feldspar and silica Relative silica SiO2 content in silicate minerals Higher in felsic Lighter color Less dense Continental Lower in mafic Darker color cid 127 More dense cid 127 Oceanic Felsic Intermediate Mafic Ultramafic This will determine behavior of magmas viscosity Magma composition Relative of silicate minerals 66 76 SiO2 52 66 SiO2 45 52 SiO2 38 45 SiO2 Changes to composition Initial composition Bowen s Reaction Series cid 127 Magmatic differentiation body s cool margins Assimilation cid 127 Magma mixing A A magma having a mafic basaltic composition erupts fluid basaltic lavas B Cooling of the magma bosy causes crystals of olivine pyroxene and calcium rich plagioclase to form and settle out or crystalize along the magma C The remaining melt will be enriched with silica and should a subsequent eruption occur the rocks generated will be more silica rich and closer to the felsic granite end of the compositional range than the initial magma Magma melts the surrounding rock and it is assimilated into the magma Two magma body s of different composition meet and mix cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 Partial melting Partial melting of a hypothetical rock composed of the minerals on Bowen s reaction series yields two products A melt having intermediate to felsic composition and unmelted residue having a mafic composition 2 13 14 Factors of magma composition Viscosity resistance to flow cid 127 Melting temperature Igneous rocks classification Texture cid 127 Phaneritic cid 127 Aphanitic Composition cid 127 Felsic cid 127 Mafic Texture mineral grain size cid 127 Mineral crystal size Slower cooling larger crystals Faster cooling smaller crystals Amount of SiO2 and volatiles Time Rate of cooling Temperature Phaneritic Coarse grained Slower cooling Intrusive cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 Mixed sizes cooling history Aphanitic Fine grained Faster cooling Extrusive Types of grain Porphyritic Vesicular cid 127 Gas bubbles cid 127 Glassy Rapid cooling Fragmental pyroclastic Angular Pegmatitic Late crystallization in fluid rich environment Nature of igneous rocks Intrusive Intrusions Plutons Extrusive Volcanic Tabular Intrusive igneous bodies Dikes cut across pre existing layers Sills horizontal window sills Columnar jointing cid 127 Massive Batholiths Xenoliths not complete assimilation Laccoliths Characteristics Coarser grain size cid 127 More resistant to erosion Inverted topography 2 18 14 SI Sessions Location Coates 220 Times Monday 4 5 30 Wednesday 5 6 50 Important conecepts Nature of volcanic eruptions Viscosity cid 127 Materials Types landforms Hazards Plate tectonics volcanoes Volcanic activity Chapter 5 Volcanoes and Volcanic Hazards cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 Active Dormant Extinct Ring of fire Plate tectonics subduction Earthquakes Volcanoes eruptions Tsunamis Nature of eruptions cid 127 What is a volcano cid 127 What causes that formation of volcano cid 127 What factors affect type of volcano or eruption Viscosity Factors affecting viscosity Temperature Composition Basaltic mafic 1 000 1 250 C Andesitic int 800 1 050 C Eruptive Temp Viscosity Rhyolitic felsic 650 900 C Greatest Composition Composition Silica content Viscosity Basaltic mafic least 50 Andesitic int Int 60 Rhyolitic felsic most 70 Greatest Dissolved gases Composition Silica content Viscosity Basaltic mafic least 50 Andesitic int Int 60 Rhyolitic felsic Most 70 Greatest Least Int Least Int Least Int Determines eruption type cid 127 Quiescent effusive Explosive Composition Basaltic mafic Silica content least 50 Gas content least 1 2 Eruptive temperatures Viscosity Pyroclastic 1 000 1 250 C Least Least cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 cid 127 Int 60 Int 3 4 800 1 050 C Int Int most 70 Most 4 6 650 900 C Greatest Greatest Andesitic int Rhyolitic felsic cid 127 Materials Lava flows cid 127 Mafic basaltic Aa sharp fragments Pahoehoe ropey Pillow lava Felsic rhyolitic Block lava cid 127 Gases H20 CO2 SO2 Pyroclastic tephra Ash cid 127 Welded tuff Lapilii Blocks Bombs Scoria Pumice Types cid 127 General anatomy of a volcano cid 127 Magma chamber underground large round Conduit leading to above ground from the chamber Vent where the conduit reaches the surface Crater the dip at the top of the volcano Caldera collapsed volcano pit Volcano morphology 1 Shield volcanoes Fluid lava flow Shallow incline Shallow magma chamber Flank eruption Summit caldera Largest type 2