Week 2 Igneous Rocks Lecture
Week 2 Igneous Rocks Lecture GEOL 1005
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This 4 page Class Notes was uploaded by Frances Cooke on Sunday November 15, 2015. The Class Notes belongs to GEOL 1005 at George Washington University taught by Glenn Havelock in Fall 2015. Since its upload, it has received 23 views. For similar materials see Environmental Geology in Geology at George Washington University.
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Date Created: 11/15/15
Igneous Rocks The rock cycle Intrusive and extrusive igneous rocks Igneous rock classification 0 Texture crystal size 0 Composition The Rock Cycle 0 Molten rock I In the mantle rocks melt usually near plate tectonic boundaries I Melts gt less dense gt rises as a plume of magma incorporating rocks assimilation I Most doesn t reach surface as it cools gt intrusive large crystals light color I Crystallization process of liquid state crystal state solid state I What reaches surface erupts in volcano I Lava flow gt dark colored extrusive o Igneous rock I Intrusive stays under surface large crystals light color I Extrusive leaves as lava flow dar small crystals 0 Sediments I Intrusions exposed by weathering and erosion I Break into small bits I Transported into rivers as sediment I Taken out to sea layered in sea bed 0 Sedimentary rock I Formed by Iithification the process of forming a rock 0 Compaction layers squeezing down 0 Cementation groundwater precipitates sand 0 Metamorphic I Heat and pressure trigger metamorphism I Melted back into magma returning to beginning of cycle 0 Not at cycles are perfect I Shortcuts of interruptions in rock cycle 0 Sedimentary rock gets uplifted into mountain instead of going deep into ground 0 Metamorphic rock might be uplifted and eroded into sediment o Igneous rock might be buried straight away and melted Igneous rocks form as magma molten rock containing solids and gases that cools and crystallizes 0 formed inside the Earth called plutonic or intrusive rocks 0 formed on the surface are volcanic or extrusive rocks formed from lava flows similar to magma but without gas or ash Plate tectonics influences where igneous rocks form Under the oceanic crust ex midAtlantic ridge divergent boundary I intrusive and extrusive rocks forming Continental crust magma created and rises through crust ex Yellowstone I both types form Continental margin heavier oceanic crust subducted under lighter continental O O O crust Plate tectonics and distribution of rock types Upwelling of magma from mantle created igneous Pressure and heat associated with convergent plate boundaries modifies adjacent rocks forming metamorphic Erosion of young mountains creates sedimentary lntrusive igneous rocks Plutons Magma chambers aka plutons Sheet intrusions cut across beddingstrata gt dike Works its way up to a bedding plane and goes along the bedding plane gt sill Mount Rushmore is carved from intrusive igneous rock granite I Rock cooled slowly at depth and since was uplifted I Overlying rocks removed by erosion I Granite too hard to be worn away like overlying sedimentary rocks Batholith an igneous intrusion in Israel Massive usually no internal bedding planes I Unlike sedimentary rock which show bedding planes 0 O O O O O 0 Each layer represents a period of time Extrusive igneous rocks Mt Etna Sicily in 2002 I Ash creates layers of sediment I Lava flows cause extrusive rocks Rock forming materials extruded from volcano I Lava outflows I Pyroclastic materials fire fragments ejected O O O O 0 Ash fine glassy fragments Cinders peasized Lapilli walnutsized material Bombs liquid lava 0 Approx 10cm long but can be up to 15 feet in diameter I Pyroclastic rocks such as tuff volcanic ash lgneous rock formation Crystallization of magma Crystals form as magmalava cools Crystal size is determined by the rate of cooling I Slow rate forms large crystals intrusive under the surface I Fast rate forms microscopic small crystals extrusive I Very fast rate form glass aka obsidian o Igneous rock classification 0 Texture I Size and arrangement of crystals I Finegrained aphanitic fast rate of cooling 0 Extrusive o Basalt formed over oceanic crust o Rhyolite formed over continental crust o Scoria vesicular I Coarsegrained phaneritic slow rate of cooling 0 Intrusive 0 Granite I Porphyritic two crystal sizes two rates of cooling o Slow then rapid cooling 0 Phenocrysts large crystals imbedded in microscopic crystals I Glassy very fast rate of cooling 0 Volcanic bombs what s thrown into air or into sea 0 Used by N Ams as arrowheads very sharp o Pumice stone contains vesicles very light I Pegmatite o Exceptionally coarse grained o Remaining melt gets more rich in H20 0 Last part of molten material to crystallize o Atoms move around at high heat and crystals grow very quickly 0 Composition I Silicate minerals 0 Dark silicates 0 Light I Four main groups 0 Graniticfelsic o Composed of lightcolored silicate quartz feldspar 0 High silica content 70 due to parent magma incorporating continental crust 0 Granite intrusive or rhyolite extrusive o Basalticmafic 0 High percentage dark silicate minerals 0 Low silica content from oceanic crust forming in volcanoes Hawaii 0 Basa o Andesitic intermediate 0 Between granite and basaltic o Mixture of light and dark silicate 0 Associated with continental margins o Andesite o Ultramafic o No extrusive version formed under crust 0 Almost 100 dark silicate minerals o Olivine and pyroxene o Intrusive rock is peridotite 0 Rare at Earth s surface 0 Six main rock types Granite diorite gabbro rhyolite andesite basalt o Porphyritic precedes any of the above names 0 Glassy obsidian and pumice
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