EOSC 114 Unit 4 Volcanoes.docx

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Department
Earth and Ocean Sciences
Course
EOSC 114
Professor
Leah May Ver
Semester
Fall

Description
Unit 4: Volcanoes  Explain what magma density and magma viscosity are o Magma: mixture of molten rock, crystals and gas below the earth’s surface o Created by the melting of pre-existing rock in Earth’s interior (mantle + crust).  Reaches the surface through fractures, erupts as lava or pyroclastic material (tephra) o In order for magma to rise, it must be  List the different categories of volcanic rocks and Less dense than crust  Runny enough to flow  Hot enough to stay liquid o Main gases: H2O< Co2, SO2, Cl2  At a depth, <10% gas content. This decreases viscosity and density  As pressure decreases, the gases become less soluble and bubbles form o Viscosity = runninses, resistance to flow.  Explain the differences between the magmas they came from  Igneous rocks come from solidifying magma Mafic Rock Felsic Rock  Rich in denser minerals (olivine,  Rich in low density minerals (quartz, pyroxene), darker in colour feldspar) ls, light colour  Mafic rocks  Felsic rocks  Basalt (extrusive), Gabbro (intrusive)  Rhyolite (extrusive), Granite (intrusive)  Dacite (extrusive), Tonalite (intrusive) Intrusive Extrusive  Big crystals = intrusive aka plutonic  Small crystals = extrusive (Granite, Gabbro). Freezes in crust,  Rapidly cooled never erupted. Cools slowly. Intermediate rocks: Dacite (ext), Tonalite (int) Andesite (ext), Diorite (int)  Explain why some magmas erupt explosively (as pyroclastic material) and some magmas erupt effusively (as lava) Basalt Andesite Dacite Rhyolite Mafic (less silica) Felsic (more silica) Intermediate Hot (1200-1400 C) Cool (600-1000 C) Low viscosity High viscosity (RUNNY) (GOOEY) NON-EXPLOSIVE EXPLOSIVE LAVA PYROCLASTICS Love gas content High gas content  Viscosity and Gas Content are related to explosively o As pressure decreases, bubbles grow  Pressure reises inside bubbles until strength of liquid magma overcome. Bubble pops, fragmentation occurs  Pyroclastic material is produced  Mafic = low gas content + fluid, quiet, effusive eruptions  Gas escapes  Felsic = high gas content + gooey, explosive  Gas kept under increasing pressure  Explain the differences between pahoehoe and a'a lavas o Both are basaltic magmas  Pahoehoe erupted at high temperature, hardens with a smooth ropy texture (moves @ km/hr)  Cooler, more viscous moves slower (m / day), rough and blocky  Aa is more viscous and chunky because of lower temp  Describe the different types of volcanic eruptions and how they are related to magma properties Hawaain Strombolian Vulcanian Pelean Plinian Phreatomag matic  Low  Basaltic –  V.  Dome  Sustained  Water + viscosity mildly explosive collap column of magma,  Effusive explosive  Explosions se ash and violent  Bombs, of ash  Explo pyroclasti  Firefount ains lavas sive c flow, v  Block explosive and ash flow Basaltic Basaltic/And Andesitic/Rhy Andesitic/rhy Different lol (Mafic) esitic olitic olitic (Felsic)  Describe the distribution of the world's active volcanoes o Located near hotspots and tectonic plate boundaries o Pacific ring of fire  List the three types of plate boundaries and the different types of volcanoes that occur at these plate boundaries o Divergent Boundaries  Mantle rises linearly, pressure decreases. Mantle can melt the overlying crust.  Mantle rises linearly and pressure decrease. Mafic magma can melt the overlying crust.  On continental crust, mafic – int – felsic  Oceanic plates – mafic only o Subduction Zones  Beneath continental crust or beneath oceanic  Beneath continental crust - Mafic-int-felsic  Beneath oceanic crust – mafic – int – felsic o Hotspots  Mantle rises in a cylindrical plume, pressure decreases  Magma can also melt overlying crust  Under continental crust – mafic – int – felsic  Under oceanic crust – mafic  Describe the type of volcanoes that occur at hot spots o Emerge out of water, form island chains  Hawaiian Islands  Describe the tectonic setting of British Columbia and determine the dominant type of volcano that occurs here o Many stratovolcanoes o Cascadia subduction zone causes continental volcanic arc  Describe the morphology, dominant rock type and typical eruption style of the different types of volcanoes o Cinder Cones – mafic, explosive  Layers of pyroclastic ejecta from fire fountaining  Mafic (basalt)  Angle of repose = 30-40 o  Small volcanoes that never grow up. Usually erupt for a few years then never again.  Small. <2km wide, <500m tall o Shield volcanoes – mafic, non-explosive  Lava erupts from a fissure, runs down, cools  Mafic lava flows  Forms pahoehoe followed by a’a  Not explosive  ~100 km wide o Stratovolcanoes – intermediate, mixed  Interbedded lava flows, pyroclastic flows and lahars  Usually int fo felsic  Frequently explosive, often viscous magma  Can be active for a long time  ~10 km wide, <5 km tall  Erupt 10-1000 years o Cald
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