EOSC 114 Final Notes; Volcanoes Landslides.docx

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Earth and Ocean Sciences
EOSC 114
Brett Gilley

Volcanoes Slide Notes 26/02/2013 13:24:00 -Slide 1- Plinean Eruption  Named after Pliny the younger  Pliny wrote a letter describing the eruption and death of his uncle, who was saving people Risk  Risk= Vulnerability X Hazard / Mitigation  Hazard = size, frequency, type of damage  Vulnerability = what is impacted and value  Mitigation = $ invested t reduce vulnerability or to control hazard Magma  Melted rock in the earth, below earth surface  Melt + crystal + bubbles  Created by the melting of pre-existing rock in earth’s interior (mantle & crust)  It reaches the surface through fractures an erupts as lava or as pyroclastic material Lava  Melted rock exposed at the earth’s surface  Melt + crystal + bubbles Volcanic system  From bottom to top o Source region  In the mantle, 1000°C  Mantle (peridotite) into magma (basaltic -> granitic)  Magma is less dense o Transport region  Magma channels, moving along cracks in the earth’s crust  Forming dikes and sills  Dikes- vertical intrusion of magma  Sills – horizontal intrusion of magma o Storage region  Magma chamber  Magma ponds below the surface in large chambers  Intrusive rock  Magma freezes in crust, is not erupted  Large crystals from magma cooling down slowly  Eruptive Region Ingeous rocks  Cooled at the surface o Extrusive/volcanic o Basalt, andesite, dacite, rhyolite (hot to cool, less to more silica, non explosive (lava) to explosive (pyroclastic)) o Small crystals/fine grained  Cooled below surface o Intrusive/plutonic o Large crystals/course grained o Slow cooling Silica Content (SiO^2)  Felsic rocks: 65-75% SiO o Light color o Rich in K, Na, Al, Si o High viscosity (sticky) o Eg. Granite, rhyolite  Mafic rocks: 45-55% o Dark color o Rich in Mg, Fe o Low viscosity (runny) o Eg. Basalt, Gabbro -Slide 2- Glass formation  Glass is a non-crystalline or amorphous, metastable solid formed by quenching a melt  Cools so fast that a regular crystal lattice cannot form Rising magma  Causes o Crystallization o Exsolve volatiles  Volatiles (gases)  Dissolved >> exsolved Physical properties of magmas  Melt density: g cm^-3 or kg m^-3 o Decreases: basal -> rhyolite, rising temperature o Increases: cooling, pressure o Controls: ascent, eruption  Viscosity: Pa s o Resistance to flow o Controls: flow, deformation, eruption o High viscosity (sticky): cooler, felsic, high silica o Low viscosity (runny): hot, mafic, lower silica - Slide 3 - Volcanic eruptions two main styles  Effusive: outpouring of molten magma from the vent; lavas o Passive eruption of magma  Lava flows (mafic-intermediate)  Lava domes (felsic-intermediate)  Gravitational collapse of lava flows/domes (blocks and ash flows) o Lava flows downslope, ponds in topographic lows o Outer crust cools and solidifies insulating the molten interior  Explosive gas driven violent eruptions -> pyroclastic deposits o Active eruption of magma  Buoyant eruption column of ash  Pyroclastic airfall  Pyroclastic flows  Ballistics proximal to vent o Pyroclastic fall  Eruption colums: 10’s o km’s  Widespread distribution of ash in the downwind direction  Ash blankets topography o Pyroclastic flows  Gas-pyroclast mixtures  Gravity-driven flows  Flow downslope, channelled in valleys  Velocity = 40-300 km/h  Temperatures = 100-600°C o Secondary processes  Syn-post eruptive deposits  Reworking deposits during eruption o Explosive volcanism = gas content + melt viscosity o Gas  Magmas produce bubbles  Bubbles expand as they rise  Foaming -> explosively  Amount of bubbles  Rate of rise  Bubble retention  Magma rises > pressure decreases > bubbles begin to form  Pressure rises inside bubbles  Pyroclastic material formed Explosivity  Magma fragments and freezes to form pyroclastic ash  Pumice > ash  Mafic > low gas content + fluid = safe  Felsic > high gas content + gooey = dangerous  Increasing energy and hazard towards bottom of volcano Types of eruptions  Hawaiian o Low viscosity basaltic magma o Low explosivity o Effusive o Lava flows and fire fountaining  Strombolian o Basaltic/andesitic magma o Mildly explosive o Bombs, lavas  Vulcanian o Viscous andesitic/rhyolitic magma o Very explosive o Sustained explosions of ash  Pelean o Dome collapse o Violently explosive o Generation of block-and-ash flows  Plinian o Andesitic/rhyolitic ash o Violently explosive o Sustained column of ash, pyroclastic flows o Pine tree shape of ash column  Phreatomagmatic o Contact between magma and water o Violently explosive Types of eruptions scale  Hawaiian, strombolian, vulcanian, pelean, plinian  Mafic > felsic  Low > high viscosity  Low > high gas content  Safe > dangerous Eruption size: VEI scale (0-8)  Volcanic explosivity index  Key characteristic’s that define VEI o Volume of ash produced o Height of eruption cloud o Duration of eruption  Logarithmic scale  Mainly depends on total volume erupted explosively  Increase in 1 unit=10times more volume erupted  Occurance: o VEI 0-3; many every year o VEI 3-5; several every decade o VEI 5-7; several every 1’000 years o VEI 7-9; 1 or 2 every 1’000’000 yrs  Examples: o VEI 1; Hawaiian o VEI 3; Vulcanian, Montserrat o VEI 5; Plinian, Mt St Helens o VEI 6; Ultra-plinian, Pinatubo, Phillippines 1991  17 million towns of SO^2  temp decrease 0.5-0.6 °C  ozone hole size increased o VEI 8; 100 Pinatubos at the same time - Slide 4 – Volcano location is controlled by:  Plate tectonic boundaries o Crust composition and melt origin will strongly influence the type of volcanism o “Pacific ring of fire” o Continental volcanic arcs: seduction volcanoes  Felsic and intermediate magmas/lavas  Felsic magma  Mafic magma o Examples:  Local continental volcanic arc  Active volcanoes (last 2 Ma)  Subduction of the juan de fuca plate under the north American plate  Cascadian subduction zone – The cascades arc  Western Aleutian arc  Hot-spots o Mafic magma from a plume of hot mantle o Not associated with plate boundaries o Plumes are stationary and pulsatory o Examples:  The Hawaiian/emperor volcanic chain Settings of volcanism  Divergent boundaries o On continental crust, eg. East African rift zone o Between oceanic plates, eg. Mid-Atlantic ridge  Subduction zones o Beneath continental crust, eg. Cascades o Beneath oceanic crust, eg. Aleutian islands  Hot spots: o Under continental crust, eg. Yellowstone o Under oceanic crust, eg. Hawaii Melting mechanism  Divergent boundaries o Mantle rises in linear pattern, pressure decreases o Mafic magma can melt overlying crust  Subduction zones o Hydration of the mantle by subducation plate o Mafic magma can melt overlying crust  Hot spots o mantle rises in cylindrical plume, pressure decrease o mafic magma can melt overlying crust Types of magma  Divergent boundaries o On continental crust, mafi-int.-felsic o Between oceanic pates, mafic  Subduction zones o Beneath continental crust, mafic-int-felsic o Beneath oceanic crust, mafic-int.-fleisc  Hot spots o Under continental crust, mafic-int-felsic Under oceanic crust, mafic Types of volcanoes  4 main types o Cinder cones – mafic, explosive - frequent  layers of pyroclastic ejecta – from fire fountaining  mafic, basalt  angle of repos : 30-40°  2 km wide, 500m high  small volcanoes that never grow up  usually erupt for a few years then never again o Shield volcanoes – mafic, non-explosive – frequent/permanent  lava erupts from fissure, runs down gentle slopes, cooling  fissure eruptions  often extent radial from volcano centre  related to regional tectonics  erupts often  mafic lava flows  generally not very explosive  100 km wide, 4-10km high o Stratovolcanoes – intermediate, felsic, mixed - frequent  interbedded lavla flows, pyroclastic flows, lahars  usually intermediate or flesic  frequently explosive, often viscous magma  may erupt many times and stay active for 100’000 rs  10 km wide, 5 km high o Calderas – felsic, explosive - rare  Supervolcanoe  Large, usually circular depression at summit of volcano  Created from large, explosive eruption of flesic pyroclastic material  Caused when roof of the magma chamber collapses  Up to 10 km across  Different from a crater - Slide 5 – Volcanic Hazard  Lava flow o Slow o Usually not dangerous o Easy to predict flow path o Usually mafic + low viscoscity o Only really hazardous to buildings & infrastructure  Fallout deposits / Pyroclastic falls o Hot ash + gas ejected from volcano o Hazard to people:
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