GEO207 Dr. Tranel Notes Volcanoes.pdf
GEO207 Dr. Tranel Notes Volcanoes.pdf GEO207
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This 6 page Class Notes was uploaded by Kimberlee Fischer on Tuesday April 5, 2016. The Class Notes belongs to GEO207 at Illinois State University taught by Dr. Tranel in Spring 2016. Since its upload, it has received 7 views. For similar materials see Natural Disasters in Geology at Illinois State University.
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Date Created: 04/05/16
GEO207 Natural Disasters Dr. Tranel Notes 3/29/16-4/5/16 VOLCANOS Volcano: Molten rock surfaces through erupting vent; Cone shaped mountain; Mountain built by repeated eruptions Magma: Melted pre-existing rock; In Earth’s interior; Moves through fractures Lava or “Pyroclastic material”: Magma at surface; present after eruption; can be effusive or explosive Igneous rocks form: -Boundaries (90%) -Convergent -Divergent – mid-ocean ridge – 80% -Divergent – continental rift – 10% -Plate Interior -Mantle plumes & hotspots – 10% Subduction margin volcanoes -Water-rich sediment -rock melts in hotter mantle Divergent margin volcanoes -Mantle near surface What is magma made of? Solid Liquid Gas Mineral Melted rock Water vapor Crystals Si & O Liquid Carbon dioxide Sulfur dioxide For magma to rise through crust, it must be: 1. Less dense than crust – low density > composition 2. Runny enough to flow – low viscosity > composition 3. Hot enough to stay liquid > composition Types of Magma Felsic Mafic Feldspar minerals Mg & Fe- rich minerals Silica rich Dark color Light color Example: Rhyolite Example: Basalt GEO207 Natural Disasters Dr. Tranel Notes 3/29/16-4/5/16 Magma composition and names: Dark color -> -> Light color Basalt Andesite Dacite Rhyolite Mafic (less silica) intermediate intermediate Felsic (more silica) Hot (1200-1400ºC) Cool (600-1000ºC) low viscosity high viscosity NON-EXPLOSIVE (lava) EXPLOSIVE (ash) Lava Flows: Rhyolitic Andesitic Basaltic -High Si -Moderate Si, Fe, Mg -High Fe & Mg -Explosive – ash -30° slopes -Non-explosive -600-1000°C -1200-1400°C -45° slopes Viscosity -Resistance to flow Viscosity is controlled by: Colder, felsic, high silica high viscosity (gooey) - Silica content Hot, mafic, lower silica low viscosity (runny) -Silica-poor magma is thin & “runny” -Silica-rich magma is thick & viscous -Temperature -Hot: lower -Cool: higher -Volatile Content Magma has <10% dissolved gas = Volatile Content (H2O, CO2, SO2, Cl) -decrease viscosity -decrease density Igneous Rocks: Intrusive – Cool slowly underground – Crystals grow large Extrusive – Cool quickly at surface – Crystals stay small Intrusive environments: -Magma solidifies beneath the surface. -High heat changes the surrounding rocks and water Extrusive Rocks Lava flows: Lava flows from volcano vent Ash: Fine particles ejected into air & fall to ground Pyroclastic debris: Volcanic blocks; rapidly move down volcano slopes; sudden eruptions Extrusive Environments: Volcanoes -Molten rock surfaces through erupting vent -Mountain built by repeated eruptions GEO207 Natural Disasters Dr. Tranel Notes 3/29/16-4/5/16 LAVA Volcanic Products: Lava Flows: molten rock Pyroclastic materials: ash, pumice, flows Volcanic Gas: carbon dioxide, sulfur Lahars: volcanic mudflows • ash & water Basaltic Lava Flows Andesitic Lava Flows Rhyolitic Lava Flows -Very hot, low silica, low -Lava mounds around vent -Lava plugs the vent to form a viscosity -Outer crust breaks to create dome rubble Higher silica, more -Highest silica, most viscous, *Pahoehoe – Ropy, glassy viscous, moves less rapidly rarely moves texture *A’a’ – Sharp, angular texture Examples where andesitic lava Examples where rhyolitic lava flows are found: flows are found: Examples where basaltic lava -Convergent boundaries -Convergent boundaries flows are found: -Continental Rifts -Mid-ocean ridges -Hotspots -Continental rifts -Convergent boundaries -Hotspots GAS AND OTHER MATERIALS Volcanic Gas- Dissolved gases in magma: higher silica = more gas Aerosols – Liquid droplets or solid particles remaining suspended in air Vesicles – Bubbles of gas travel through magma; bubbles frozen in lava create holes Volcaniclastic deposits: fragmented material ejected from the volcano, called Pyroclastic Debris Volcanic ash – Powdery glass shards Lapilli: smaller chunks that are pea to plum sized material Blocks and bombs: can be sized anywhere from apple to refrigerator dimensioned materials Tephra: Unconsolidated Pyroclastic grains; Ash, Lapilli Tuff : Coherent rock; ash, Lapilli Pyroclastic falls (ash fall) -Occur during explosive volcanic eruptions, ash falls downwind of the volcano Magma fragments freeze to form pyroclastic ash HAZARDS to people: -Breathing in ash – can be deadly -Total darkness – very scary -Roofs collapse – most dangerous problem, most fatalities are people trying to shelter HAZARDS to aircraft: -Engines suck in ash and STOP – not good -Windshields are scratched and break -Turbulence Satellites now watch for ash clouds GEO207 Natural Disasters Dr. Tranel Notes 3/29/16-4/5/16 Volcaniclastic Deposits Lahars – Wet debris flows Pyroclastic flow – Avalanche of hot ash & Lapilli Ignimbrite – Sheet of tuff deposited in pyroclastic flow Pyroclastic flows: Include avalanches, hot pyroclastic material, air, gas, gravity driven, 50-500 km/hr TYPES OF VOLCANOES Volcano Architecture Magma Chamber: Fractured zone where magma pools Conduit: Opening to surface Fissure: Conduit: Vertical crack Crater: Circular depression around vent Caldera: Circular depression formed after collapse of volcano center Four types of Volcanoes -Shield Volcanoes -Cinder Cones -Stratovolcanoes -Calderas Shield Volcanoes -Broad, gentle domes -Formed by: Low viscosity basaltic lava flows, large Pyroclastic sheets *Shield volcano = Erupts continuously Cinder Cones -Cone shaped piles of tephra -30-35o slopes -Symmetrical with deep summit craters Stratovolcanoes -Composite Volcanoes -Large and cone shaped -Alternating lava and tephra frequently explosive, often viscous magma Calderas -Large -Explosive -Felsic Pyroclastic material -Magma chamber roof collapses ERUPTIONS Types of Eruptions Effusive – Lava flows, streams, pools; Basaltic and low viscosity Explosive – Ash clouds, avalanches, pyroclastic flows; Pressure builds from expanding gas How a volcano erupts: Eruption Styles and the Role of Water Content -Concentration of water in magma largely determines peaceful or explosive eruption -Basaltic magma can erupt violently with enough water -Rhyolitic magma usually erupts violently because of high water content, high viscosity (secondary role) GEO207 Natural Disasters Dr. Tranel Notes 3/29/16-4/5/16 Styles of volcanic eruptions -Non explosive Icelandic and Hawaiian; Somewhat explosive Strombolian; Explosive Vulcanian and Plinian What determines eruption style? Viscosity – Lava easily moves away from vent, no pressure build up Gas pressure – If gas is trapped in more viscous material, it escapes more forcefully Environment – Water cools lava on ocean floor more quickly than air on dry land Volcanic Explosivity Index (VEI) -Volume of ash -Column height -Duration Volcanic Activity Extinct Dormant Active Erupted in the past – Will never Have not erupted for 10,000 Are erupting – Have erupted erupt again – Devil’s Tower years – Have potential to erupt recently – Will erupt soon – in future – Yellowstone Kilauea, Mt. Rainier, Mt. St. Helens Volcano Monitoring and Prediction -Geology and mapping; Hazard Maps -Seismology - earthquakes -Gas emission; Fumaroles -Ground deformation (volcano changes shape) -Satellite observation Step 1. mapping previous hazards 1. Compile existing data 2. Geological mapping: examine type & distribution of deposits from past eruptions to predict future Geochemistry - (determine silica content → explosivity) *Geochronology - (age of deposits/eruption rate) Step 2. Seismic monitoring -Baseline monitoring essential to long-term -Ideal seismic networks -6 or more local seismic stations -within 15 km; locate depth of ‘quakes -Magma rises, Rock cracks, Surface cracks Step 3. Volcano deformation -Tilt measurement -Lateral movements -Vertical movements Volcano seismicity: Pre-eruption quakes – swarms – M < 5 – increase in number -close to eruption location; becomes shallower GEO207 Natural Disasters Dr. Tranel Notes 3/29/16-4/5/16 Stratovolcano inflation & intrusion -Magma moves upward -Volcano changes shape fat skinny Mt St Helens *InSAR measurements by satellites detect changes in elevation. *Brighter color = greater elevation change Tilt measurement & GPS -TM - angle of slope -GPS –position -traditional surveying