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Chapter 4

Chapter 4 - Volcanoes

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Rob Milne

Risks and Disasters – Chapter 4  Mount Unzen, 1991 o Most destructive of Japan’s 19 active volcanoes o 200 years ago – erupted and killed 15 000 people o June 3, 1991 – erupted forced 12 000 people to evacuate  End of 1993 – produced 0.2 km3 of lava and more than 10 000 superheated flows of hot gas, ash and large rock fragments  Produced lahars, special channels constructed to contain them  Introduction to Volcanoes o Located near plate boundaries o 2/3 of all active volcanoes on land located along Ring of Fire o Volcanoes lie above subduction zones bordering Pacific, Nazca, Cocos, Philppine and Juan de Fuca plates o Magma  Composed of melted silicate minerals and dissolved gasses  Contains:  Oxygen (O)  Silicon (Si)  Aluminium (Al)  Iron (Fe)  Magnesium (Mg)  Calcium (Ca)  Sodium (Na)  Potassium (K)  Si and O most abundant, creating silica 2SiO )  Volcanic rocks  Basalt – 45-52% SiO (most common) 2  Andesite – 52-63% SiO2  Dacite – 63-68% SiO2  Rhyolite – >68% SiO2(most rare) o Volcano types  Different shapes and eruptive styles based on viscosity of magma  Magma with higher silica content are more viscous than magma with low silica content  Low silica content tend to explode (basalt/andesite)  High silica content will flow (rhyolite/dacite) Silica Volcano Type Shape Content Viscosity Rock type Eruption Type Shield Volcano Gentle arch, Low Low Basalt Lava flows, some dome shaped explosive activity Cone shaped Andesite or Combination of flow Composite Volcano with steep Intermediate Intermediate sides dacite and explosive Volcanic Dome Dome shaped High High Rhyolite Highly explosive Steep cone, Cinder Dome with summit low low Basalt Explosive crater  Maars  Interaction between groundwater and magma  Conversion into steam creates violent explosion forms a crater similar to a meteorite impact  Ice-contact volcanoes 1 Risks and Disasters – Chapter 4  Sublgacial volcanoes  Produces huge floods due to large quantities of ice being melted  Odd shapes due to rapid chilling of lava and pyroclastic material as it comes into contact with water and ice o Volcanic Features (general overview)  Craters, calderas and vents  Craters – depressions at tops of volcanoes formed by explosion or collapse of summit area  Calderas – circular or oval depressions that form during explosive ejection of magma and subsequent collapse  Volcanic vents – openings through which lava and pyroclastic debris erupt o Some circular, some long cracks (fissures)  Hot springs and geysers  Hot spring – hot water that gets discharged when ground water comes into contact with hot rocks  Geysers – groundwater that boils in underground chamber to produce periodic releases  Resurgent calderas and supereruptions  Super eruptions are very rare, extremely violent eruptions  Occurs when large volume of magma rises to shallow depths in curst over mantle hot spot  Resurgent calderas – sites that are still capable of eruptions due to magma being present beneath calderas floor o Volcanoes and plate tectonics (general overview)  Subduction zones  Oceanic crust is denser and covered by thick wet sediment  Stratovolcanoes occur at subduction zones, o Most common type of volcano found around the pacific rim  Mid-ocean ridges and continental drift zones  Plates move away from one another, magma mixes very little with other materials, made entirely of low-viscosity basalt  Hot spots beneath the ocean  Can also be found where hot mantle material wells up beneath plate at a stationary point rather than boundary  Upwelling mantle material creates volcano o Creates a series of volcanoes, rather than just one  Hotspots beneath continents  Caldera-forming eruptions occur in tectonic setting, associated with dcitic and rhyolitic explosions (very explosive)  Geographic regions with active volcanoes o Most volcanoes along the ring of fire o Highest risk of volcanic activity is in the mountainous regions of the northwestern side of N.A o Young volcanoes in Canada restricted to BC and southern Yukon o Volcano that poses greatest threat to Canada is Mount Baker in N. Washington  Eruption could spread ash over Vancouver, Abbotsford, Chilliwack and other cities  Ash would paralyze air and ground traffic  Would form large lahars, melt water/ice on summit  Overflow water in Baker Lake and cause dam to fail  Effects of volcanoes o 50-60 volcanoes erupt worldwide 2 Risks and Disasters – Chapter 4 o Usually located in sparsely populated areas, very little damage o Past 100 years, 100 000 people killed  Hazards associated with volcanoes o Primary (direct effects)  Lava flows, ash fall, volcanic bombs, pyroclastic flows, pyroclasitc surges, lateral blasts and poisonous gases o Secondary (indirect effects)  Lahars, debris avalanches, landslides, groundwater/surface water contamination, floods, fires, tsunami o Eruptions can cool atmosphere for year or so o Volcanic Explosivity index – quantifiable index of volcanic eruption size  Volume of material erupted  Height of eruption cloud  Duration of main eruptive phase o Lava flows  Most common products of volcanic activity  Happens when magma erupts from volcanic vents  Three major types – basaltic, andesitic and rhyolitic  Can be fluid/rapid or slow/viscous  Basaltic lavas  Low viscosity  Harden with a smooth ropy texture (pahoehoe)  Cooler temperature basaltic lavas o Harden with rough, blocky texture (aa) o Lateral blasts  Gas, ash and rock fragments can be blown horizontally from side of mountain at start of eruption  Ejects at tremendous speeds and very destructive o Pyroclastic flows and surges  Most lethal phenomena  Avalanches of hot gas, ash and rock fragments  Can move at speeds up to 150 km/h  Destroy buildings and objects by impact, burial or incineration  No chance of survival  1902 – Mount Pelee killed 30 000 people  Pyroclastic surge – explosive interaction of water and magma, more dangers than flows, 360 km/h o Ash fall  Finely broken rock and gas blown into atmosphere during explosions  Carried downwind and rans out to produce ash fall  Carpets hundreds – thousands of square km of land  Creates hazards  Vegetation may be destroyed, can be beneficial in long-term, enriches soil and increases soil moisture  Surface water may be contaminated, may kill fish by clogging gills o Increase acidity of water  Buildings may collapse due to added weight  Respiratory illnesses  Can damage mechanical and electrical equipment  Affects flying aircrafts 3 Risks and Disasters – Chapter 4 o Poisonous gases  Number of gases – water, carbon dioxide, carbon monoxide, sulphur dioxide, hydrogen sulphide, chlorine and hydrofluoric acid  Can reach toxic levels  Lake Nyos – deep crater lake with dormant volcano, released misty cloud of carbon dioxide glass killing 1742 people and estimated 3000 cattle  Degassing pipes installed to help release some of the carbon dioxide in the air  Sulphur dioxide can react with water to produce sulphuric acid -> acid rain  Damages crop, increases acidity in water levels  Volcanoes produce smog (vog) o Edifice or sector collapse  Flank of volcano may collapse during eruption  Can increase size and strength of an eruption  As magma moves up, inflates like a balloon and slopes become oversteepened and unstable during magma’s ascent  If large collapse, known as a sector collapse o
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