Vulcanicity

Vulcanicity

1. Case Studies

   • Mt Nyiragongo
     • Congo, January 2002
     • Associated with African rift valley
     • Lava is very fluid and can travel downhill at >90km
     • Eruption was unexpected
     • Destroyed at least 1/3 of Goma
       • Over 200,000 inhabitants
       • Water and power supplies destroyed as were medical facilities
       • Abandoned properties were looted
     • Death toll = 147
       • Over 350,000 had to flee
       • Thousands required medical attention
     • 'Red Alert' issued for Goma enabling evacuation
       • UN humanitarian aid brought in 2 days after eruption
       • Camps were set up
   • Mt Etna
     • Sicily 1991-93
     • Lava began to pour from Eastern side of volcano late 1991
       • Quickly advanced towards settlement of Zafferana
         • Protective methods had to be put in place
           • Large earth barrier to hold back lava for several months
             • Lava spilled over Spring 1992
               • Small barriers put in place across valley
                 • Concrete blocks were dropped from helicopters to block the lave flow
                   • Engineers blasted openings to create a new direction of flow
                     • Eruption ended 10 months later in early 1993
     • In 2002 a serious eruption occurred
       • Ski village was destroyed and clouds of ash rained down on the city of Catania
   • Soufriere Hills
     • Montserrat 1995-97
     • Steam and ash reached over 2,500m
     • March 96, ash cloud, dome growth and small pyroclastic flows
       • 97, dome growth, small explosions, ballistic projectiles
         • 25th June, large explosions, pyroclastic flows
           • 4-5 millions m3 of material flowed and damaged houses, killing some people
             • Minor activity since
             • Approx 10,000 people evacuated to Antigua
       • Capital buried by 10m of ash and mud
         • New permanent housing being built
         • Capital move to Salem
     • 40% of island safe to live on
       • British government set up exclusion zones

2. Tectonic Theory

   • Wegener
     • 200 millions years ago - pangea stretches pole to pole
       • 180 - pangea splits in Gondwana and Laurasia
         • 65 - major continents moved apart
           • Now - modern day positions
     • Evidence from landforms
       • Continents seemed to fit together
       • Mountain ranges etc lined up
     • Evidence from fossils
       • Fossils from one continent matched fossils from another
       • Glossopteris and Mesosaurus
     • Evidence from climate
       • Some fossils did not match current climate
       • Glacier scratches in South Africa
   • Palaeomagnetism
     • Lava erupts on ocean floor, iron-rich minerals in lava align with magnetic field of earth
       • Lava cools, sets in with alignment
         • Record polarity of earth at time of cooling
           • Showed earth's polarity changes every 400,000 years, new rock settles evenly either side
     • Striped pattern mirrored on either side of mid-atlantic ridge suggests ocean crust is spreading away from boundary
       • Oceanic crust gets older with distance from mid-atlantic ridge
         • Older crust continuously moved by new crust
           • Led to discovery of large subducting plate boundaries - driving drifting of continents

3. Plate Margins

   • Constructive
     • Mid-oceanic plate ridge
       • Can be 2 oceanic places/2 continental plates moving apart
         • Found under ocean
           • Magma escapes from mantle, touches ocean, cools and forms new land
             • Can break water to form new islands eg. Iceland
               • Gentle earthquakes, less violent more constant volcanic eruptions
   • Destructive
     • Convergent
       • Oceanic and continental crust collide
         • Denser oceanic crust subducted
           • Heat from mantle + friction cause oceanic plate to melt in subduction zone
             • Continental plate cannot be destroyed so forced to make fold mountains
               • Oceanic plate becomes less dense, magma rise to surface making volcanoes
                 • Large earth quakes, powerful eruptions
   • Collision
     • 2 continental plates collide
       • Forced up wards and downwards creating fold mountains
         • Large eathquakkes
           • No volcanoes because no plate destroyed
   • Conservative
     • 2 continental plates move side by side
       • No major landforms
         • Crust not being destroyed, no magma
           • Huge earthquakes

4. Features

   • Instrusive
     • When magma forced to surface, a lot of mass doesn't reach that level
     • Molten magma intrudes existing rock, cools, becomes igneous intrusive rock
     • Batholiths
       • Formed deep below surface
         • As magma cools, large crystals formed in rock
           • Dome shaped, exposed by alter erosion
             • 100s km in diatmetre
               • Unaffected by characteristics and structure of existing rocks
     • Dykes
       • Vertical intrusions, horizontal cooling cracks
         • Cut across bedding planes
           • Often occur in groups (dyke swarms)
     • Sills
       • Horizontal intrusions, vertical cooling cracks
         • Commonly made up of dolerite
   • Extrusive
     • Basic (shield)
       • Occur at constructive margins or hotspots
         • Lava flows out central vent
           • Spreads over wide area before solidifying
             • Result = cone with long, gentle sides made up of many layer of lava
     • Acid dome
       • Acid lava solidifies on exposure to air
         • Produces steep sided, convex cone
     • Composite
       • Classic shaped volcano results from alternating eruptions
         • Ash then lava
           • Forms layers
             • Found at destructive boundary
     • Fissure
       • Occur at constructive boundaries
         • Rather than central vent, lava can be ejected from fissures
           • Lava forms large plateaus filling in hollows
     • Caldera
       • Build up of gases, extreme explosion, clears magma chamber, removes summit
         • Side of volcano collapses inwards
     • Ash or cinder cone
       • Ash or cinder builds up into symmetrical cone
         • Found at bottom of shield volcanoes or calderas

5. Eruptions

   • Shield volcanoes
     • When gases escape magma, non-violent eruptions
       • Critical sudden pressure change between vent and atmosphere
         • Type of magma and chemical composition are important
           • When silica is below 55% - gases easily escape, basaltic lava is fluid and very hot, will flow for long distances before cooling to solidify
             • Cooling on the surface forms a skin which red-hot lava flows
               • Supply of magma also important
                 • New magma readily available along constructive margins
                   • Frequent/continuous eruptions keep vents open, easier for lava to pour
   • Composite
     • Lava is viscous so gas cannot escape easily
       • Gas explodes out as it emerges into atmosphere
         • Shatters lava from volcanic bombs to ash and dust
           • Ash and dust can be ejected into upper atmosphere, circulating around the world
             • Violent explosion will carry a mixture of hot lava and ash up to 200kmh
               • As eruption ends, remaining viscous lava oozes out of vent, piles as dome above
                 • Lava in crater piles up, forms a volcanic plug
                   • Needs to be cleared before next eruption