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VOLCANOES www.geolsoc.org.uk/volcanoes Arthur’s Seat from Blackford Hill Image copyright: Rob Purvis (Wikimedia Commons) A volcano is a rupture in the Earth’s crust which allows magma/ash/gases to escape from beneath the surface. When magma reaches the surface of the Earth, it is called lava. Most volcanoes occur along or near the margins of tectonic plates, where plates move away from each other or collide. At divergent plate boundaries, oceanic plates move away cools to form new crust. That means most volcanic activity occurs under the sea. Where there is a lot of activity from each other and hot magma rises and, volcanic material can build up above sea level, forming an island such as Iceland. At convergent plate boundaries, oceanic “lithosphere” (the crust and upper mantle) sinks back into the mantle, often forming a deep ocean trench. The volcanoes that can form near to these trenches are amongst the most dangerous on Earth, because they are typically explosive and many are located near populated coastlines. In the middle of plates volcanoes sometimes occur. Some geologists think this is because of hot mantle plumes, which create chains of volcanoes as the plate moves over them – but not everyone agrees! The Hawaiian islands are the best known example – the Big Island of Hawaii rises 10 kilometres above the ocean floor. Volcanoes in the UK? Great Britain no longer has active volcanoes as we are now in a tectonically quiet part of the world. But we had an eruptive past. You can still see evidence of volcanoes in the Lake District, Snowdonia, NW Scotland and Northern Ireland, from eruptions that happened millions of years ago. Arthur’s Seat in Edinburgh is part of a 350 million year old volcano. DID YOU KNOW? Ash blasted into the atmosphere during an eruption may cause intense thunder and lightning. Major volcanic eruptions eject sulphurous gases high into the atmosphere, causing temporary cooling at the Earth’s surface. Volcanoes also emit carbon dioxide - but only about 1% of what human activities have. VOLCANOES www.geolsoc.org.uk/volcanoes What comes out of volcanoes? Significant historical eruptions Volcanic eruptions are classified on a numerical scale called the Volcanic Explosivity Index (VEI), from 0 to 8 Year Location 1793 on scale the Volcanic eruptionsare areclassified classified onaanumerical numerical scalecalled called theVolcanic Volcanic Eight month long eruption; Europe-wide Iceland (LakiVolcanic fissue)eruptions Explosivity ExplosivityIndex Index(VEI), (VEI),from from00to to88 environmental disaster. A fifth of Iceland’s Year Location Major features population was killed mostly as a result of famine. Year Location Major features 1815 1883 1902 1980 1985 1991 Major features Significant Significanthistorical historicaleruptions eruptions 1783 1783 Iceland Iceland (Laki (Lakifissure) fissure) Eight Eightmonth monthlong longeruption; eruption; environmental Europe-wide environmental Indonesia (Tambora, Sumbawa The largest eruptionEurope-wide in recorded history: Magnitude disaster. disaster.AAfifth fifthof ofIceland’s Iceland’s Island) 7. It triggered the “Year without summer” population was mostly population wasakilled killed mostlyin 1816 as a result of famine. as a result of famine. 1815 Sumbawa largest in 1815 Indonesia Indonesia (Tambora, Sumbawa The largest eruption in (which killed Indonesia (Krakatau, between(Tambora, Massive pyroclasticThe flows anderuption tsunamis; Island) recorded Island) recordedhistory: history:Magnitude Magnitude7.7. Java and Sumatra) around 36,000 people) smallerthe waves recorded ItIttriggered “Year without triggered the “Yeareven without aasummer” summer”in in1816 1816 in the English Channel. 1883 Massive 1883 Indonesia Indonesia(Krakatau, (Krakatau,between between Massivepyroclastic pyroclasticflows flowsand and Java and Sumatra) tsunamis; (which killed around Java and Sumatra) tsunamis; (which killed around 29 000 inhabitants 36,000 of St Pierre killed in seconds by Martinique (St Pierre) 36,000people) people)smaller smallerwaves waves recorded in the English pyroclastic flows. even even recorded in the English Channel. Channel. 1902 Martinique Pierre) 29 inhabitants of Pierre 1902 Martinique(St (StAn Pierre) 29000 000an inhabitants ofSt Stled Pierre eruption triggered by earthquake to the USA (Mount St Helens) killed killedin inseconds secondsby bypyroclastic pyroclastic largest debris avalanche in recorded history. Covered flows. flows. 1980 USA (Mount St Helens) An eruption triggered by an State inAnfine grey dust. 1980 USA (Mount StWashington Helens) eruption triggered by an earthquake earthquakeled ledto tothe thelargest largest debris avalanche in debris avalanche inrecorded recorded Sudden melting of the icecap on the volcano’s Columbia (Nevada del Ruiz) history. history.Covered CoveredWashington Washington summit produced mudflows killing 23 000 people State grey Statein infine fine greydust. dust. 1985 Sudden 1985 Columbia Columbia(Nevada (Nevadadel delRuiz) Ruiz) Suddenmelting meltingof ofthe theicecap icecap on summit with pyroclastic Philippines (Mount Pinatubo) Major explosive eruption onthe thevolcano’s volcano’s summit falls and produced producedmudflows mudflowskilling killing23 23 ash falls. 000 000people people 1991 Major 1991 Philippines Philippines(Mount (MountPinatubo) Pinatubo) Majorexplosive explosiveeruption eruptionwith with pyroclastic pyroclasticfalls fallsand andash ashfalls. falls. Did Didyou youknow? know? DID YOU KNOW? Despite Despitethe thedanger, danger,millions millions of people live of people liveclose closeto toactive active Despite the danger, volcanoes, millions of sometimes people volcanoes,sometimes live close to active volcanoes, through throughlack lackof ofchoice. choice. Chemical weathering sometimes through lack of choice. Chemical weatheringof of deposits volcanic depositscan canrelease release Chemical weatheringvolcanic of volcanic minerals that become aa minerals that become deposits can release minerals that natural fertilizer. One natural fertilizer. One become a natural fertilizer. One intensely example exampleisisthe the intensely example is the intensely cultivated area cultivated area cultivated areaaround around Italy, which around Naples, Italy,Naples, which has the Naples, Italy, whichhas hasthe the eruptions of Mount eruptionstoofthank Mount eruptions of Mount Vesuvius to thank for Vesuvius tofruit thank forits its for its rich soils and Vesuvius top quality and rich soils and top quality rich soils and top quality vegetables. fruit fruitand andvegetables. vegetables. Lava: the chemical composition of the lava will determine whether an eruption is explosive or more gentle. When gases can escape easily the eruption will be less violent What Whatcomes comesout outof ofvolcanoes? volcanoes? Explosive particles: hot fragments Lava: of Lava:the the chemical composition ofthe the of chemical lava thatcomposition are ejected by lava will determine whether an lava will determine whether an volcanoes cool as they fall eruption eruptionisisexplosive explosiveor ormore moregentle. gentle. through the air. These ‘pyroclasts’ When gases can escape easily When gases can escape easilythe the range in less size from very large eruption will eruption willbe be lessviolent violent bombs and blocks to ash or Explosive Explosiveparticles: particles:hot hotfragments fragmentsof of tephra lava lavathat thatare areejected ejectedby byvolcanoes volcanoes cool coolas asthey theyfall fallthrough throughthe theair. air. Pyroclasticrange flows:ininasize mixture These from These‘pyroclasts’ ‘pyroclasts’ range size fromof hot very and to verylarge large bombs andblocks blocks toash ash orat gasbombs and tephra that can flowor tephra tephrahurricane velocity. It was pyroclastic from Pyroclastic flows: mixture of hot Pyroclastic flows:aaflows mixture ofVesuvius hot gas tephra that can flow wiped gasand andthat tephra thatout canPompeii flowat at and hurricane velocity. pyroclastic hurricane velocity.ItItwas was pyroclastic Herculaneum in AD79. flows flowsfrom fromVesuvius Vesuviusthat thatwiped wipedout out Pompeii Pompeiiand andHerculaneum Herculaneumin inAD79. AD79. Lahars: Slurry flows of ash and Lahars: Slurry ash Lahars:tephra, Slurryflows flows offlow ashand and a whichof from tephra, which from aavolcano. tephra,volcano. whichflow flow from volcano. They have roughly the They consistency Theyhave haveroughly roughlythe consistencyof of consistency the of concrete. concrete. concrete. Can weCan predict an eruption? we an Can wepredict predict aneruption? eruption? Because Becausethey theyare arehazardous, hazardous, Becausevolcanoes they are hazardous, volcanoes volcanoesneed needto tobe bemonitored. monitored. need toUnlike be monitored. Unlike earthquakes, ititisisearthquakes, possible Unlike earthquakes, possibleto to predict a volcanic eruption. it is possible to predict a volcanic predict a volcanic eruption. eruption. Earthquake (seismometers) Earthquake sensors (seismometers) Earthquake sensorssensors (seismometers) and and gravity meters can track the and gravity meters can track the gravity meters can track the movement of movement magma and movementof andgas gasto tothe the magmasurface. and gas to of themagma surface. surface. The of be Theswelling swelling ofaavolcano volcano can be The swelling of a volcano can becan monitored monitored using tiltmeters, laser monitored using tiltmeters, laser using tiltmeters, laser rangefinders and rangefinders rangefindersand andglobal globalpositioning positioning global positioning system (GPS) equipment. system (GPS) equipment. system (GPS) equipment.All Allthis this All this technology technology means prediction means prediction technology means predictionisis is more ––but still gettinggetting more accurate – but is still getting moreaccurate accurate butisisnot still not perfect. perfect.not perfect. Soufriere Hills volcano in Montserrat erupting in Soufriere Hills volcano in Montserrat Soufriere Hills volcano in Montserrat eruptingerupting in 1995.in 1995. Image Voight Imagecopyright: copyright:Barry Voight Image1995. copyright: Barry Voight Barry (Wikimedia Commons). (Wikicommons). (Wikicommons).
