Презентація на тему «Volcanic Eruptions and Hazards» (варіант 1)

content: Volcanic Eruptions and Hazards

content: What is a volcano?A volcano is a vent or 'chimney' that connects molten rock (magma) from within the Earth's crust to the Earth's surface.The volcano includes the surrounding cone of erupted material.ventconemagma chamberconduit

content: How and why do volcanoes erupt?Hot, molten rock (magma) is buoyant (has a lower density than the surrounding rocks) and will rise up through the crust to erupt on the surface.Same principle as hot air rising, e.g. how a hot air balloon worksWhen magma reaches the surface it depends on how easily it flows (viscosity) and the amount of gas (H2O, CO2, S) it has in it as to how it erupts.Large amounts of gas and a high viscosity (sticky) magma will form an explosive eruption!Think about shaking a carbonated drink and then releasing the cap.Small amounts of gas and (or) low viscosity (runny) magma will form an effusive eruptionWhere the magma just trickles out of the volcano (lava flow).

content: Explosive EruptionsMt. RedoubtExplosive volcanic eruptions can be catastrophicErupt 10's-1000's km3 of magmaSend ash clouds >25 km into the stratosphereHave severe environmental and climatic effectsHazardous!!!Above: Large eruption column and ash cloud from an explosive eruption at Mt Redoubt, Alaska

content: Three products from an explosive eruptionAsh fallPyroclastic flowPyroclastic surgeExplosive EruptionsPyroclastic flows on Montserrat, buried the capital city.

content: Direct measurements of pyroclastic flows are extremely dangerous!!!

content: Effusive EruptionsEffusive eruptions are characterised by outpourings of lava on to the ground.Hawaii

content: Practical Exercise 1.What controls the violence of an eruption?How fast is magma ejected out of the volcano?

content: Volcano Monitoring and Hazard Mitigation

content: 92,000 Tambora, Indonesia 181536,000 Krakatau, Indonesia 188329,000 Mt Pelee, Martinique 190215,000 Mt Unzen, Japan 1792Volcanic FatalitiesBut, volcanoes cause fewer fatalities than earthquakes, hurricanes and famine.Courtesy of www.swisseduc.ch

content: Pyroclastic flowLahars/Mud flowsPyroclastic fallLava flowNoxious GasEarthquakesVolcanic HazardsCourtesy of www.swisseduc.ch

content: Pyroclastic FlowFor example, eruption of Vesuvius in 79 AD destroyed the city of Pompeii

content: Pompeii (79AD)On August 24, 79AD Mount Vesuvius literally blew its top, erupting tonnes of molten ash, pumice and sulfuric gas miles into the atmosphere. Pyroclastic flows flowed over the city of Pompeii and surrounding areas.

content: Pompeii (79AD)Pyroclastic flows of poisonous gas and hot volcanic debris engulfed the cities of Pompeii, Herculaneum and Stabiae suffocating the inhabitants and burying the buildings.

content: Pompeii (79AD)The cities remained buried and undiscovered for almost 1700 years until excavation began in 1748. These excavations continue today and provide insight into life during the Roman Empire.

content: Vesuvius todayVesuvius remains a hazardous volcano with heavily populated flanks:around 1.5 million people live in the city of Naples aloneNaples is situated approx. 30 km from VesuviusPyroclastic flows can flow up to 100 km from source!Bay of NaplesVesuviusNaplesCourtesy of www.swisseduc.ch

content: An eruption of Mt Peleé in 1902 produced a pyroclastic flow that destroyed the city of St. Pierre.beforeafterMt Peleé, Martinique (1902)

content: 29,000 people died…. Only 2 survived! Why?

content: How do pyroclastic flows cause devastation?

content: Pyroclastic Flow - direct impactCourtesy of www.swisseduc.ch

content: Pyroclastic Flow - burial

content: Pyroclastic Flow - burns

content: Pyroclastic Flow - laharsHot volcanic activity can melt snow and iceMelt water picks up rock and debrisForms fast flowing, high energy torrentsDestroys all in its path

content: Pyroclastic FallAsh loadCollapses roofsBrings down power linesKills plantsContaminates water suppliesRespiratory hazard for humans and animals

content: Lava FlowIt is not just explosive volcanic activity that can be hazardous. Effusive (lava) activity is also dangerous.

content: Lava Flow - Heimaey, IcelandIceland, January 23,1973.Large fissure eruption threatened the town of Vestmannaeyjar.

content: Lava Flow - Heimaey, IcelandThe lava flows caught the inhabitants by surpriseBefore the eruption was over, approximately one-third of the town of Vestmannaeyjer had been destroyed

content: Lava Flow - Heimaey, IcelandHowever, the potential damage was reduced by spraying seawater onto the advancing lava flows.This caused them to slow and/or stop, or diverted them away from the undamaged part of the town.

content: Practical Exercise 2.Assessing Volcanic Hazards

content: So….How do we minimize the risk of active volcanoes?

content: Volcano MonitoringVolcano Observatories are set up on all active volcanoes that threaten the human population. These are designed to monitor and potentially to predict the eruptive behaviour of the volcano in question.

content: Volcano MonitoringSeismicityDeformationGas Output(on volcano and remote sensing techniques)These three things are the most important precursors to an eruption.

content: Seismic ActivityEarthquake activity commonly precedes an eruptionResult of magma pushing up towards the surfaceIncrease volume of material in the volcano shatters the rockThis causes earthquakes

content: Seismic ActivityEarthquake activity is measured by SeismographsSeismographs are stationed on the flanks of the volcanoThese record the frequency, duration and intensity of the earthquakes and report it back to the volcano observatory.

content: Deformation Monitoring“Tiltmeters” are used to measure the deformation of the volcanoThe tiltmeters measure changes in slope as small as one part per million. A slope change of one part per million is equivalent to raising the end of a board one kilometer long only one millimeter!

content: Deformation MonitoringTilltmeters can tell you when new material enters the magma chamber.Note the presence of earthquakes in relation to the deformation. Often it is a combination of events that fore-warns of an eruption.AB

content: Gas MonitoringCommonly gas output from a volcano increases or changes composition before an eruption.As magma rises to the surface it releases (exsolves) much of its gas content.This can be measured

content: Gas MonitoringGas samples are collected from fumaroles and active vents.Gas levels may also be monitored by remote sensing techniques

content: In Summary..Volcanoes are extremely hazardous.However, the volcano can be studied, monitored and understood.Each volcano is different, and offers a unique set of dangersPlans may be emplaced to help control potential damage.

content: Post-lecture Question:What should geologists do about volcanic eruptions in the future?Study volcanoes to find out more about how and why they eruptMonitor the volcanoesDevelop hazard mitigation plansUnderstand the population around volcanoes, i.e. why do people choose to live near volcanoes?Education

content: Additional (Optional) Material

content: Are there other volcano related hazards?

content: Noxious Gas1,700 people living in the valley below Lake Nyos in northwestern Cameroon mysteriously died on the evening of August 26, 1986.

content: Noxious GasLake Nyos is a crater lake inside a dormant volcano.The lake had become laden with carbon dioxide gas.This gas had suddenly bubbled out of the lake and asphyxiated nearly every living being in the surrounding valley.

content: Noxious GasA management plan has been developed to remove gas from the lake to prevent a further tragedy.An artificial vent to the lake surface was created with pipe.Water is pumped from the bottom of the lake to the surface through the pipe, where it can degas.

content: Noxious Gas

content: Noxious Gas

content: Noxious GasThe Lake Nyos incident was not unique.Two years earlier, Lake Monoun, 60 miles to the southeast, released a heavy cloud of toxic gas, killing 37 people.A third lake, Lake Kivu, on the Congo-Rwanda border in Central Africa, is also known to act as a reservoir of carbon dioxide and methane, which is a valuable natural gas that is gathered from the lake and used locally.

content: EarthquakesLarge volumes of magma moving through the shallow crust can cause large earthquakes.This can lead to building collapse, slope failure and avalanches

content: EarthquakesDestruction after a volcanic induced earthquake in Japan