Презентація "Volcanic Eruptions and Hazards"

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Volcanic Eruptions and Hazards

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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.
magma chamber

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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 works
When 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 eruption
Where the magma just trickles out of the volcano (lava flow).

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Explosive Eruptions
Mt. Redoubt
Explosive volcanic eruptions can be catastrophic
Erupt 10’s-1000’s km3 of magma
Send ash clouds >25 km into the stratosphere
Have severe environmental and climatic effects
Above: Large eruption column and ash cloud from an explosive eruption at Mt Redoubt, Alaska

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Three products from an explosive eruption
Ash fall
Pyroclastic flow
Pyroclastic surge
Explosive Eruptions
Pyroclastic flows on Montserrat, buried the capital city.

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Direct measurements of pyroclastic flows are extremely dangerous!!!

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Effusive Eruptions
Effusive eruptions are characterised by outpourings of lava on to the ground.

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

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Volcano Monitoring and Hazard Mitigation

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92,000 Tambora, Indonesia 1815
36,000 Krakatau, Indonesia 1883
29,000 Mt Pelee, Martinique 1902
15,000 Mt Unzen, Japan 1792
Volcanic Fatalities
But, volcanoes cause fewer fatalities than earthquakes, hurricanes and famine.
Courtesy of www.swisseduc.ch

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Pyroclastic flow
Lahars/Mud flows
Pyroclastic fall
Lava flow
Noxious Gas
Volcanic Hazards
Courtesy of www.swisseduc.ch

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Pyroclastic Flow
For example, eruption of Vesuvius in 79 AD destroyed the city of Pompeii

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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.

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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.

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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.

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

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

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

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How do pyroclastic flows cause devastation?

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Pyroclastic Flow - direct impact
Courtesy of www.swisseduc.ch

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Pyroclastic Flow - burial

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Pyroclastic Flow - burns

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Pyroclastic Flow - lahars
Hot volcanic activity can melt snow and ice
Melt water picks up rock and debris
Forms fast flowing, high energy torrents
Destroys all in its path

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Pyroclastic Fall
Ash load
Collapses roofs
Brings down power lines
Kills plants
Contaminates water supplies
Respiratory hazard for humans and animals

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Lava Flow
It is not just explosive volcanic activity that can be hazardous. Effusive (lava) activity is also dangerous.

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

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Lava Flow - Heimaey, Iceland
The lava flows caught the inhabitants by surprise
Before the eruption was over, approximately one-third of the town of Vestmannaeyjer had been destroyed

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Lava Flow - Heimaey, Iceland
However, 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.

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Practical Exercise 2.
Assessing Volcanic Hazards

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How do we minimize the risk of active volcanoes?

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Volcano Monitoring
Volcano 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.

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

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Seismic Activity
Earthquake activity commonly precedes an eruption
Result of magma pushing up towards the surface
Increase volume of material in the volcano shatters the rock
This causes earthquakes

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Seismic Activity
Earthquake activity is measured by Seismographs
Seismographs are stationed on the flanks of the volcano
These record the frequency, duration and intensity of the earthquakes and report it back to the volcano observatory.

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Deformation Monitoring
“Tiltmeters” are used to measure the deformation of the volcano
The 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!

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Deformation Monitoring
Tilltmeters 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.

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Gas Monitoring
Commonly 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

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

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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 dangers
Plans may be emplaced to help control potential damage.

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Post-lecture Question:
What should geologists do about volcanic eruptions in the future?
Study volcanoes to find out more about how and why they erupt
Monitor the volcanoes
Develop hazard mitigation plans
Understand the population around volcanoes, i.e. why do people choose to live near volcanoes?

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Additional (Optional) Material

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Are there other volcano related hazards?

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Noxious Gas
1,700 people living in the valley below Lake Nyos in northwestern Cameroon mysteriously died on the evening of August 26, 1986.

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Noxious Gas
Lake 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.

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Noxious Gas
A 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.

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Noxious Gas

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Noxious Gas

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Noxious Gas
The 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.

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

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Destruction after a volcanic induced earthquake in Japan