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

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Volcanic Eruptions and Hazards
What is a volcano?
vent
cone
magma chamber
conduit
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).
Explosive Eruptions
Mt. Redoubt
Above: Large eruption column and ash cloud from an explosive eruption at Mt Redoubt, Alaska
Three products from an explosive eruptionAsh fallPyroclastic flowPyroclastic surge
Explosive Eruptions
Pyroclastic flows on Montserrat, buried the capital city.
Direct measurements of pyroclastic flows are extremely dangerous!!!
Effusive Eruptions
Effusive eruptions are characterised by outpourings of lava on to the ground.
Hawaii
Practical Exercise 1.
What controls the violence of an eruption? How fast is magma ejected out of the volcano?
Volcano Monitoring and Hazard Mitigation
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
Pyroclastic flowLahars/Mud flowsPyroclastic fallLava flowNoxious GasEarthquakes
Volcanic Hazards
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Pyroclastic Flow
For example, eruption of Vesuvius in 79 AD destroyed the city of Pompeii
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.
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.
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.
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
Vesuvius
Naples
Courtesy of www.swisseduc.ch
An eruption of Mt Peleé in 1902 produced a pyroclastic flow that destroyed the city of St. Pierre.
before
after
Mt Peleé, Martinique (1902)
How do pyroclastic flows cause devastation?
Pyroclastic Flow - direct impact
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Pyroclastic Flow - burial
Pyroclastic Flow - burns
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
Pyroclastic Fall
Ash loadCollapses roofsBrings down power linesKills plantsContaminates water suppliesRespiratory hazard for humans and animals
Lava Flow
It is not just explosive volcanic activity that can be hazardous. Effusive (lava) activity is also dangerous. 
Lava Flow - Heimaey, Iceland
Iceland, January 23,1973.Large fissure eruption threatened the town of Vestmannaeyjar.
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
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.
Practical Exercise 2.
Assessing Volcanic Hazards
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.
Volcano Monitoring
SeismicityDeformationGas Output (on volcano and remote sensing techniques)
These three things are the most important precursors to an eruption.
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
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.
Deformation Monitoring
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.
A
B
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
Gas Monitoring
 Gas samples are collected from fumaroles and active vents.  Gas levels may also be monitored by remote sensing techniques
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.
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?Education
Additional (Optional) Material
Are there other volcano related hazards?
Noxious Gas
1,700 people living in the valley below Lake Nyos in northwestern Cameroon mysteriously died on the evening of August 26, 1986.
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.
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.
Noxious Gas
Noxious Gas
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.
Earthquakes
Large volumes of magma moving through the shallow crust can cause large earthquakes.This can lead to building collapse, slope failure and avalanches
Earthquakes
Destruction after a volcanic induced earthquake in Japan
If you release the pressure of a magma chamber (by cracking the surrounding rock or breaking through to the surface) the gas dissolved in the magma will start to exsolve (separate from the melt forming bubbles). These bubbles, called vesicles, rapidly expand and rise through the magma. (Think about shaking up a bottle of carbonated drink to build up the pressure and then taking the top off the bottle to quickly release the pressure - what happens?)The rapid escape of gas (volatiles) causes magma to fragment and erupt explosively.Presenter: read through the facts about explosive eruptions. Give examples of the volumes and distances. For example, the local swimming pool might be approximately 25km from the school. So get the children to think about how far that is to drive from the school to the pool and then imagine that distance going straight up into the air!
See worksheet: Volcanoes, Exercise 1.
Volcanoes can be extremely hazardous natural phenomena. That is why it is important to understand as much as possible about them, in an attempt to understand the processes involved in eruptions and try to minimize the risk to life and property.
Many volcanoes are found in heavily populated areas. Volcanic soil is very fertile and rich in minerals so people move on to the sides of volcanoes to plant crops and graze livestock.This puts them in great danger if there is an eruption.Large individual volcanic eruptions can causes numerous fatalities. **CLICK FOR POP-UP** However, such large, catastrophic eruptions occur relatively infrequently so volcanoes causes less fatalities than earthquakes, hurricanes and famine.Note: there are many small-scale eruptions occurring all over the world every day. But most are either in remote locations, are too small to cause too much damage or are well-managed and only a minimal risk to humans.
There are many hazards associated with volcanic eruptions: see list. (these are presented in turn in following slides)
These ancient cities remained buried and undiscovered for almost 1700 years until excavation began in 1748. These excavations continue today and as entire cities are preserved as they were on the day in 79AD when they were hit by the pyroclastic flows from Vesuvius, they provide a uniquely preserved snapshot of Roman life and culture.
Mt Vesuvius is still classed as an Active Volcano.The major Italian city of Naples lies approximately 30 km from the volcano vent. 1.5 million people live in Naples and as pyroclastic flows can travel as far as 100km from source, the city is built well within the danger zone! Will there be a repeat of what happened to Pompeii in 79AD?
We have seen the effect of pyroclastic flows entering a town. But what are the exact processes that cause the damage?
The debris left behind from a passing pyroclastic flow can bury structures.These example are from a 1997 eruption of the Soufrière Hills volcano on Montserrat. The pyroclastic flows produced buried the capital city of Plymouth. The white clock tower in the top photo was on the top of the town hall, which used to stand several storeys above street level.
Pyroclastic flows have temperatures commonly in excess of 400 degrees Celsius. Hot enough to burn forests and wooden structures.Here we see burnt trees after an eruption from Mt St Helens in the USA.
Moving on from pyroclastic flows, there are other hazards associated with volcanic eruptions. Such as Pyroclastic Fall:An explosive eruption will produce an eruption column of hot gas, ash and debris ejected kilometres into the air. As this debris falls back down to the ground it can cause a lot of damage.Like too much snow on a roof, too much ash raining down from an eruption column can cause the roof to collapse.Ash loading on power lines will cause them to fall.As little as 1 centimetre of ash accumulated on the leaves of a plant will stop it from being able to photosynthesize and therefore the plant will die.Lots of fine ash falling in lakes, rivers and water reservoirs will cause contamination making it unfit to drink, or to live in if you are a fish etc.Very fine ash particles, if inhaled by humans, can cause extensive damage to the lungs causing a respiratory disease called silicosis.
The people eventually stopped the advance of the lava flows by pumping seawater from the nearby harbor and spraying it on to the lava flows. This caused the front of the lava flow to cool quickly and stop moving.  This formed a barrier for the lava behind it.The residents of the town were able to stop and (or) divert the lava from the rest of the town.
See Volcanoes Exercise 2.
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.Changes in the seismic activity (especially an increase) may forecast an eruption.
When magma rises towards the surface the decrease in pressure causes it to lose some of its gas content. As gas is released from the magma it often vents at the surface, leaking out of small cracks in the ground or from the large volcanic vent.A dormant volcano will commonly vent gas even when there is no eruption going on. This is because the magma is deep down in the crust, still releasing gas but not in the position to erupt at the surface.A change in the amount of gas or the chemistry of the gas being released is another precursor to an eruption. An increase in the amount of magma in the chamber will produce an increase in the amount of gas. Also, the new magma may have a slightly different chemical composition to the old magma and therefore release different abundances of gas types (CO2, SO2, H2O). For example, an increase in the ratio of carbon to sulfur can be used to indicate the arrival of a new batch of magma at the summit reservoir.
Gas (volatile) composition and abundance can be measured directly from the volcano by gathering samples from the vents and fumeroles.Or, using remote sensing techniques. For example, the amount of sulfur dioxide (SO2) released by a volcano can be measured indirectly by a correlation spectrometer or COSPEC. The spectrometer compares the light coming through the volcanic plume to a known spectra of sulfur dioxide, thereby measuring the SO2 levels in the plume. 
Presenters: Ask the students this question. Get them to list 5 things geologists can do to help understand and mitigate the hazards of future volcanic eruptions.See if they can come up with the same answers as provided (or perhaps they can think of more!).