Презентація на тему «Volcanoes: Eruptions and Hazards»

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Volcanoes: Eruptions and Hazards
What is a volcano?
vent
cone
magma chamber
conduit
What causes volcanoes?
The Formation of Magma Mantle rock melts when the temperature increases or the pressure decreases.
What causes volcanoes?
Where Volcanoes Form Tectonic Plate Boundaries!!!
Plates
Earthquakes
Volcanoes
Volcanoes and Earthquakes are found on places where plates meet
What causes volcanoes?
What causes volcanoes?
Hot Spots
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).
How and why do volcanoes erupt?
The Composition of Magma Determines whether it is explosive or not!
  Pyroclastic material Rock fragments created by 		     eruptionsmagma explodes from volcano and solidifies in the airexisting rock is shattered by powerful eruptions
Volcanic blocks
Volcanic bombs
Lapilli
Volcanic ash
What Erupts from a Volcano?
What Erupts from a Volcano?
Lava can be 	thick or thin.
Blocky lava
Pahoehoe
Aa
Pillow lava
How do volcanoes affect the Earth?
Flows and Fallouts hot ash can flow really quicklyKnock down buildingsDam rivers (flooding/drought)Kill crops and livestockClimatic ChangesAsh & Gases can block sunlightDrop average global temperature noticeably
Types of Volcanoes
Shield volcano
Cinder cone volcano
Composite volcano
Craters, Calderas, and Lava Plateau
Crater From explosions of material out of the vent and the collapse of material back into vent
Caldera Much larger depression that forms when magma chamber empties and its roof collapses 
Lava Plateau Forms when lava erupts from long cracks, or fissures, and spreads out evenly (thousands of km)
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
Courtesy of www.swisseduc.ch
Pyroclastic flowLahars/Mud flowsPyroclastic fallLava flowNoxious GasEarthquakes
Volcanic Hazards
Courtesy of www.swisseduc.ch
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.
How do pyroclastic flows cause devastation?
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. 
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.
SeismicityDeformationGas Output (on volcano and remote sensing techniques)
These three things are the most important precursors to an eruption.
Volcano Monitoring
 Gas samples are collected from fumaroles and active vents.  Gas levels may also be monitored by remote sensing techniques
Gas Monitoring
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.
There are lots of uncertainties about why volcanoes form, because scientists have to rely on models based on rock samples and other data.
There are lots of uncertainties about why volcanoes form, because scientists have to rely on models based on rock samples and other data.
The Earth is made of many different and distinct layers. The deeper layers are composed of heavier materials; they are hotter, denser and under much greater pressure than the outer layers. Core: The Earth has a iron-nickel core that is about 2,100 miles in radius. The inner core may have a temperature up to about 13,000°F (7,200°C = 7,500 K), which is hotter than the surface of the Sun. The inner core (which has a radius of about 750 miles (1,228 km) is solid. The outer core is in a liquid state and is about 1,400 miles (2,260 km) thick. 
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!
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.
We have seen the effect of pyroclastic flows entering a town. But what are the exact processes that cause the damage?
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.
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.