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 A volcano

استعرض الموضوع السابق استعرض الموضوع التالي اذهب الى الأسفل 
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التخصص : جيولوجيا إقتصادية
عدد المساهمات : 53
نقاط : 121
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تاريخ التسجيل : 11/10/2010
الموقع : صنعاء

مُساهمةموضوع: A volcano   الأربعاء 4 أبريل - 22:12

A volcano

is a place on the Earth’s surface where hot, molten rock (called magma) breaks through. As we will see there are many different types of volcanoes and material that is erupted.
However, in general a volcano is classed as “active” if it erupts lava, rock, gas or ash, or if it shows seismic (earthquake) activity.
A volcano is dormant if it hasn't erupted for a long time (less than 1 million years) but could again one day.
An extinct volcano will never erupt again.

Presenters
go through the different parts of a volcano

Magma:
Molten rock beneath the surface of the earth.


Magma chamber:
The subterranean cavity containing the gas-rich liquid magma which feeds a volcano.


Conduit:
A passage followed by magma in a volcano.


Vent:
The opening at the earth's surface through which volcanic materials issue forth.


Cone:
A volcanic cone built entirely of loose fragmented material (pyroclastics) and (or) lava flows erupted from the vent. Erupted material builds up with each eruption forming the cone.


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).
– Why Do Volcanic Eruptions Occur?
• High temperature of the Earth’s interior
• Melting of lower crust and mantle = molten rock = magma
• At depths > 20 km the temperature = 800-1,600 degrees Celsius
• The density of the magma is less than the crustal rock, therefore it rises to the surface
• Source of this heat?
• Residual from the cooling of the Earth (& solar system)
• Radioactive decay
• Convection in the mantle
- Brings hot rock up from near the interior of the Earth and returns cooler material towards the centre of the Earth for reheating.
- Shock/impact melting
- E.g. meteorite impacts produce instantaneous heat and melting from high energy collisions
- Two styles of volcanic eruption: Explosive and Effusive (see next slides for further descriptions of each)
- Explosive: where rapidly escaping gas bubbles (= vesicles) rip apart the magma, fragmenting it.
- Effusive: where the magma leaks out onto the surface passively as lava flows.
- NOTE: Some effusive eruptions involving highly viscous lava may turn into explosive eruptions. If the magma is too viscous (sticky) it can block up the volcanic vent, trapping gas inside the volcano. If this gas builds up enough to break through the blockage an extremely dangerous explosive eruption may form.
- Some of the most explosive eruptions have formed this way, e.g. Pinatubo 1991

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!

Explosive Eruptions
• 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
• Hazardous!!!



Explosive Eruptions
• Three products from an explosive eruption
– Ash fall
– Pyroclastic flow
– Pyroclastic surge


Ash fall:
The fallout of rock, debris and ash from an explosive eruption column.

An explosive volcanic eruption will propel large volumes of volcanic rock, ash and gas into the atmosphere. The larger (most dense) particles will fall out of the air quickly and close to the volcanic vent. The smaller particles (ash) can be suspended in the atmosphere for days to weeks before they fall back to Earth. Whilst in the atmosphere the wind can transport the ash particles large distances. For example when Mt Pinatubo (Philippines) erupted in 1991 ash was blown all the way around the entire globe!

Pyroclastic flow:
Pyroclastic flows are hot, turbulent, fast-moving, high particle concentration clouds of rock, ash and gas.
Pyroclastic flows can reach > 100 km from a volcano.
They can travel 100s km/h and are commonly >400°C.
They will destroy everything in their path including buildings, agriculture and forests. Although because they contain a high concentration of particles and a low concentration of gas, they are dense and usually are confined to, and flow along, topographic lows (valleys).
It is extremely important to understand them as they are often the most hazardous component of an explosive eruption.


Pyroclastic surge:
Pyroclastic surges are low particle concentration (low density) flows of volcanic material. The reason they are low density flows is because they don’t have a high concentration of particles and contain a lot of gas.
Pyroclastic surges are very turbulent and fast (up to 300 km per hour).
They overtop high topographic features, and therefore are not confined to valleys.
Pyroclastic surges usually do not travel as far as pyroclastic flows, but pyroclastic surges can travel up to at least 10 kilometers from the source.




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

• A volcanic eruption dominated by the passive outpouring of lava onto the Earth’s surface is called an effusive eruption.

• This happens either because there is not enough gas (volatiles) in the magma to break it apart upon escaping, or the magma is too viscous (sticky) to allow the volatiles to escape quickly.

• Remember: molten rock is called “magma” when it is underneath the ground. It is called “lava” once it has been erupted onto the surface.

• Lava flows generated by effusive eruptions vary in shape, thickness, length, and width depending on the type of lava erupted, discharge rate (how fast it comes out of the vent), slope of the ground over which the lava travels, and duration of eruption.
• Although not generally as hazardous as explosive eruptions, lava flows can burn and bury buildings and forests and do pose a danger to people living on or near an active volcano.


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
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.




Volcanic Hazards
There are many hazards associated with volcanic eruptions
• Pyroclastic flow
• Lahars/Mud flows
• Pyroclastic fall
• Lava flow
• Noxious Gas
• Earthquakes

haps the biggest hazard are pyroclastic flows. As mentioned earlier these are hot, fast moving, high particles concentration flows of gas, rock and ash (something you don’t want to get in the way of!).
A famous historic example of an explosive eruption that produced devastating pyroclastic flows was the 79AD eruption of Mt Vesuvius in Italy that buried the ancient Roman city of Pompeii…


Dangerous Volcanoes

1 Volcanoes have been erupting since shortly after the earth was formed. They are still erupting today. About twenty volcanic eruptions occur each day on earth. Some of these eruptions are quiet. Others are violently explosive. Either way, volcanoes can be very dangerous.

2 What are the dangers of volcanic eruptions? Most people think the biggest danger is lava flows. However, lava does not usually injure people or animals. The lava flows slowly. Most anything that moves can get out of its way. But lava can run over houses and roads, causing much destruction.

3 Pyroclastic flows are mixtures of hot ash and gas. They are more dangerous than lava flows. Pyroclastic flows can travel very quickly down the slope of a volcano, destroying everything in their path.

4 Ash and other tephra from volcanic eruptions can cover a wide area. It can be heavy enough to collapse buildings. It can also destroy plants and trees. Poisonous gases that spew out of volcanoes are very unsafe. Eruptions can also cause giant mudslides. They can even trigger a tsunami, a giant, destructive ocean wave.

5 Since 1700, more than 250,000 people have been killed by volcanic activity. Entire towns have been wiped out. Many acres of forests and croplands have been ruined

How do pyroclastic flows cause devastation?
We have seen the effect of pyroclastic flows entering a town. But what are the exact processes that cause the damage?
Pyroclastic Flow - direct impact
The direct force of a pyroclastic flow traveling at 10’s of metres per second and carrying boulders as large as houses is extremely damaging.
On the volcanic island of Montserrat in the West Indies (top photos), metre-scale blocks of volcanic rock when embedded 10’s of centimetres into concrete walls by the force of the passing pyroclastic flow. The top right hand photograph shows steel reinforcement bars that were once in the wall of a house bent in the direction of the passing flow (from right to left of screen).

As we previously saw, the entire city of St Pierre in Martinique was swept into the ocean.

Pyroclastic flows have temperatures commonly in excess of 400 degrees Celsius. Hot enough to burn forests and wooden structures.


Pyroclastic Flow – lahars
• ‘Lahar’ is an Indonesian term that describes a hot or cold mixture of water and rock fragments flowing down the slopes of a volcano and (or) river valleys.

• Heavy rain after an eruption or hot volcanic activity melting snow and ice will provide a large volume of water that will flow down the sides of the volcano. This water picks up the newly erupted material forming fast flowing torrents of water, mud, ash, rock and debris.
• Lahars can flow great distances and be very destructive. The bottom photo shows a lahar knocking down a concrete bridge.
• 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



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.

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

Lava flows although generally slower moving and less catastrophic than pyroclastic flows still remain dangerous.
• Lava flows have temperatures in excess of 200 degrees Celsius. Therefore will burn any flamable material it contacts with.
• Thick lava flows will bury all in it’s path including infrastructure (buildings, roads, waterways etc.) and agricultural land…

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.
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.


What is volcano monitoring?
Scientists set up “laboratories” or “volcano observatories” on the sides of active volcanoes to look for signs that the volcano is active and may have an eruption soon.
What are they looking for?
As magma moves through the Earth’s crust it can alter it’s environment producing sign’s that it is on its way to the surface…these signs are called “precursors” to an eruption.
Precursors include:
• Increased earthquakes in the area (increased seismicity)
• Swelling and cracking of the ground (deformation)
• Change in the amount of or chemistry of the gas coming out of the volcano
Change in the groundwater levels or chemistry.
• Seismicity
• Deformation
• Gas Output
– (on volcano and remote sensing techniques)

These three things are the most important precursors to an eruption.

Seismicity (earthquake activity), ground deformation and gas output are the 3 most important precursors to an eruption….
For example, the Montserrat volcano observatory in the West Indies will increase its alert level and warn the population if two out of these three precursors occur. This warning and increase in alert level may involve relocation of people and livestock to safer areas and (or) a ban on boats sailing within 4 km of the volcano.

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


Volcanoes and seismic activity (earthquakes) -
what’s the link?
Many erupts are preceded by increased levels of seismic activity. The earthquakes are caused by fracturing and brittle failure of the subsurface rocks as new magma pushes it’s way up towards the surface.

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.


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!


Why is measuring ground deformation on a volcano important?
Tiltmeters can tell the scientists when new magma has entered a magma chamber in the volcano.
• Figure A shows a volcano in a dormant (resting) stage. Titlmeters on the sides of the volcano meassure a shallow slope angle.
• When new magma enters the magma chamber (Figure B) the chamber swells to accommodate the larger volume. This causes the sides of the volcano to bulge out. It’s like blowing up a balloon: the more air you put into the middle the bigger the out skin of the balloon gets. The tiltmeters will now record a new steeper slope angle on the outside of the volcano.
If fresh magma enters the magma chamber, this again may be a precursor to an eruption. New magma will increase the pressure in the magma chamber causing more fracturing of the surrounding rock (this will produce earthquakes) and potentially the formation of a conduit to the surface. It is common for ground deformation and seismic activity to occur at the same time before an eruption occurs.

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
• 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.


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.

What should geologists do about volcanic eruptions in the future?
1. Study volcanoes to find out more about how and why they erupt
2. Monitor the volcanoes
3. Develop hazard mitigation plans
4. Understand the population around volcanoes, i.e. why do people choose to live near volcanoes?
5. Education

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!)


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.

• 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.

• 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 avalanc[/left]hes

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A volcano
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