Plate tectonics and associated hazards part 1: volcanoes

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  • Created by: Ikra Amin
  • Created on: 29-01-15 15:00

different lava types associated with diff PB's

Magma: INTRUSIVE, hot molten rock which is underneath the surface.

Lava: EXTRUSIVE, hot molten rock which is on the surface.

Key types of ignenous rock (rocks formed from the cooling of magma):

  • Mafic/basic = basalt 
  • Intermediate = andesite 
  • Silicic/acid = rhyolite                                Look at the silica content for all these (pg 5 in s/g)

These chemical differences cause the resulting magma (and therefore lava) to have different characteristics.

  • If a magma/lava has a high % of silica, it is not going to flow very easily. It is classed as being VISCOUS (sticky) and explosive as gases build up.
  • If you lower the silica content the magma/lava becomes more runny (non-viscous) and less explosive as gases can escape.
  • Three well known types of lava are: Basalt which is a basic/mafic rock; Andesite, which is
  • an Intermediate rock and Rhyolite, which is an Acidic rock.
  • Basalt is associated with magma coming straight from the mantle and is linked to constructive plate boundaries andHot Spots like Hawaii, both lead to non-violent eruptions which occur frequently.
  • Andesite and rhyolite are linked to destructive plate margins and infrequent, violent eruptions
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type of magma/lava

Type of magma/lava: Basaltic/basic

  • 48% silica content
  • Associated with CONSTRUCTIVE PB
  • Frequent and non explosive (gases can escape)
  • Non violent eruption and low silica content

Type of magma/lava: Andesitic/intermediate

  • 62% silica content
  • Associated with DESTRUCTIVE PLATE MARGINS
  • Infrequent and violent eruption (gases trapped and can't escape)
  • Higher silica content and pressure builds up 

Type of magma/lava: Rhyolitic/acid

  • 71% silica content 
  • Associated with DESTRUCTIVE PLATE MARGINS
  • Highest silica content
  • Infrequent violent eruptions (gases trapped and can't escape)
  • Pressure builds up
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forms of intrusive activity-dykes,sills,batholiths

IGNEOUS INTRUSIONS

Some Magma will not reach the surface. Instead it cools within the earth’s crust to form igneous intrusions (whereas lava is extrusive)

• The rocks are crystalline in nature. If magma is trapped underground in an igneous intrusion, it cools slowly because it is insulated by the surrounding rock. Crystals have time to develop

• The size of crystals depends on rate of cooling. Batholiths are the largest type of igneous intrusion so cool more slowly & form rocks with larger crystals (e.g. granite)

• Sills and dykes are minor igneous intrusions and so form rocks with smaller crystals because they cool more quickly (Dolerite & basalt)

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sill

Igenous intrustion: Magma which cooled in the Earth's surface forms igneous intrusions

Example: Drumadoom, Isle of Arran OR Whin Sill, Northumberland

Explanation of formation: Magma is forced along between the layers of a sedimentary rock. They sit parallel or discordant to the layers in the sedimentary country rock.

Description of characteristics:

  • Dolorite rock (igneous rock that is formed)
  • 20-30m across
  • Goes through country rock (sedimentary rock)
  • Sill forming sea cliff
  • Eroded land surface
  • Concordant/parallel= follows the rock
  • Often causes contact metamorphism, but they are small so have less heat so less country rock affected
  • Small crystals due to quick cooling
  • Flat-topped cliffs or islands

Sills can be at an angle but don't have to be discordant

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Dyke - pg 8-10 for photos

Examples: Kildonan dyke swarm, Isle of Arran

Explanation of formation: Cut through layers in sedimentary country rock, so are discordant. Formed when magma is forced along cracks or weaknesses in the rocks. Magma cools to form "walls" of igneous rock which can vary in width. When the intruding magma is under high pressure, it creates cracks of its own to travel through the rock so often many cracks are formed. This produces many dykes, and an intrusive pattern called a dyke swarm.

Dykes and sills are much smaller than batholiths, so the magma cools more quickly. This produes medium to fine grained igneous rocks, with crystls less than 5mm in size. Common rock types incl. dolerite (medium grained) and basalt (fine grained) and both have a low silica content.

Characteristics:

  • Lots of dykes in 1 place = dyke swarms
  • Discordant - cuts through layers of rock but not parallel
  • 1-20m width
  • Made of basalt
  • Small crystal size due to quick cooling

Dykes have to be concordant

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Batholith

Examples: Dongei Batholith, Ireland OR Coastal Batholith, Peru

Explanation of formation:

  • Vast amount of magma required (doesn't cool and intrude at once) formed by several smaller intrusions occuring in the same place overtime. 
  • The layers build up to form batholiths (also called layer intrusion).
  • Batholiths generated during a period of mountain building when continents are colliding
  • As 2 continental plates collide, they push both upwards to create mountains and thicken the crust downwards into the mantly, creating 'mountain roots'
  • Heat and pressure due to thickening of the crust so the lower crust melts
  • The magma is thick and stick (viscous) so doesn't flow easily

Characteristics:

  • Igneous rock type = grainte
  •  >100km across
  • Large crystal size due to slower cooling
  • Upwards dome
  • Can see it due to erosion
  • Found in centre of mountain ranges (fold mountains) - earth's crust folds
  • A large body of magma collecting in 1 place
  • Discordant to country rock
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major forms of EXTRUSIVE volcanic activity(types o

Major forms of extrusive volcanic activity (types of volcano)

The different types of magma/lava result in different types of volcanoes and eruption events. Some are violent eruptions from steep sided volcanoes; others are gentle eruptions from fissures (cracks) in the ground.

Categorising/Classifying volcanoes can be done in different ways. Let’s first split them into explosive and non-explosive.

EXPLOSIVE VOLCANOES: Explosive are linked to CONVERGENT plate boundaries where SUBDUCTION is occurring. Magma produced is HIGH in silica content and the lava is viscous (Sticky). The lava will be andesitic (common) or rhyolitic (less common) Volcano type 1: Rhyolitic (acid) domes. (pg 12 for diagrams)

  • E.g. Casa Grande, Arizona, USA. 
  • Tall and steep because fprmed from viscous lava which is slow moving so cools closer to the vent 

Volcano type 2: Composite (or Strato-) volcanoes.

  • Mount Etna, Sicily
  • These are “textbook” shaped and make up 60% of the earth’s individual volcanoes. They can become complex in shape due to secondary/parasitic cones. They are usually made of andesitic lava and ash
  • Has volcanic bombs, ash clouds
  • Made up of alternating layers of lava flows and volcanic ash
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explosive volcanoes cont

Volcano type 3: Ash/cinder cones (Can't be a caldera because magma chambers can't be partially empty)

  • E.g: Mt Paracutin in Mexico.
  • These are similar to composite/strato-volcanoes in shape, but don’t have the lava layers  They are made up entirely of ash, cinder and volcanic bombs blasted from the crater. With both ash and strato-volcanoes, 50% of the ash is from pyroclastic flows. 
  • An explosive eruption can produce a pyroclastic flow. This is a fast moving avalanche of volcanic materials (e.g. Cinders, ash, lapilli (small stones), pumice and volcanic bombs ) as well as hot gases which move very quickly down the volcano
  • Pyroclastic flows: hot rocks and ash mixed with gas which goes down mountain sides. It;s dense cloud so 'hugs' ground. Can get 600 degrees celcius and go 200 km/h 

Volcanic bomb: Volcanic bombs are rocks larger than 64 mm diameter, formed when a volcano ejects viscous fragments of lava during an eruption. They often cool into solid fragments before they reach the ground. They are tear drop shapdes because when the blob of lava is going through the air it gets shaped/moulded into streamlined shape.

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explosive volcano cont..

Volcano Type 4: Calderas

Example: The Santorini caldera in the Aegean Sea

These are large circular depressions (1-20km across) caused by the collapse of the top of a volcano following an explosive eruption. They are much bigger than a typical volcanic crater. They tend to form in a three stage process:

  1. Top of cone begins to collapse and the eruption is violent. - Volcano is explosive because magma is viscious (Sticky - andesitic or rhyolitic), high silica content (over 60%), gases are trapped in magma - build up of pressure. 
  2. Caldera formed as top of cone sunk into magma. - Large volumes of magma are released which empties the magma chamber. The top of the volcano caves in due to magma chamber not being full so there's unsupported rock.
  3. Rainwater may collect in the caldera to form a form. A new volcano cone may also form in the caldera.

The Santorini caldera in the Aegean Sea about 100 kilometres north of Crete (Eastern Mediterranean), was flooded with sea water after its eruption in 1630BC. It was the 2nd largest eruption in historic times (behind Tamboro).

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non-explosive volcanoes

The magma is low in silica and so non-viscous (runny), This is basaltic lava. Obviously, the lava cannot build up into a steep sided volcano as it will simply flow outwards in layers (lava flows). This leads to two possible outcomes:

Volcano Type 5: Lava Plateaux/Fissure Eruptions

These eruption events are usually associated with FISSURE eruptions. Here the lava simply spills out from a fissure (linear crack in the crust) to form sheets of lava over the pre-existing landscape. In Iceland the Laki lava flow of 1783-4 covered 560 sq km, killed 1/3rd of Iceland’s population and caused two years of extremely cold winters and summers globally. They are closely associated with constructive plate boundaries.

Volcano Type 6: Shield Volcanoes These are characterised by their gentle slopes of less than 10° leading away from a central vent. Hawaiian volcanoes such a Mauna Loa are good examples and many can be found in Iceland.

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Volcano Explosivity Index (VEI)

The VEI is a classification according to the power of the eruption (Explosivity). See the table on pg 18-19

Essential notes on VEI

• Uses a scale rating of 0 to 8 based on increasing explosivity

• 0 being non –explosive to 8 being megacolossal

• Most non- explosive volcanoes score either 0 or 1

• Most explosive volcanoes score over 2

Other way of classifying volcanic activity:

ACTIVE, DORMANT AND EXTINCT.

Active: any volcano which has erupted in the last 10,000years. This includes erupting and dormant volcanoes.

Dormant: Dormant volcanoes are those that are not currently erupting, but could become restless or erupt again.

Extinct: Extinct volcanoes are those that scientists consider unlikely to erupt again.

Look @ pg 21

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Minor forms of volcanic activity

EXTRUSIVE - ON THE SURFACE

Geysers e.g. Geysir and Strokkur in Iceland (Every 5 mins with water and steam reaching a height of about 20m); or Old Faithful in Yellowstone

Sequence of events in a geyser eruption:

  1. Rainwater percolates through porous lava e.g. basalt and becomes groundwater.
  2. the groundwater is heated up to 250-300 deg C. by the hot rock surrounding the shallow magma chamber. The water is now above boiling point (superheated), but it cannot turn to steam due to the high pressure conditions.
  3. The superheated water rises (lower density) through the plumbing system (cracks/ fractures in the rock). Silica in the rock is dissolved by the water and deposited closer to the surface along the walls of the plumbing system – causing constriction and adding to the high pressure conditions
  4. Close to the surface, the water boils due to lower pressure. This “bubbles” to the surface and some of the pressure is released on the superheated water below. The superheated water in the ground can now boil and turn to steam (gas) and expands quickly. This forces water and steam to erupt at the surface. The sudden change from water to steam is known as the “flash point” ..... The water “flashes” to steam.

Requirements of geyser are:

  • a geothermal heat source
  • silica rich rock (e.g. rhyolite) and permeable rock and a water source
  • a constricted plumbing system  and high pressure to allow superheatng rock
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minor forms of volcanic activity cont..

2) Hot Springs e.g. South Iceland; Rotorua in New Zealand.

These occur where groundwater is constantly heated by hot rocks beneath the surface. Unlike a geyser, the hot water reaches the surface under lower pressure. Many such pools of shimmering blue steaming water can be found in the Geysir area of Iceland.

Requirements for a hot spring:

  • a geothermal heat source
  • permeable rock
  • plumbing system
  • lower pressure - lower pressure means the groundwater doesn't become superheated so there's no steam pressure to trigger an eruption unlike a geyser.
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3) mud pot

E.g. South Iceland; Rotorua NZ.

Similarly, if the surface is not rocky, but deposits of clay, then boiling mud pots will form. The acidic gases which rise to the surface attack surface rocks, forming clay. The clay-rich soil mixes with the surface water to produce muddy, steam heated slurry or mud-pool

Requirements for a mud pot:

  • geothermal heat source
  • permeable rock
  • plumbing system
  • lower pressure
  • surface deposits of clay
  • hot water mixes with surface clay deposits to form a pool of boiling mud
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4) fumaroles

e.g. “Big Boiler” is the name given to the largest fumarole in Lassen Volcanic National Park, California

In many volcanic areas, instead of water being heated and emerging at the surface as boiling water, steam and/or gases can emerge from small vents commonly less than 10 cm across. These are FUMAROLES. They are often very numerous, and Yellowstone Park alone has thousands.

So the requirements for fumaroles are:

  •  a geothermal heat source
  • permeable rock
  • plumbing system
  • high pressure
  • very little water

Solfatara e.g. Naples, Iceland, Yellowstone

A solfatara is a type of fumaroles. It is a natural steam vent rich in sulphur gases which colour the surroundings with yellow sulphur. They produce a distinctive rotten egg smell

So the requirements of a solfatara are:

  • a geothermal heat source
  • permeable rock
  • plumbing system
  • high pressure
  • very little water
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primary effects of volcanic eruptions

Hazards which are a DIRECT result of the volcanic eruption

Lava flows: Layers of molten lava can destroy almost everything in their path. Trees and buildings are burnt and transport links are severed. Can threaten whole settlements.

Pyroclastic flows: Fast moving clouds of fragmented material varying from bombs and blocks to ash mixed with hot gases. Sometimes referred to as a nuee ardente or "glowing cloud"

Tephra: Solid material of varying grain size, from volcanic bombs to ash, ejected into the atmosphere.

Volcanic gases: Volcanoes emit a variety of dangerous gases. Hydrogen sulphide is the most noticeable (rotting eggs), but others incl. CO2, CO, SO2 and Chlorine. 

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Secondary effects of volcanic eruptions

Landslides: Collapse of volcanic flank as eruption takes place. Eruption of Mt. St. Helens in 1980 triggered the largest landslide ever witnessed.

Lahars: Volcanic mud flows caused by the ash erupting from a volcano mixing with river water. Worst cases are when there is high rainfall or snow melt as eruption occurs. Lahars can occur days, weeks or even years after the eruption as ash layers are "reworked" by heavy rain. 

Tsunami: Oversized waves in the sea or large lakes triggered by the collapse of volcano flanks or caldera forming volcanic events such as that which created Santorini caldera. That single event is believed to have wiped out 80% of the Minoan people, at the time a great civilisation. 

Flooding: Melting glaciers/ice caps can release huge quantities of water in very short periods. Mount St Helens in 1980 and Grimsvotn in Iceland in 1996 are good examples.

Climate change: The large volumes of ash and gas ejected into the atmosphere can reduce global temperatures by blocking out sunlight. The 1783 Laki eruption had a global impact which led to reduced crop yield and in 1991, the eruption of Mt. Pinatubo led to a 0.5 deg C fall in mean global temps.

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different methods of volcanic hazard management

The use of the three P’s is a good starting point to consider management of any hazard:

  • P – Prevention
  • P – Prediction
  • P – Protection

Prevention

This means preventing the hazard from occurring. At present there is no way of preventing a volcanic eruption. In reality, there is little chance of this happening in the future.

Prediction

As technology has improved, so has our knowledge of volcanoes and hence our ability to predict their eruption cycle. As this prediction is inextricably linked to the availability of technology and an educated population, there is a clear divide between Rich and Poor countries.

Monitoring Sakrujima in Japan

  • Aircraft and satellites measure heat, gas and ground movemen
  • Observation borehole measures changes in temp and quality of hot spring water and underground gas composition
  • Underground observation tunnel monitors EQ's and changes in the earth's crust and temp
  • Remote sensing of the chemical composition of escaping gases
  • Earth's electrical resistance and local magnetic field measured
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Predict eruption of Sakurajima

Method 1-GROUND DEFORMATION:

  • The volcano sweels (suggesting magma is moving up under the volcano). 
  • Rate of swelling is measured using tilt meters and GPS that measures horizontal and vertical movement to a mm.
  • Thermal monitoring using IR band satellite imagery can detected magma movement just below the volcanoes surface.

Method 2-SEISMICITY:

  • Seismic observations use seismographs to monitor increased seismicity of 3 types:
  • Short-period EQ's are caused by the fracturing of brittle rock as magma forces its way upwards.
  • Long-period EQ's are believed to indicate increased gas pressure in a volcanoes magma chamber. 
  • Harmonic tremours are the result of magma vibrating in the vent as it moves upwards.

Method 3-MONITORING GAS EMISSIONS:

  • As magma rises into magma chambers, gases escape (mainly sulphur dioxide) and if its quantity is escaping, volcanic gas increases this can signal the start of a major eruptive sequences.
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protection

This refers to the strategies which are employed to reduce loss of life and injury, by taking action before, during and after an eruption. It is sometimes referred to as MITIGATION (mitigating the effects). There are close links between Prediction and some of the Protection strategies.

Education

  • This could also be called community preparedness. Most countries which have active volcanoes make the population aware of the risks through schools or advertising campaigns. These are often focussed on the areas of greatest risk.
  • Land use mapping of the danger areas (usually using evidence from past eruptions) can help to reduce future loss of life and economic losses. In some less developed countries such as the Philippines, lack of education can cost life.
  • During the 1991 eruption of Mount Pinatubo, it was discovered that the tribal groups who lived on the slopes did not know what a volcanic eruption was. Sadly, hundreds lost their lives during the violent pyroclastic flow which accompanied the eruption.
  • Evidence that people living near Sakurajima were prepared through education; they had bunkers (concrete); big signs; kids wear helmets; all graves have a roof; drainage channels to divert lahars.
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protection continued..

EVACUATION

  • “If you are given enough warning, you have time to escape” Many lives have been saved thanks to evacuation plans put in place by local or national authorities.
  • Over 250,000 people were evacuated from Mount Pinatubo in 1991 and the death toll of 800 would have been far higher without this.
  • Once evacuated, aid is often required, especially for those re-housed in temporary camps. The United Nations (UN) has a number of agencies which can provide aid e.g. Office for the Coordination of Humanitarian Affairs (OCHA), World Health Organisation (WHO) and UNICEF (the UN Children’s Fund).
  • In many wealthy countries, they have there own disaster relief agency, for example FEMA in the USA (Federal Emergency Management Agency).

ENGINGEERING

This may involve developing hard engineering structures to divert lava flows or lahars. What has been built in the Sakurajima area to decrease the impact of lahars: Diversion channels - divert to sea.

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case studies

We will look at how the effects of a volcanic eruption and responses to them differ due to contrasts in levels of wealth. The two case studies we will look at are:-

  • Nevado del Ruiz, Colombia which represents a volcanic event from an LEDC
  • Eyjafjallajokull, Iceland which represents a volcanic event from a MEDC

In each case, the following should be examined:-

  • The nature of the volcanic hazard
  • The impact of the event
  • Management, of and responses to, the hazard

We should also be aware that huge variations in development can exist within a country e.g. between urban and rural environments. Similarly, in MEDCs contrasts in wealth occur e.g. between run down inner city areas & suburban areas. Picking out such variations will gain extra credit in exams as it shows “INSIGHT”.

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Nevada Del Ruiz, Colombia - LEDC

Date of eruption: 13th November 1985 at 9am. 

Nature of hazard:

  • Part of the Andes Fold mountains which have resulted from the subduction of the Nazca plate (part of the Pacific Ocean) beneath the South American plate.
  • The volcano had been dormant for 69 yrs, but havig erupted 7 times in the previous century.
  • On 13th November 1985 at 9.00pm  (local time), an eruption of VEI scale 3 sent andesitic ash over 30km into the atmosphere.
  •  The summit of the volcano was covered by a glacier and large meltwater valleys lead away in a radial pattern.
  • The nearest major settlement was Armero, containing 28,000 people, but this was over 40km from the volcano.
  •  Already raining heavily so rivers were swollen.

Why people of Armero died; 

  • Volcanic ash and glacial meltwater and rain ==> Lahars
  • Greatest lahar risk - East of mountain - following mountains 
  • 40 miles away - town was locaed downstream of a confluence which would mean a lahar would double in power. Town was burried in the past. 
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NDR cont

Why people of Armero were taken by surprise resulting in so many deaths (23,000 in total):

  • Dormant for 69 years
  • They were skeptical because it was 40 minutes away
  • 9am in the morning, people asleep 
  • Bad weather at the same time (heavy rainfall) 

Could loss of life be avoided? 

  • Yes, if evacuation had occured earlier. Hazard map, they know Armero was at risk. Armero had been burried by a lahar before and also diversion channels could be put in. 

Reasons why no protective measures taken: 

  • Didn't know when it was going to happen but govt. would only evacuate when they knew it was definetly going to happen 
  • Mexico City EQ happened at the same time
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Eyjafjallajokull

2010 eruption in Iceland.

1st eruption - fissure eruption 

2nd eruption - moved to main crater 

Negative impacts of the eruption:

  • evacuation
  • flights grounded
  • reservoir was blocked
  • generator had brokeen down
  • crops destroyed - short term 
  • turned day to night
  • inhalation of ash - a health risk

Positive impacts of the eruption:

  • tourism of 1st eruption
  • flood defences built
  • carrots and grapes managed to grow
  • community spirit amongst locals
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cont..

Protective measures taken:

  • roads cut away to protect bridges
  • animals put inside
  • gas masks given to prevent inhalation of ash (ash went 5 miles high)
  • dredging of ash to prevent flooding 
  • bulldozers clear ash
  • large reservoir created

1st eruption in March 2010 (fissure) 

2nd eruption in April 2010 (phreatic explosive)

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the NATURE of the volcanic hazards

Nevada Del Ruiz (1985 - LEDC)

  • Subductive plate 
  • Nazca and South American plate
  • Dormant for 69 years
  • Andesitic with high silica content and therefore explosive

Eyjafjallajokull (2010 - MEDC)

  • Constructive plate
  • North Atlantic and Eurasian plates, also a hotspot
  • Small eruption in March
  • In April, a 4 VEI eruption
  • Basaltic - low & silica (48%) and non viscous
  • Lava, ash, volcanic bombs
  • No deaths
  • Dormant for 200 years. People there are used to volcanoes, there is an eruption every 2-3 yrs so prediction and protect are excellent and the local population come to expect eruptions.

BOTH:

  • Covered by ice on the summit 
  • Settlements far away. Home to a few people
  • Dormant for a long time 
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Impact of the volcanic events

Nevado Del Ruiz - LEDC 

  • Lahars (very powerful lahars were caused in which 28,000 people were killed)
  • Pyroclastic flows rolled down the flanks of the volcano
  • More lahars in 2006

Eyjafjallajokull - MEDC

  • River levels rose
  • Some roads were blocked by falling ash 
  • Ash fell on fields (crop issues)
  • There was a large ash cloud that interruped international travel (in turn affective economy as unable to fly out trades etc.)
  • 800 evacuated from towns south of the volcano
  • Animals evacuated
  • Cost £2.5 billion to airlines and £1 billion to UK economy
  • Passengers stranded abroad
  • Jokulhlaups - glacial bursts caused flooding
  • Health problems - exacerbates asthma and can bring on emphysema
  • Insurance companies did not cover people for having to stay abroad-hotel bills etc mounted up
  • Look at page 48-53 for more notes on ven diagrams

BOTH:

  • Melting of ice on top of volcano - could impact to greater risk of lahars and flooding
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Management, and response to volcanic events

Nevado Del Ruiza - LEDC

  • Evacuation called off last minute
  • Reassuring messages over the radio
  • International support from US - $1 million
  • Occured same time as Mexico City EQ
  • 12 hours to reach Armero 

Eyjafjallajokull - MEDC

  • 800 people evacuated 
  • Locals issues with face masks
  • A reservoir was created
  • Some roads were closed
  • Met ofice contact with Iceland to produce forecasts
  • Flights cancelled and air traffic diverted - new safety rules made with new tolerance levels. It's safe for planes to fly if there are less than 2mg of particles per m cubed
  • Breaches made in the south orbital road to minimise damage to the road so that it was only smaller sections that would need replacing
  • Reservoir dug to hold the meltwater when glacial bursts occur to protect the town of Vik
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iceland

Iceland has an excellent prediction and protection plan in terms of hazard management. Therefore no

one died in the event. Ash has covered the fields and there is some concern over crop growing and

animals grazing on the ash. The other concern is that Katla could erupt as it is thought that the magma

chambers may be interlinked or the moving magma or earthquakes under Eyjafjallajökull may trigger

movement further east near Katla. This has the potential to be more devastating as it is 10 times larger

than Eyjafjallajökull.

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