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Unit 2 Section B - Causes and effects of volcanoes and responses to them

Page history last edited by K J Hutchinson 14 years, 2 months ago

Lesson 1 - What happens in an eruption?


Learning objectives:

- to be able to describe at least 3 differences between composite and shield (basic) volcanoes

- to be able to describe at least 3 primary effects of a volcanic eruption

- to be able to describe at least 3 secondary effects of a volcanic eruption


Key words 


A volcano is an opening in the ground where magma forces its way to the surface. Magma which reaches the earth's surface is called lava. Volcanoes can be active (erupting), dormant (sleeping) or extinct (no eruption for 10,000 years and unlikely to erupt again). You need to know about two different types of volcano - composite volcanoes and shield (basic) volcanoes. 


The first video (the one on the left) is a catchy song that tells you the basics about an eruption. THe second video (the one on the right) is much more useful as it highlights some of the key differences between composite and shield volcanoes. It also introduces some of the hazards of volcanoes as well as briefly discussing some of the benefits that they bring to an area.


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Composite volcanoes


Composite volcanoes happen where the lava is acidic. The sticky acidic lava pours slowly down the side of the cone and cools quickly to produce a steep sided volcano. Alternate layers are formed because each eruption first produces rock fragments which are later covered by lava. This kind of volcano is found at destructive plate margins.



Shield (basic) volcanoes


Shield volcanoes are enormous features built up only from layers of lava. They produce lots of lava but they tend not to erupt violently. Shield volcanoes form when the lava is basic (the opposite to acidic). You get these types of volcanoes along constructive plate margins and also where there are hotspots. Basic lava is runny so it flows quite a long way before it cools.



The diagram below shows how much bigger shield volcanoes are than composite volcanoes.



The effects of volcanic eruptions


The effects of volcanic eruptions can be divided into primary and secondary effects. The primary effects are immediate and come from the eruption itself whereas the secondary effects result from the primary effects. You need to be able to describe at least 3 primary and 3 secondary effects of volcanic eruptions. A number of these effects are shown on the diagram below., Can you categorise them into primary and secondary effects?




Primary effects of a volcanic eruption


You need to learn the definitions of the terms volcanic gases; lava flows; pyroclastic flows; tephra.


Volcanic gases - All magma contains dissolved gases that are release during and between eruptions. These gases are mainly steam, carbon dioxide and compounds of sulphur and chlorine.


Lava flows - These are streams of molten rock.


Pyroclastic flows - These are high speed avalanches of hot ash, rock fragments and gas which move down the sides of a volcano. These flows occur when the vent area or ash column collapses.


Tephra - The explosive power of an eruption causes old lava to be blasted into tiny pieces and hurled into the air. The fragments are tephra. 


Secondary effects of a volcanic eruption


Make sure that you know at least 3 secondary effects of an eruption in detail.


Lahars - These are mixtures of water, rock, ash, sand and mud that originate from the slopes of a volcano. Lahars often happen because of heavy rainfall eroding volcanic deposits or heat from a volcanic vent suddenly melting snow and ice.


Landslides - Heat from cooling magma can cause hydrothermal alteraton of the rocks, turning sections of them into clay. This weakens the rocks and increases the risk of slope failures.


Flooding - Explosive eruptions can change thge surface areas around a volcano and disrupt drainage patterns, leading to long-term flooding.


Other secondary effects include:


Food / water supply interrupted.


Businesses forced to close.

Cost of insurance claims.


Long-term issues with the tourism industry. 


Useful links:

BBC Bitesize on types of volcano 

BBC Bitesize on why volcanoes form and how they erupt

BBC Bitesize on effects (positive and negative) of eruptions


Lesson 2 - The Mount St Helen's eruption May 1980 - an eruption in an MEDC


Learning objectives:

- to be able to recall basic factual information about the eruption - date, time, location, number of deaths, direction of blast

- to be able to give at least 3 primary and secondary effects of the Mount St Helen's eruption


Mount St Helens is a mountain in the Cascades range in North West USA. The volcano is in Washington State, on the west coast of the USA (see map below). The Cascades have formed because the area is a destructive plate margin. The small Juan de Fuca plate (oceanic) is being subducted under the large North American plate (continental). This is shown in the diagram below.




The Cascades have experienced a lot of earthquakes and eruptions over the years because they are on a plate boundary. The chart below shows eruptions in the area in the last 4000 years.



Mount St Helens erupted on May 18th 1980. This followed a period of activity which began in March 1980 with an earthquake measuring 4.0 on the Richter scale. There was 3 months of activity as magma rose inside the mountain, creating a large bulge on the north side of the mountain. This was due to a blockage in the main vent which prevented the magma rising through the vent in the normal way. The technical name for the bulge is a cryptodome.


On May 18th, an earthquake measuring 5.1 on the Richter scale caused a landslide on the northern flank of the volcano. This explosed the cryptodome and resulted in a sudden release of pressure and a huge eruption in the form of a lateral (sideways) blast. The Plinian eruption lasted for 9 hours (see images below).




The plume of ash erupted for more than 9 hours. It spread north-eastwards, eventually reaching 30 kilometres into the sky. 540 million tonnes of ash were pushed into the atmopshere and noticeable ash fell on 11 American states. Just three days after the eruption, air pollution monitoring systems detected ash in east coast cities such as New Yoprk (over 4000 kilometres away). The ash circled the globe in 17 days.


The blast zone covered over 700 square kilometres and left a lunar landscape. Trees were flattened and ash covered the area. This is shown below.



Primary effects of the Mount St Helens eruption


  • Ash column of over 80,000 feet in height.
  • Pyroclastic flows moving at up to 670 miles per hour flattened over 600 square kilometres of forest.
  • Poisonous gases released. 


Secondary effects of the Mount St Helens eruption


Make sure that you can describe at least 3 of these effects. Give as much detail as you can!


  • Glaciers melted on the volcano, mixing with ash and mud to form lahars.
  • 200 homes, 27 bridges, 15 miles of railway and 185 miles of roads were destroyed.
  • 7000 big game animals perished (deer, elk and bear). 
  • People across north-western America were told to stay indoors and wear gauze masks.
  • The ash made roads slippery and reduced visibility. Many roads were closed, trains halted and aircraft grounded.
  • Fish, in some hatcheries, perished as ash fell into lakes and streams, clogging their gills and raising the temperature of the water.
  • Crops were destroyed, or subsequently produced low yields, because ash settled on leaves, impeding photosynthesis.
  • Electricity supplies were interrupted and sewers were blocked, and the ash damaged car engines.
  • The town of Yakima, as an example, some 150km away, was blanketed in 1.5cm of ash
  • Skies were turned grey as far away as as Spokane, Washington State, 400km away.
  • 57 people lost their lives in the blast, including 84-year-old Harry Truman who had refused to evacuate his lodge near Spirit Lake. 


Harry Truman's story



84-year-old Harry R. Truman died in the 1980 blast. After consistently refusing to leave his lodge at the foot of the mountain beside Spirit Lake he had been granted special permission to remain inside the Red Zone (evacuation zone). He stayed while about 2000 people were evacuated from the area in the last few weeks before the eruption. His body was never found.




They had to change their postcard!


The eruption of 1980 completely changed the shape of Mount St Helens. The volcano had previously been nicknamed 'America's Mount Fuji' because of its symmetrical shape which was similar to Mount Fuji in Japan. The blast changed this and the tourist operators in the area literally had to change their postcard (see below). The Mount St Helens song (clip below) also refers to this change.


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Useful links:

Panoramic image of the Mount St Helens crater

National Geographic Magazine's 30 year anniversary edition about Mount St Helens

National Geographic interactive about the blast zone 

USGS Mount St Helens factsheet

BBC Bitesize case study of Mount St Helens


Lesson 3 - The Mount Pinatubo eruption June 1991 - an eruption in an LEDC


Learning objectives:

- to be able to recall basic factual information about the eruption - date, time, location, number of deaths

- to be able to give at least 3 social, economic and environmental effects of the eruption


In an exam, you might be asked to locate a volcano or other feature. Think about scale, distance and direction. Try to zoom in from international to national to local scale when giving locations.


Mount Pinatubo is in Asia. It is on the island of Luzon in the Philippines, about 200km northwest of the capital Manila.




Prior to the 1991 eruption, Mount Pinatubo had not erupted since 1380 (so it was a dormant volcano). The volcano was monitored by the Americans from the nearby Clark airbase using scientific equipment such as seismometres, tiltmeters and GPS.


From 9th June 1991 there were many eruptions and earthquakes with the largest eruption being at midday on 12th June. Watch the video clip below for a summary of the eruption.


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Environmental (physical effects)


The explosion sent a cloud of steam and ask up to 30 km into the sky turning day into night.

Volcanic bombs were thrown into the air.

Up to 50cm of ash fell on nearby farmland, villages and towns. SOme ash even reached Australia!

A typhoon then passed over the ash cloud and the heavy rainfall mixed with the ash in the sky creating thick mud which fell to the ground.

A lahar was generated - this travelled as far as Angeles City over 20 km away.

The weight of the ash caused buildings to collapse, including 200,000 homes.

Water supplies were contaminated.

The 1991 rice crop was ruined and planting was impossible for a number years following the eruption.


Social effects


Over 200,00 homes collapsed under the weight of the ash, leaving people homeless.

Huige shanty-type refugee camps were set up and disease spread rapidly in them (especially diahorrea, chickenpox and malaria) leading to 600 deaths.

Many schools collapsed under the weight of ash.

Destruction of rice crops led to food shortages.

People had no electricity for weeks.

Many people moved from rural to urban areas to seek shelter and food.


Economic effects


Many factories collapsed under the weight of ash.

Roads were unusable and bridges were destroyed - they were very costly to rebuild.

Over 1 million farm animals died either due to starvation or by drinking contaminated water supplies. This had a huge economic cost.

The total cost of damages and repairs was estimated at $450 million.

There is high unemployment in the area and the Clark Airbase remains closed.


Managing the eruption


The eruption claimed a relatively small number of lives as it had been accurately predicted (by the Americans) allowing the immediate area to be evacuated in advance.


On 5th June, alert level 3 was reached and villages on the volcano slopes were evacuated. This included members of the Aeta tribe.

On 7th June, alert level 4 was reached and all villages within 10 km of Mount Pintaubo and the 15,000 people at Clark Airbase were evacuated.

On 9th June, alert level 5 was reached and all people living within 20 km of the summit were evacuated.


In total, about 1 million people were evacuated before the eruption on 11th June. It is estimated that only 6 people died from the primary effects of the eruption (pyroclastic flow), although 70 more died later when they were suffocated by lahars. Many more deaths occurred due to the spread of disease in camps set up to house the homeless.


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The ash that was ejected into the atmopshere circled the earth within a few days and blocked out some of the sun's heat for several months, lowering world temperatures. It is believed that the eruption may have delayed global warming for a few years. 


What did people do after the eruption?


The responses can be divided into two groups - short and long term responses. Make sure you can give examples from each category,


Short term responses:

  • Shanty-type refugee camps set up to shelter the homeless.
  • Emergency aid brought into the area (eg food, drinking water, tents, blankets) from other parts of the Philippines and from further afield.
  • Disaster relief from other countries (from Governments and NGOs such as ActionAid, Oxfam) sent to the area.
  • Some infrastructure repaired (eg roads cleared of ash and mud, electricity supplied again after 3 weeks).

Long-term responses:

  • The Philippines is an LEDC and had little money to spend on rebuilding the area devastated by the eruption.
  • New schools and houses have been built but this has taken time.
  • New villages away from the danger area are slowly being built.
  • Local authorities have tried to encourage new investment in the area (there is high unemployment eg Clark Air Force base still closed).
  • Repairs to the infrastructure damaged by the eruption continues


Thanks to Slideshare user jmoncur for this very informative PowerPoint presentation about the eruption.



Useful links:

Notes from The Student Room

'The sleeping giant awakens' - detailed information about the eruption

USGS factsheet

USGS factsheet about lahars at Mount Pinatubo


Lesson 4 - Why do people live in danger zones?


Learning objectives:

- to be able to describe the advantages of living in areas at risk from earthquakes and/or volcanic eruptions

- to be able to explain why people choose to stay in, or are unable to move away from, an area at risk from earthquakes and volcanic eruptions


500 million people live close to active volcanoes across the world and they form a major tourist attraction. The exam specification is very specific about the fact that you need to know how geothermal energy, tourism and fertile soils can attarct people to live in risk areas so make sure you remember these three factors!


What are the advantages of living in earthquake zones and areas close to volcanoes?


Geothermal energy

Geothermal energy is where steam from water heated by hot rocks below the surface of the earth is used to drive turbines and generate electricity. This is a very cheap and sustainable way of producing electricity since it uses a renewable source. More than 70% of the homes in Iceland are heated by geothrmal energy.



Volcanic landscapes often have very beautiful scenery. They attract a whole range of visitors from students on geography fieldtrips to artists, photographers, walkers etc. Tourists migh come to watch eruptions, see geysers, relax in hot springs or have mud baths (we hope to do all of these things on the GCE Geography fieldtrip to Iceland!). The tourism industry that builds up in the area can provide many jobs and these encourage people to live in the area. Thse include jobs as gudies, in visitor centres, in hotels and other types of accommodation as well as in gift shops, cafes. bars etc.


Fertile soils

The volcanic rocks are very rich in minerals. They form fertile soils when weathered, and these are ideal for growing crops. On Mount Etna in Sicily, yields of grapes are five times higher than the national average because the soils are so fertile.


Raw materials and minerals

Many valuable minerals are erupted from volcanoes and these can be collected and used. Gold, silver, copper and tin are found in the remains of extinct volcanoes. In active areas, heated groundwater concentrates traces of these minerals into rich veins which can then be mined. 


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Why do people still live in danger zones?


Many people choose to live in these areas because of the benefits listed above but a great deal of others stay because they unable to relocate away from the danger zones. For many people, the advantages of living in a danger zone by far outweigh the risk of coping with the possibility of an earthquake or eruption. Make sure you know the reasons listed below as well as the benefits listed above!


- some people think that disasters only affect other people and will never harm them - they ignore the dangers and assume they will be OK

- some people come from families who have lived in danger areas for many many years - long before they knew that they were hazard zones - and they don't want to break family ties by moving away

- some people think that scientists will be able to predict any seismic activity long enough in advance for the area to be evacuated - so they feel safe to live there

- some people simply may not be able to afford to move anywheere else

- some of the settlements in danger zones eg. Mexico ity, San Francisco, Los Angeles, have gorwn into enormous megacities and there just isn't space to rebuild them elsewhere away from danger


Approaches to living in danger areas


There are three main approaches - the fatalistic approach, the acceptance approach and the approach


The fatalistic approach - If it happens, it happens - it's all part of living in this area

The acceptance approach - hazards are a part of everyday life here are we have learned to live with them - the benefits of living here outweigh the disadvantages

The adaptation approach - events can be predicted and warnings given - this makes it safe to live here


These approaches are summarised in this PowerPoint from Richard Allaway of www.geographyalltheway.com - thanks for sharing this Slideshare presentation.



Useful links:

The Geography Site's explanation of why people live near volcanoes

USGS site 'The plus side of volcanoes'

BBC article 'Living in a volcano's shadow'

GCSE Geobytes (St Ivo School) article on why people live in hazard zones


Lesson 5 - Preparing for a volcanic eruption


Learning objectives:

- to know how volcanoes can be monitored

- to know how people can plan and prepare for a volcanic eruption

- to know what actions people can take following an eruption


Volcanoes can be monitored in a number of ways. The technology is well-developed but is very expensive, so its use tends to be limited to more economically developed countries. The image below fro the USGS summarises the main ways in which they collect monitoring information.


Scientists look for increases in seismic activity, releaees ofvolcanic gases and changes in temperature as well as changes to the shape of the volcano.



Gas emissions - As magma nears the surface and its pressure decreases, gases escape. One the main volcanic gases is sulphur dioxide. An increase in sulphur dioxide therefore suggests more magma near the surface.


Ground deformation - Swelling of the volcano signals that magma has accumulated near the surface. Scientists monitoring an active volcano will often measure the tilt of the slope and track changes in the rate of swelling.


Thermal monitoring - Both magma movement and changes in gas release can lead to temperature changes at the volcano's surface.


Hydrology - Borehole measurements are often used to monitor changes in a volcano's subsurface gas pressure and temperature. Increased gas pressure will make water levels rise and suddenly drop right before an eruption, and thermal focusing (increased local heat flow) can reduce or dry out acquifers.


How can people plan and prepare for a volcanic eruption?


If an eruption is predcited, access to the volcano may be restricted or the area might be evacuated (i.e. people moved away to a place of safety). The case study of Mount Pinatubo above shows how these techniques can save a great many lives. We also looked at the example of Chances Peak in Montserrat - the island is zoned into evacuation 'no-go' zones and areas where people can stay but need to be ready to leave quickly. When the volcano started erupting in 1995, access to the south of the island was restricted and around 5000 poeple were evacuated to the northern part of the island where a new port area was built at Little Bay to allow people to leave the island. The volcano is still erupting, and the island zone map and hazard level classification syhstem is shown below.


Hazard Level 1 2 3 4 5
Typical Activity More than one year with no measured activity.

No activity that threatens the north or west.

Low measured activity.

Mild activity that threatens the west.
Significant change of measured activity.
High measured activity.
Lava extrusion that threatens the north or west.
Large unstable dome to the north or west.
Threat of large pyroclastic flows to the north or northwest.
Threat of lateral blast or sector collapse.
Zones map Hazard Level 1 Hazard Level 2 Hazard Level 3 Hazard Level 4 Hazard Level 5
Zones A Unrestricted Unrestricted Unrestricted Unrestricted Controlled access
B Unrestricted Unrestricted Unrestricted Controlled access Controlled access
C Unrestricted Daytime access Controlled access Controlled access Essential workers
F Unrestricted Daytime access Daytime access Controlled access Controlled access
V Daytime access to some areas Controlled access Essential workers Essential workers Essential workers
Maritime Exclusion Zones W Unrestricted Daytime access Daytime transit Essential workers Essential workers
E Unrestricted Essential workers Essential workers Essential workers Essential workers


When lava flows are threatening a town or village, it may be worth the expense of building earth or rock walls to divert lava flow or even using planes and helicopters to drop water to cool the lava and slow it down. This happened in Iceland in 1983. The miltary might be called ino to use bombs and dynamite to alter the flow of the lava (these techniques have been used by the Italian authorieis when managing Mount Etna).


In areas where volcanoes give out large amounts of ash, building design can help to limit the damage caused by an eruption. Flat-roofs are a major risk as ash settles on them and becomes very heavy, causing collapse. Sloping roofs are much better, particularlly if they can be made from smooth materials such as metal and glass which are more likely to shed the volcanic ash. People living in affected areas should also make sure that doors and windows have a good seal and that filter systems are installed to limit ash damage to computer and electrical systems.


Residnets should keep an emergency kit - water, tinned food, wind-up radio, tape, towels, googles, torch, breathing mask - in case the volcano erupts. Campaigns in areas subject to eruptions have helped to educate people about the importance of keeping an eruption kit. In some areas, practice drills are held to make sure that everyone knows what to do in an eruption. Evacuation routes can be planned and tested, and signposts can help to inform people of escape routes. Volcanic hazard maps can also be used to help predict the likely consequences of an eruption. The map below is for Mount Merapi in Indonesia.



What to do following an eruption


  • Evacuate, if advised to do so.
  • Be prepared to stay indoors and avoid downwind areas if ashfall is predicted.
  • Do not approach the eruption area.
  • Be aware of stream and river channels when evacuating.
  • Move toward higher ground if mudflows are approaching.
  • Keep your radio on for reports about what action you need to take. 

    Useful links: 

    USGS webpage - 'How we monitor volcanoes'

    Montserrat Volcano Observatory for up to date information about the hazard map 


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