1. Volcanoes

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3. What is a volcano?
A volcano is an opening or vent in the earth’s surface through which molten material erupts and solidifies as lava.
Volcanic vent

4. Where are volcanoes found?
Around which plate do we find most volcanoes?

5. Label this cross section of a volcano
Volcanic bombs, ash, lava, gases
Magma chamber
Parasitic cone
Crater
Main vent

6. Cross section of a volcano
Volcanic bombs, ash and gases
Crater
Parasitic cone
Main vent
Magma chamber

7. Why do they happen?
A destructive plate boundary is found where a continental plate meets an oceanic plate.
The oceanic plate descends under the continental plate because it is denser. As the plate descends it starts to melt due to the friction caused by the movement between the plates. This melted plate is now hot, liquid rock (magma). The magma rises through the gaps in the continental plate. If it reaches the surface, the liquid rock forms a volcano.
Destructive Plate Boundary

8. Destructive plate boundary

9. Why do they happen?
At a constructive plate boundary, two plates move apart.
As the two plates move apart, magma rises up to fill the gap. This causes volcanoes at this type of boundary. However, since the magma can escape easily at the surface, the volcano does not erupt with much force.
Constructive Plate Boundary

10. Constructive plate boundary

11. At destructive plate boundaries the lava is viscous (thick like treacle) and it cannot flow very far from the volcano’s vent. This makes the volcano steep sided.
At constructive plate boundaries the lava is runny and it can flow far away from the volcano’s vent. This makes the volcano gentle sided.

12. Volcano Shapes
Why are these volcanoes different shapes?

13. Do all volcanoes erupt?
Active volcano – liable to erupt e.g. Mt Etna.
Dormant (sleeping) volcano – a volcano which has not erupted for many years. For example, Mt Pinatubo erupted in 1991 after 500 years of dormancy.
Extinct volcano – a volcano which has not erupted for many thousands or millions of years e.g. Edinburgh.
However, it is often very difficult to tell whether a volcano will erupt again…El Chichon, Mexico erupted in 1982 after being dormant for approximately 1200 years!

14. The Hawaiian islands are a chain of volcanoes in the Pacific Ocean.
Look at their location on the map below. Why is this an unusual place for them to be located?

15. Hot spot volcanoes
This ‘conveyor belt’ animation is a simplified version of the formation of hot spot volcanoes/ chains of islands.
Internet Links – US Geological Survey Hawaiian volcanoes
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In the animation above, why are the volcanoes to the left of the ‘hot spot’ extinct?

16. located on the ‘Ring of Fire’
Mt St Helen’s eruption (May 1980)
Mt St Helens
located on the ‘Ring of Fire’
While all care is taken to ensure web links contain useful information, Boardworks does not take responsibility for the content or accuracy of external web sites.
Internet Links

17. Mt St Helens – causes of the eruption
North American Plate
Juan de Fuca plate
Which is the oceanic plate?

18. Mt St Helens – the eruption
Animation showing the Mt St Helen’s eruption with text.

19. Mt St Helens – consequences of the eruption
10km
Interactive map:
Redrawn map, showing compass point, scale line, Mt St Helens, North and South Toutle rivers and area of total destruction. Buttons (North Toutle, trees flattened, Portland, Mt St Helens, Spirit Lake, ash cloud) reveal extra information when clicked…e.g. click on North Toutle button and ‘mudflows 200m thick in the upper Toutle valleys’ appears.

20. Mt St Helens
AFTER
BEFORE

21. What damage did the eruption cause?
Up to 70mm of ash falls across Washington and East Montana… 7000 school students have an early summer vacation due to problems on the roads
An increase in tourism
Mudflows block and divert rivers. The US army takes 18 months to dig out the debris from the River Toutle.
Two million birds, animals and fish are killed.
Communications routes threatened – railway and road bridges destroyed, Portland harbour blocked
A decrease in tourism
Hot mudflows raise the temperature of the rivers and lakes to 30ºC
60 people die from the eruption.
Gophers, through their tunnelling, mix the soil with the ash. This increases the fertility of the soil.
Deer mice, chipmunk, vole, gophers, salamander survive the blast. Their population steadily increases due to the absence of predators.
Damage to crops=$175million
The post office and Lone Fir Motel shut down in Cougar.
Separate the above into the short term and long term impacts of the eruption.

22. Deer mice, chipmunk, vole, gophers, salamander survive the blast
Deer mice, chipmunk, vole, gophers, salamander survive the blast. Their population steadily increases due to the absence of predators.
Why do you think animals such as the vole and gopher survived the blast?
What benefits might the volcanic activity have brought to Mt St Helens?

23. Why do people live in volcanic areas?
This lava is weathered (broken down) to form a fertile soil,
Can you think of any other reasons?
Geothermal energy (heat from the earth) can be produced in many volcanic areas.
Tourists are attracted to areas of volcanic activity.

24. Nevada del Ruiz, Colombia (November 1985)
Nevada del Ruiz is located in the Andes. The volcano had been giving signs of increased activity for some time and on the 13th November the volcano erupted, producing a cloud of steam, ash and rocks. The heat melted snow on the volcano and the water produced combined with ash, creating a mudflow. This mudflow rushed down the valley at 60km/hour towards the town of Armero and engulfed the town with mud up to 5m deep. It was just before midnight and most people were in bed. By the time the rescue team reached Armero, the next day, the mudflows had set hard.
23,000 dead
5,000 injured 5,000 homes destroyed
The eruption started at 9.08pm. If the mudflows reached Armeno at 11.30pm, how far is Armero from Nevada del Ruiz?
What could have been the volcano’s ‘signs of increased activity’?
Why wasn’t Armero evacuated when the volcano had shown these ‘signs of increased activity’?

25. Internet links!
Volcano World - a fun and informative web site
Virtual field visits
The Michigan Technological University Volcanoes Page
Global Volcanism Program
Fallout: Eye on the Volcano
Savage earth
While all care is taken to ensure web links contain useful information, Boardworks does not take responsibility for the content or accuracy of external web sites.