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

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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:



Lesson 2 - The Mount St Helen's eruption


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

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