Volcanoes

Volcanoes are found at:

• Divergent plate boundaries - where plates are moving away from each other.
• Convergent plate boundaries - where plates are pushing into each other.
• Hotspots - where columns of rising magma melt and weaken the crust.

Volcanic eruptions are unpredictable but scientific monitoring can identify signs that an eruption is likely to occur:

• increases in gas emissions
• change in volcano's shape
• increased seismic activity
• increased thermal activity

The threat of an eruption and its impact can be minimised:

• evacuation plans put in place
• exclusion zones keep people out of dangerous areas, or prevent building in areas at risk
• volcanic alert systems notify people of potential or ongoing eruptions

Watch this video to revise the key points about volcanoes in the Environmental Hazards section of National 5 Geography.

Volcanoes usually form along plate boundaries, where tectonic plates are either moving towards or away from one another:

• Divergent plate boundaries - also called a constructive plate boundary, where plates are moving away from each other, for example Iceland.
• Convergent plate boundaries - also called a destructive plate boundary, where plates are pushing into each other, for example Mount St Helens in the USA.

Volcanoes can also form away from plate boundaries over hotspots, for example the Canary Islands or Hawaiian Islands.

Over 450 of the world’s active and dormant volcanoes (around 75%) are located around the Pacific Ring of Fire which runs around the Pacific Ocean, passing through a number of countries including Japan, New Zealand and Mexico.

When two plates move apart, the cracks in the crust allow liquid rock, called magma, to rise from the mantle through the Earth’s crust, to the surface.

When this molten rock reaches the surface it is called a volcanic eruption.

• Destructive plate boundaries involve two plates moving towards each other.
• The heavier plate is subducted (forced down below the lighter plate), into the mantle, where it melts.
• The molten rock rises up to the surface through cracks in the rocks creating a volcanic eruption.
• These eruptions are usually very explosive because they are mixed with gases.

A hot spot is a place where a static column of rising magma occurs in the mantle. This magma plume is hotter than the mantle around it.

The magma pushes up the crust above it creating a dome. The crust weakens and can eventually crack resulting in a volcanic eruption.

As the tectonic plate moves over the hot spot, a chain of volcanoes or volcanic islands can form, for example the Canary Islands or Hawaiian Islands.

• Magma at the surface is called lava and it cools and solidifies into solid rock.
• This process can repeat itself over many years forming a cone-shaped mountain or volcano.

Volcanoes have common features:

• magma chamber - this is where the molten rock is stored beneath the ground at great heat and pressure
• main vent - the channel through which magma rises to reach the Earth's surface
• secondary vent - some magma may escape through the side of the volcano, particularly if the main vent becomes blocked
• crater - this is found at the top of the volcano and the magma usually erupts from here
• cone - this is formed from the material ejected during eruptions and grows in size with each eruption

When a volcano erupts there can be lava, ash, steam or gaseous emissions:

• lava flow - lava is the name for magma that has reached the Earth's surface. Laval flows are rivers of molten rock that flow down the sides of a volcano.
• ash cloud - in an explosive eruption, dissolved gases expand, breaking the magma into tiny particles of rock and glass. These rise up into the air as a cloud.
• pyroclastic flow - a fluid mix of of hot steam, ash, rock, volcanic gases and dust. This hugs the ground and flows at very high speed down the sides of a volcano.
• sometimes eruptions are explosive and lava is thrown out as volcanic bombs.

Countries can try to reduce the risk of damage from volcanic eruptions by attempting to predict when they might occur, protecting their buildings and preparing their population for what to do in the event of an eruption.

The extent to which a country can do this depends on their level of development.

• A high income country like Iceland can spend more than a low income country like Democratic Republic of Congo, meaning the effects would be reduced more.

Prediction

Volcanic eruptions are unpredictable, so volcanoes can be monitored to estimate when and where they are likely to erupt. A variety of techniques are used:

Planning

The threat of an eruption and its impact can be minimised:

• exclusion zones can be put in place to keep people away when there is a threat of eruption. Longer term exclusion zones can prevent buildings from being constructed in areas as possible risk of eruptions.
• evacuation plans are put in place so that people know the safest routes away from danger.
• volcanic alert systems notify people of potential or ongoing eruptions.
• survival kits - people are encouraged to put together kits with bottled water, non-perishable foods, face masks and a battery-powered radio

Measures can be taken during an eruption to minimise danger and damage:

• Bulldozers and diggers can be used to create earth barriers to redirect lava flows away from buildings and infrastructure.
• Spraying water onto lava is used to cool it, so that it becomes more solid and slower.
• Flights can be stopped or diverted to prevent aircraft being endangered or damaged by ash clouds.

The Cumbre Vieja eruption took place on La Palma, one of the Canary Islands, which lie in the Atlantic Ocean.

The Canary Islands are an autonomous region of Spain.

The Cumbre Vieja volcano on La Palma erupted from the 19th September to the 13th December 2021.

Causes

The Canary Islands have over 30 volcanoes, ten of which lie on the island of La Palma.

The volcanoes are located above a hot spot rather than along a plate boundary.

The Canary Island hot spot is located on the African plate, which is estimated to be moving between 2cm to 3cm per year.

Before the eruption

Multiple earthquakes were detected in early September 2021. These were caused by the force of magma moving upwards and fracturing the crust.

Monitoring this seismic activity showed the focuses of earthquakes had risen from a depth of 20 km to just a few kilometres. This showed how close the magma was to the surface.

Satellites and GPS showed the landscape had changed shape, rising by up by 15 cm.

Impact of the eruption

The volcanic eruption began on 19th September, releasing columns of gas and pyroclasts, and flows of lava. The eruption continued until 13th December:

• Rather than one crater, emissions came from 33 separate fissures during the course of the eruption.
• A new volcano (called Tajogaite) nearly 200 m high and 700 m in diameter was built up.
• The column of gas and ash above the eruption reached up to 8.5 km above sea level.
• Ash from the eruption reached other Canary Islands, including Tenerife and Gran Canaria, up to 200 km away.
• Lava flows covered over 12 km² of land
• Around 10 km^2 of farmland, including 4 km² of banana plantations were destroyed, damaging the economy of the island.
• More than 7,000 people had to leave their homes.
• 2,800 buildings were destroyed, including over 1,000 homes, factories, a church and school.
• One man died due to inhaling toxic gases while in the exclusion zone.
• Spanish authorities estimated the direct cost of the eruption was €862.7 million.

Management

• Scientific monitoring allowed the Canary Island Volcano Emergency Plan (PEVOLCA) to be activated before the eruption.
• This allowed the rapid evacuation of 300 people in the local area
• Monitoring of the lava flow showed that possible blocking of a major highway, leading to the evacuation of a further 700 people from the Los Llanos de Aridane coastal region.
• A exclusion zone of 2.5 km around the centre of the eruption was established to protect people from pyroclastics and volcanic gases.
• The hospital in La Palma cancelled routine activity to be ready to take on emergency treatment, although this was not needed.
• Flights to and from the island were halted to prevent ash clouds from damaging planes or causing engine failure.
• After an increase in gas emissions in December, 33,000 people were ordered to stay indoors.
• In early 2022, €5.4 million from the EU Solidarity Fund (EUSF) was paid to Spain to cover costs of immediate emergency and recovery work.

Causes

Mount St. Helens, Washington State, began a series of eruptions in 1980 when a massive landslide and powerful explosive eruption created a large crater, and ended six years later after more than a dozen eruptions of lava built a dome in the crater.

The first sign of activity began in the spring of 1980 with a series of small earthquakes began. After thousands of additional earthquakes and steam explosions, a cataclysmic eruption occurred on 18 May 1980.

Mount St Helens lies close to a destructive plate boundary where the smaller Juan de Fuca plate is being forced into the mantle by the larger North American plate.

Friction and heat cause the plate to melt and, as it melts, molten rocks are formed. The molten rock builds up until it has the chance to reach the surface through cracks in the Earth’s crust.

Before the eruption

Beginning in March 1980, an increasing number of small earthquakes were recorded by seismographs. The earthquakes became more frequent and some were more severe.

Eruptions of steam began from the main crater in late March.

A bulge began to form on the north face of the mountain, reaching over 2.4 km in diameter.

Scientists predicted correctly that the growing bulge would cause a landslide that would weaken the mountain and lead to an eruption.

A state of emergency was declared and authorities were able to evacuate people from the areas surrounding Mount St Helens, and they set up an exclusion zone around the volcano. Emergency services were also on hand to rescue those people needing help.

Impact of the eruption

• The mountain was reduced from a height of 2950m to 2560m as the eruption created the largest landslide ever recorded.

• All plant and animal life within a 25km radius of the volcano was killed, including fully grown trees.

• Mudflows poured down the valleys choking rivers with rock debris, killing fish and ripping trees from their roots.

• Sixty one people died due to mudflows, being crushed to death and poisonous gases, while 198 had to be rescued.

• Mudflows destroyed bridges, houses and logging camps.

• The explosion flattened buildings and trees and knocked out power supplies and telephones.

• Ash clouds resulted in airline flights being cancelled.

• Ash caused £100 million of damage to farm machinery and crops.

In this video clip, Professor Iain Stewart tells the story of Mount St Helens' 1980 eruption.

An earthquake-triggered landslide unleashed a sideways blast of hot gas and ash from this famous stratovolcano volcano (also known as a composite volcano) in the state of Washington, USA.

Volcanoes are found at:

• Divergent plate boundaries - also called a constructive plate boundary, where plates are moving away from each other.
• Convergent plate boundaries - also called a destructive plate boundary, where plates are pushing into each other.
• Hot spots - where columns of rising magma melt and weaken the crust.

Volcanic eruptions are unpredictable but scientific monitoring can identify signs that an eruption is likely to occur:

• increases in gas emissions
• change in volcano's shape
• increased seismic activity
• increased thermal activity

The threat of an eruption and its impact can be minimised:

• evacuation plans put in place
• exclusion zones keep people out of dangerous areas, or prevent building in areas at risk
• volcanic alert systems notify people of potential or ongoing eruptions
• building more eruption-resistant buildings
• earth barriers can be built to divert lava flow away from buildings and infrastructure
• water can be sprayed onto the advancing lava to cool it and slow its movement
• flights can be stopped or diverted away from ash clouds that could damage aircraft.