Geography

Volcanoes

Volcanoes

Key Words:

Term	Definition
Pyroclast/Pyroclastic Material	Any material ejected from a volcano including ash, dust and rocks
Subduction	When one plate is brought underneath the other
Volcanologist	Studies volcanoes
Plutons	Granite structures
Caldera	Cauldron like volcano
Silica	The name given to a group of minerals composed of silicon and oxygen, the two most abundant elements in the earth's crust

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Geography

Volcanoes

What's covered:

• How and why volcanic activity occurs
• The location of volcanoes and how they can be predicted
• Features of volcanic activity, both above and below ground
• The effects of volcanoes, both positive and negative
• The effects of volcanic activity on the Irish landscape

Common topics in the exam:

• Labelling a diagram of a volcano
• Positive and Negative effects of volcanoes
• Distribution of volcanoes – link to plate boundaries
• Intrusive and extrusive landforms

Figure 1: A volcanic landscape

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Volcanoes

• A volcano forms when molten magma forces its way to the Earth's surface through a narrow tube called a vent and reaches the surface through an opening called a crater.

• Once magma reaches the surface it is called lava.

• Magma is stored underground in a magma chamber – it can reach 1,000°C here so the rock is in a molten state.

• During an eruption hot ash, volcanic rock bombs, dust and lava erupt – collectively known as pyroclastic material.

Figure 2: Volcanic activity at plate boundaries

Location of volcanoes:

• Most famous example – Pacific Ring of Fire – contains 75% of the world's volcanoes.

Divergent plate boundaries:

• Where two plates pull apart

• E.G the Mid Atlantic Ridge

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Geography

Volcanoes

• A fissure/crack is created in the Earth's surface and magma makes its way to the surface through this fissure.
• Volcanic eruptions are gentle as the magma has a larger opening to reach the surface.

Convergent plate boundaries:

• Two plates collide.
• E.G Nazca and South American plates created the Andes – 2,000 active volcanoes there.
• There is only a small opening between plates.
• As subduction occurs the magma reaches the surface under extreme pressure, therefore volcanic eruptions are often violent.

Hotspots:

• Where a weakness occurs in the centre of a plate due to a superheated plume of molten magma.
• There are 120 hotspots in the world.
• Hotspots are stationary so as the plates move, new islands are created over the hotspots.
• Over time the hotspots become less and less active and eventually extinct.
• They leave a trail of extinct volcanoes behind them – e.g Hawaii.

The quantity of silica present in the magma determines whether or not the volcanic eruption will be explosive or not. A high silica content creates thick lava that traps the gases, creating an explosive eruption.

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Geography

Volcanoes

Types:

Active: erupts regularly – e.g Mount Etna in Italy – erupted in 2015 20 years after the last eruption, spewing lava 7km up into the air.

Dormant: Has not erupted for a long time but may erupt again e.g Mount St. Helens in the USA erupted in 1980 after 120 years.

Extinct: Has not erupted in recorded history – e.g Slemish Mountain in Co. Antrim.

Volcanic Material:

Lava

Acidic

• High Silica content – over 70% - less hot - 800°C
• Does not flow far as it is thick
• Gases cannot escape, becoming trapped which creates violent eruptions
• Creates steep volcanic structures at convergent plate boundaries

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Volcanoes

Basic

• Lower silica content – 55% - hotter – 1,000°C
• Flows greater distances as it is thinner
• Eruptions are less violent as gases are not trapped
• Creates shallow volcanic structures at divergent plate boundaries or hotspots

Pyroclastic Material:

Collective name for material forcefully ejected from volcanoes – ash, cinders, dust, pebbles, rocks. Might also be called Tephra.

Pyroclastic flow is when the material flows down the side of a volcano during an eruption – if it happens on a snow-capped volcanic mountain then the melted snow and pyroclastic material mixes to form a mudslide known as a lahar.

Predicting Volcanoes

Seismic Activity:

• Volcanic activity can also cause earthquakes – a sudden increase in the frequency or force of seismic activity could indicate an eruption is going to happen.

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Geography

Volcanoes

Heat and Gases:

• Rock temperatures around a volcano are constantly monitored. As the magma nears the rock the temperatures increase.

• The quality of air is also monitored. Increased levels of sulfur dioxide indicates that an eruption is imminent.

Changes in the shape:

• Tiltmeters can be installed to identify bulging/change in shape of the side of a volcano.

• Before the eruption of Mount St. Helens in 1980, the side of the volcano bulged, swelling by over 100m.

Volcanic Landforms

When magma reaches the surface, it is referred to as lava. This cools and solidifies to form Basalt.

Extrusive structures are those found on the Earth's crust.

If magma cannot reach the surface it will force its way into folds and faults in the Earth's crust. It will cool and solidify to form plutonic/intrusive rock such as Granite.

Intrusive structures are those found within the Earth's crust.

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Volcanoes

Extrusive Structures

Central Vent Eruption

Most volcanic mountains are central vent eruptions.

1. Composite Volcano

• These have alternate layers of lava and pyroclastic material that build up to form a steep sided cone-shaped mountain.
• Most of the material falls close to the crater. If this becomes blocked, the entire top of the volcano can blow off, creating a huge depression called a caldera.

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Geography

Volcanoes

2 Dome Volcano

• These are also steep sided, rounded volcanoes but are formed by slow moving acidic lava that erupts from the volcano.

• The material does not flow far so the crater can often become blocked, creating very violent eruptions such as Mount St. Helens.

3 Shield Volcano

• Have a very wide base with shallow, sloping sides.

• It is created by the fast-flowing basic lava that erupts from the volcano.

• They are common at hotspots, e.g Hawaii.

Fissure Eruptions

Figure 4

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Volcanoes

• Lava can gently pour through a fissure onto the surface.
• Tend to occur at divergent plate boundaries where basic lava flows through the openings and build up.
• The Derry-Antrim plateau was formed this way.
• A fissure is also active at the Mid-Atlantic Ridge.

Hotspots

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Volcanoes

• Hotspots are unusually warm areas of magma found beneath the Earth's surface, within the mantle.

• They have a fixed location and the plates move over them.

• The superhot magma plumes rise through fissures to form an active volcano directly above on the Earth's surface.

• When the plates move away from the hotspot the volcano becomes inactive.

• A string of extinct volcanoes are then left behind as the plates move away from the hotspot – the oldest volcanoes are furthest away from the hotspot.

• Hawaii is an example of land created by a Hotspot.

Figure 6: Hawaii

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Geography

Volcanoes

Case Study: Mount St. Helens

Case Study of an extrusive volcanic landform (Central Vent Eruption)

What/Why?

• Dormant volcano erupted on 18th May 1980 in Washington State, USA after 120 years of inactivity

• Volcano is on a convergent plate boundary – Pacific Plate is being subducted underneath the North American Plate

• Earthquake measuring 5 on the Richter Scale occurred directly below the volcano causing a landslide of 175 – 250 km per hour – largest/fastest in recorded history

• Side of volcano began to bulge as magma began to rise

• Eruption could be heard 1,120km away as the force of the eruption blew the top off the volcano resulting in a nuée ardente (ash cloud) being blasted 25km into the atmosphere

• The pyroclastic flow (ash, gas, rocks) overtook the landslide and destroyed 600km2 of forest

What/Negative Effects?

• 61 people killed as 300km of road and 27 bridges were destroyed

• An estimated 7,000 game were killed (deer, elk, bears) as well as other birds and mammals when the forests were destroyed (Washington State Dept. of Game)

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Volcanoes

• Approx. 12 million salmon were killed, estimated 40,000 of them were young (Dept. of Fisheries)

• Height of Mount St. Helens reduced by 401 metres following the eruption – now down to 2,549 metres

• $1 billion worth of damage caused

Positive Effects

• Area is now a National Volcanic Monument with tourists accommodated via walking trails and campsites

• Soil is now enriched with nutrient rich volcanic ash making it very fertile

Case Study: Antrim-Derry Plateau

Case Study for extrusive volcanic landform (fissure eruption), can also be used talking about Basalt in the Rock Cycle

What/Why?

• Basalt Plateau extending 4,000km2 and is above sea level in parts up to 350 metres

• Formed in Co. Antrim where the European and American plates began to pull apart

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Geography

Volcanoes

Causes

• Earth's crust was stretched when plates pulled apart causing fissures to form which allowed lava to pour onto the surface

• Successive lava flows built up to form a flat basalt plateau over 15 million years

• New continental crust was formed as the plates continued to move apart

• As Antrim was pushed further away from the divergent plate boundary, volcanic activity ceased

• Giant's Causeway is the most famous section with 60,000 hexagonal columns of basalt formed when magma cooled and contracted slowly above the sea level

Figure 7: Giant's Causeway

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Volcanoes

Intrusive Structures

Figure 8: Intrusive Features

Batholith

• A Batholith is the largest intrusive volcanic landform, with a surface area of 100km2

• It has a domed top with gently sloping sides

• Formed when magma was forced into the crust, cooling and solidifying to form intrusive rock such as Granite.

• It appears on the surface due to Denudation (weathering and erosion) exposes the batholith and plates colliding (results in buckling and compression which creates fold mountains – example is Leinster Batholith from Kilkenny to Dublin)

• They are usually surrounded by metamorphic rocks.

• Wicklow mountains were formed during the Caledonian folding/orogeny – the core of the Leinster Batholith in the Wicklow mountains is Granite. The area surrounding the edge of the batholith is metamorphic rock such as quartzite and schist.

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Geography

Volcanoes

Sills

• Formed when magma is injected horizontally between the layers of sedimentary rock that are found close to the surface.

• The magma squeezes into the bedding planes, melting surrounding rock, then cools to form granite.

Dykes

• Form when magma is injected vertically into fissures in the surrounding rock strata.

• Overtime the magma widens this fissure and eventually cools, forming a vertical layer of granite.

Laccoliths

• Form when magma pushes an overlying rock upwards.

• They have a domed top with a flat base.

• The magma then cools and solidifies to form granite.

Lopoliths

• Form when magma pushes an overlying rock upwards.

• They have a domed top with a flat base.

• The magma then cools and solidifies to form granite.

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Case Study: The Leinster Batholith

This case study can be used for an intrusive volcanic landform OR a distinctive rock landscape for granite in Ireland

What/Why?

Exposed during the Caledonian fold movement 400 MYA – largest Batholith in Ireland and Britain – stretching 120 km from Dublin to Kilkenny – 1,500km2 and includes the Dublin and Wicklow Mountains

Causes

American and Eurasian plates collided, forming Leinster's fold mountains

Part of these plates melted and subducted, allowing large volumes of magma to force its way into the crust

Once this magma cooled and solidified, it formed a dome-shaped batholith underground

This is made of granite – weathering and erosion removes outer layers of rock to expose the granite

The hot magma underground deforms the other rocks, creating layers of metamorphic rock – Lugnaquilla in the Wicklow Mountains has a cap of resistant schist rock on top

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Geography

Volcanoes

Effects of Volcanic Activity

1 Positive

Fertile Volcanic Soils

• Rich, fertile soils called laterite soils, rich in minerals formed from ash and cinder

• Common in many countries such as Brazil and are good for growing coffee

• Fruit and Veg are also grown on the soils in Sicily near Mount Etna

• Also called Terra Rossa soils

Tourism

• Volcanoes are a major source of income from tourists in certain areas, e.g. Mount Vesuvius, Pompeii – around 3 million visitors every year

• Leads to spin off industries such as bus companies, accommodation, restaurants, souvenir shops which generates a lot of employment

• Tourists are also attracted to Geysers and hot springs such as Old Faithful, Yellowstone National Park, USA and Blue Lagoon in Iceland

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Volcanoes

Geothermal Energy

• Water is often heated to over 1,000°C due to the rocks beneath it being in contact with Magma – this can be used to create and harness energy

• The water does not evaporate, instead creating steam which rises, along with hot water, and are gathered by turbines which harness the energy

• This energy is then turned into electricity – in Iceland, 90% of homes are generated by electricity from Geothermal Energy

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Geography

Volcanoes

2 Negative

Lahars

• Mudflow mixed with ice and snow from the top of a volcano
• Can have devastating effects on the surrounding area with the speed that they travel
• Mount Pinatubo in the Philippines erupted in 1991 resulting in the deposition of ash and rocks on the volcano – this then mixed with the snow and ice to from fast moving Lahars killing 847 people – would have been much worse had thousands not been evacuated prior to this

Loss of Life and Destruction of Property

• Eruption of Vesuvius in 79AD caused the death of 20,000 people and the complete destruction of the towns of Pompeii and Herculaneum
• Toxic gas was also thrown into the air such as Carbon Monoxide and Sulfur which are harmful to humans
• Bring in info from Case Study on Mount St. Helens here also