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Volcanoes. Find out more about them in this presentation. Credits. End. Volcanoes. Find out more about …. Information on what causes a volcano, the different layers in the Earth, Plate Tectonics, types of volcanoes and well known eruptions. Why they occur. Where they Occur.

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Find out more about them in this presentation.





Find out more about ….

Information on what causes a volcano, the different layers in the Earth, Plate Tectonics, types of volcanoes and well known eruptions

  • Why they occur
  • Where they Occur

World-wide examples – Pacific Ring of Fire, plate boundaries – Scottish examples and S.W. England.

  • What effect they have

Destructive events, Value to farmland, Value to industry

Terms used (magma, lava, vents, ash, gas), geysers, distinctive rock types, mineral veins and gemstones,

  • Other features
  • Exercises

See if you can answer the questions and solve the puzzles.

  • Useful websites

Links to websites with lots more information about volcanoes.

volcanoes why they occur
Volcanoes: why they occur
  • What causes a volcano?
  • Different types of volcanoes and famous eruptions
  • The different layers of the Earth
  • Plate Tectonics
volcanoes what causes a volcano
Volcanoes: what causes a volcano
  • The earth is unstable.
  • Below the surface of the earth’s crust there is movement, known as plate tectonics.
  • The energy generated can cause molten rock to seek a path to the surface.
  • The force of molten rock or explosive material forms a volcano.
the lava begins to flow
The lava begins to flow
  • There is lots of energy released
  • Gas and ash explodes into the air and lava flows down the sides of the volcano.
why is the volcano forming 1
Why is the volcano forming? 1

Images from scienceclips.com

  • Earth movements in the Crust causes the molten rock to seek ways to reach the surface
  • The pressure causes the molten rock to find a route upwards.
why is the volcano forming 2
Why is the volcano forming? 2
  • Once the surface is breached there could be explosions and clouds of ash and volcanic bombs.
  • This depends on how much gas is trapped within the molten rock.
why is the volcano forming 3
Why is the volcano forming? 3
  • On other occasions the molten rock has more liquid than gas and the lava flows out onto the surface covering a large area.
why is the volcano forming 4
Why is the volcano forming? 4
  • If the molten rock doesn’t reach the surface it will cool down to form hard rock, known as granite.
  • Over millions of years the softer rocks are eroded to reveal the granite.
different types of volcano
Different types of volcano




They differ in the force of their eruption and the same volcano can show more than one type over its active period.

This, in turn, is linked to their location on a tectonic plate.

Live web cameras at http://vulcan.wr.usgs.gov/Photo/volcano_cams.html

Image from the USGS website.

shield volcano

Images from the USGS website.

Shield Volcano
  • Very slow, but easily flowing lava covering a wide area forms shield volcanoes as in the Hawaiian islands.
  • This happens where an oceanic plate moves slowly over a hot spot in the earth’s crust.
  • This type has alternate layers of ash and lava.
  • Sometimes there can be different layers of lava on top of each other, but with years in between eruptions.
cinder volcano
Cinder Volcano

Image from the USGS website.

This is a very dangerous and explosive form caused by tremendous pressure releasing vast quantities of ash clouds which can move at great speed.

Larger pieces of molten rock are known as volcanic bombs.

When the ash cools it can form pumice, a rock full of air bubbles.

cinder volcanoes 2
Cinder Volcanoes (2)
  • There are some famous examples
    • Vesuvius (AD 79)
    • Krakatoa (1883)
    • Mount Pelee (1902)
    • Mount St. Helens (1980)

Image from the USGS website.

These eruptions were caused by a massive build up of pressure on a lava dome causing an explosion throwing ash and gas in a very fast-moving cloud over the surrounding land. Some explosions destroyed part of the volcano.

vesuvius italy ad79
Vesuvius, Italy AD79
  • A famous eruption which destroyed the Roman town of Pompeii.
  • In recent years the ash has been removed to reveal the town and its occupants. This is evidence of how such eruptions take people by surprise.

Image from the USGS website 1944 eruption

krakatoa indonesia 1883
Krakatoa, Indonesia, 1883

This eruption caused the island of Krakatoa to blow apart and spread ash around the world.

The eruption caused water to fill the gap then rebound out as a tidal wave (tsunami) causing great loss of life.



Images from the USGS website.

Image from Sherston CD.

mount pelee martinique
Mount Pelee, Martinique
  • This volcano erupted in 1902 killing almost 30,000 people on the island of Martinique in the Caribbean Sea.
  • Such a high number was the result of the sudden explosion of ash in a fast-moving cloud (160 km per hour) with gases, the speed overwhelming the people.
mount st helens washington usa
Mount St. Helens, Washington, USA


Find out more about this fantastic event.

Before the eruption geologists had time to study the volcano, but even they were surprised by the speed and ferocity of the event.



mount st helen s map
Mount St Helen’s Map

The blast caused damage over several miles.

Millions of trees were bowled over.

Many houses were destroyed and a number of people killed.

inside the earth
Inside the earth

There are four layers

  • The Earth’s Crust
  • The Mantle
  • The Outer Core
  • The Inner Core

From the Sherston Clip Art CD ROM.

plate tectonics 1
Plate Tectonics (1)

Image from the USGS website.

The Earth’s Crust and the top part of the Mantle is composed on a number of separate plates.

Each plate moves slowly over the mantle and its movement results in volcanic activity on the plate boundaries. Note the three types of volcano.

plate tectonics 2
Plate Tectonics (2)

Animation from the USGS website.

The different plates have moved over vast distances in the past 600 million years.

Some plates collide (converge), some move apart (diverge) and some slide past each other (transform).

plate tectonics 3
Plate Tectonics (3)

There are four variations linked to volcanic activity

  • Converging plates
  • Diverging plates
  • Transform plates
  • Hot Spots where an

oceanic plate is very thin

Earth tremors and earthquakes also occur.

Image from the Sherston Secondary Clip Art CD ROM.

Plate Tectonics animationhttp://www.ucmp.berkeley.edu/geology/tectonics.html

plate tectonics converging plates
Plate Tectonics - Converging Plates

Fold mountains

Plates moving towards each other results in one plate sliding below the other.

When this happens the rock are melted by the very high pressure and the molten rock forces its way upwards with volcanic activity if it reaches the surface.

Image from the Sherston CD ROM

Upper Mantle

plate tectonics converging plates 2
Plate Tectonics - Converging Plates (2)

Where an oceanic plate collides with a continental plate it sinks and there is a lot of volcanic activity.

This is the situation on the west coast of North America.

Image from the Sherston CD ROM

plate tectonics converging plates 3
Plate Tectonics - Converging Plates (3)

Volcanic eruptions over the past 4000 years in western U.S.A.

Image from the USGS website.

plate tectonics diverging plates
Plate Tectonics – Diverging Plates

When two plates diverge or move away from each other this allows molten rock to break through the Earth’s Crust spreading lava in long fissures or through vents to form volcanoes.

Animation from the USGS website.

The lava forms a ridge, usually in the middle of oceans such as the Atlantic, forming the Mid-Atlantic ridge.

Sometimes this activity rises above sea level to form islands such as Iceland.

plate tectonics transform plates
Plate Tectonics – Transform Plates

This is where two plates slide past each other.

The plates rub against each other and, when the tension builds up, the pressure is released by an earthquake. Some of these result in volcanic activity, but this is less common.

plate tectonics hot spots
Plate Tectonics - Hot Spots

At a few points on the Earth’s Crust the plate is very thin.

This happens on an oceanic plate.

The best example is the range of islands stretching out from Hawaii which have formed shield volcanoes.

hot spot formation
Hot Spot Formation

Image from the USGS website. Satellite view


In 1963 a Canadian scientist (J Wilson) studied the islands running from Hawaii across the Pacific Ocean almost to the coast of Japan.


He noted that the islands were a string of volcanoes with the younger, active ones being the Hawaiian islands and the older, extinct volcanoes stretching north west for 3000 km.

This is over 3000 km from the closest plate boundary, so what is happening?

hot spot formation 2
Hot Spot Formation (2)

Images from the Sherston Secondary

Clip Art CD ROM.

The oceanic plate is very thin and a magma chamber is close to the surface.

It breaks through releasing lava onto the seabed which grows in time to form seamounts (underwater mountains) and islands if the volcano is above sea level.

When the plate moves then the volcanoes move away from the hotspot and become inactive.

hot spot formation 3
Hot Spot Formation (3)

Image from the USGS website.

Note the move from the younger, active volcano to the older, extinct volcanoes.

hot spot formation 4
Hot Spot Formation (4)

Image from the USGS website.

Other hotspots on the ocean floor. The lines show the boundaries of different tectonic plates.

volcanoes where they occur
Volcanoes: where they occur

Although they appear all over the world their location can be linked to plate tectonics. Some active zones are

  • Pacific Ring of Fire
  • Southern Europe
  • East Africa
  • Mid Oceanic Ridges (Atlantic)

Although there are no active volcanoes in Scotland a lot of areas are the result of volcanic activity in the past. A separate file is available on Igneous Activity in Scotland.

Main Menu

volcanoes where they occur35
Volcanoes: where they occur

Image from the USGS website.

volcanoes what effect they have
Volcanoes: what effect they have

Volcanoes are a natural hazard yet in many parts of the world there are a large number of people living very close to active volcanoes.

In other areas extinct volcanoes have been used by people and wildlife in a number of ways.

volcanoes what effect they have37
Volcanoes: what effect they have

Let’s look at some effects:-

  • Hazards faced during an eruption
  • Why people live close to active volcanoes
  • How extinct volcanic landscapes have been used.

Image from the USGS website (modified)




Eruption Cloud

Eruption Column




Pyroclastic Flow

Acid Rain

Water and sulpher gas mix to create fumaroles.

Many volcanoes are so high they are covered in snow and new volcanic activity melts the snow causing mud flows and landslides.

Sometimes the lava builds up to form a lava dome which can explode with dramatic effect, such as at Mount Pelee and other volcanoes mentioned in this study.

Lava Dome

If there is a lot of gas present the lava explodes out of the vent to cause ash clouds and other features.


Larger pieces of lava are known as volcanic bombs.

Lava can flow over many kilometres and the eruption cloud can extend around the world.

Molten rock reaches the surface to form destructive lava flows.




Lava Flows




why people live close to active volcanoes 1
Why people live close to active volcanoes (1)

Where hot springs reach the surface people have used the mineral rich water for a wide range of products.

They have also helped heat homes and provide electricity from power stations using the steam to drive the turbines as happens in Iceland.

Many poorer people in over populated countries have no alternative place to live and so take a risk living on the dangerous slopes

Lava and ash are rich in minerals. Once they cool and are broken down by the weather fertile soil forms.

The steep slopes are ideal for groves of citrus fruit crops and vineyards, especially if the volcano is located in a warm climate zone.

If you look at a world map of active volcanoes you will notice that they are found in densely populated countries such Japan, Indonesia, and Italy.

So why would the people live there?

why people live close to active volcanoes 2
Why people live close to active volcanoes (2)

Volcanic materials ultimately break down and weather to form some of the most fertile soils on Earth, cultivation of which has produced abundant food and fostered civilizations. -- Excerpt from: Tilling, 1985, Volcanoes: USGS General Interest Publication

The western USA, the fields of Japan, Indonesia, New Zealand, Hawaii and Italy all lie in areas where volcanoes are active now or have been in the past few hundred years.

other benefits
Other benefits

Tourism benefits from visitors being attracted to the magnificent scenery. This results in holiday centres comprising hotels, chalets, food and clothing stores and other services such as petrol stations and transport links being established.

Many volcanic areas are listed as National Parks.

Industry benefits from using aggregates of igneous rocks in road building, gravel for paths, granite for houses and other chemicals for cleaning agents. Mineral veins have encouraged mining of precious metals.

how extinct volcanic landscapes have been used
How extinct volcanic landscapes have been used.

The Ochil Hills, near Stirling

Some of the best examples of an extinct volcanic landscape are to be found in many parts of Scotland.

Some places are easy to spot whilst others look like normal hills until the rocks are studied.

There are many parts of the world no longer near a plate boundary, but when they were volcanoes formed on the surface and igneous rock spread out into the existing rocks forming sills, dykes, laccoliths and batholiths.

Let’s take a closer look at some Scottish examples.

volcanoes other features
Volcanoes: Other Features

Choose from

  • Parts of a volcano
  • Geysers
  • Gemstones
  • Mineral veins
  • Basalt and Granite
  • Calderas

Image from the Microsoft’s DGL website.

Main Menu

parts of a volcano
Parts of a Volcano

Image from the USGS website.

Typical parts of a volcano showing a composite volcano with layers of ash and lava.

You can also see a side vent which hasn’t quite reached the surface and streams of superheated water.

The mixing of lava with the existing rocks can form mineral veins when cooling takes place.



Image from the USGS website.


A geyser is a hot spring which sends extremely hot water bursting out of the earth. Sometimes there are spectacular fountains created. In the Yellowstone National Park in the U.S.A. the geysers are very active.

Another well known location is Iceland, on the Mid-Atlantic Ridge where the how springs are used to create bathing pools and to produce hydro electricity.

Water is heated below the surface by igneous activity.



Images from Sherston Secondary Clip Art CD ROM.


Geothermal areas such as California, Hawaii, Iceland and New Zealand harness the heat and energy held in the trapped, hot water.

Water is heated below the surface by igneous activity.



Colourful, precious stones created by igneous activity.


Think of diamonds, emeralds and other stones and you may be surprised to know that these precious gems are the result of chemical reactions caused by the great heat associated with molten rock mixing with other rocks. When the rock cools crystals form, often in gas bubbles called geodes.

Then if the stone is polished and cut into thin slices it becomes really attractive as shown by these agate examples from the Creetown Museum in South West Scotland.

An agate as it appears within a rock. It doesn’t look very impressive until it is polished. Then ……


mineral veins
Mineral Veins

Modified image from the USGS website.


Igneous activity can form new chemical compounds which are valuable to mankind. The hot magma melts the surrounding rock and when it cools the minerals separate out into mineral veins.

Cornwall was famous for its tin mines over a century ago.

Magma chamber


Veins of copper, tin, gold, silver and zinc are found in areas of past volcanic activity.



basalt and granite
Basalt and Granite
  • Basalt has cooled down quickly on the surface or on the sea bed and is broken down easily.
  • Granite is a very hard rock as it cools slowly underground. It is used in buildings. Aberdeen is known as the Granite City.

Basalt is formed from lava flows and granite from the slow cooling of igneous rocks well below the surface.



Wizard Island a cinder volcano just under 800 m high.


Most volcanoes have a crater near their summit but a caldera is a much larger feature.

It is formed when the magma deep down moves causing the ground to collapse down the vent into the space.

Crater Lake, Oregon.

This caldera is over 9 km wide.

Visit http://www.crater-lake.com


volcanoes exercise 1
Volcanoes: Exercise 1

Image from the USGS website (modified)

Use the list below to label the diagram.

Lava Eruption Cloud

Acid Rain Magma

Bombs Mud flow

Lava Dome Landslide

Fumaroles Ground Water

Pyroclastic Flow

volcanoes exercise 2
Volcanoes: Exercise 2

Find out

  • Why minerals are separated into veins when the molten rock cools down?
  • How a geode forms.
  • Why converging plates result in more explosive volcanoes than a diverging plate.
  • Why earthquakes and volcanoes are found in the same locations across the world.
volcanoes exercise 3
Volcanoes: Exercise 3

There are a number of exercises found on the US Geological Website.

Just click on


volcanoes useful websites 1
Volcanoes: Useful Websites 1

The British Geological Society


The United States Geological Society – a fantastic site


Volcano World http://www.volcanoworld.org/

Movie clips of volcanoes http://www.volcanoworld.org/vwdocs/movies/movie.html

 Link to Mt. St. Helenshttp://www.volcanoworld.org/vwdocs/msh/msh.htm

There are a large number of websites devoted to a study of volcanoes. Some are listed here to get you started.

volcanoes useful websites 2
Volcanoes: Useful Websites 2

Plate Tectonics animation http://www.ucmp.berkeley.edu/geology/tectonics.html

  Plate tectonics explained – quite advanced


Volcanic definitions.



Created by Jim Birney,

Education Adviser,

Fife Education Service,

Scotland. (Jim.Birney@fife.gov.uk)

Images marked USGS are public domain images from the United States Geological Service.

Images marked Sherston are from the Sherston Secondary Clip Art CD ROM with their kind permission.

Other images are taken from Free Clip Art websites such as scienceclips.com and Microsoft’s DGL website.