Chapter 10
Vulcanicity
Chapter 10: Vulcanicity
Vulcanicity
• Process whereby magma flows into the earth’s crust or
onto the surface
• When magma reaches the surface, it is called lava
• The type of landform that arises depends on:
1. Location where the magma cools or solidifies
2. Nature of the magma (viscosity, temperature)
3. Means by which the magma
reaches the surface
Lava – molten rocks
Chapter 10: Vulcanicity
Location where the magma cools
• When magma enters lines of weakness such as faults
in the earth’s crust, it may cool underground to form
intrusive landforms
• When the magma (lava) cools and solidifies on the
surface, it forms extrusive landforms
Chapter 10: Vulcanicity
Intrusive landforms
1. Batholith
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Large body of igneous rock that was formed when
magma cooled deep within the Earth’s crust
2. Sill
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Sheet of igneous
rock that was formed
when magma cooled
between older layers
of rock
 Roughly parallel
to the surface
 Does not cut across
pre-existing rock
layers
A batholith in Sierra Nevada
Chapter 10: Vulcanicity
3. Dyke
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Mass of intrusive igneous
rock that cuts across preexisting rock layers
Thickness varies from a few
centimetres to many metres
4. Laccolith
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Igneous intrusion that was
formed between two layers of
sedimentary rocks and forced
the overlying strata to arch up
Usually dome- or mushroomshaped
Formed at shallow depths by
viscous magma
A dyke
Chapter 10: Vulcanicity
Viscosity of the magma
• Viscosity refers to the magma’s resistance to flow
• It depends on the silica content and the temperature of
the magma
• Basic magma has low silica content, higher
temperatures and lower viscosity (i.e. more fluid)
 When the magma reaches the surface, it moves
quickly and covers large areas before solidifying
• Acid magma has high silica content, lower temperatures
and higher viscosity (i.e. less fluid)
 The lava moves slowly and solidifies quickly
Chapter 10: Vulcanicity
Means by which the magma reaches the surface
• Acid magma is associated with explosive eruptions
• As the magma rises from deep within the earth, the
dissolved gases in the magma expand due to the lower
pressure
• Because of its viscosity, acid magma does not allow
these gases to escape
• The gases build up and
eventually lead to
explosive eruptions
• Such eruptions generate
a lot of ash and cinders
Mt St Helens during an eruption
Chapter 10: Vulcanicity
Extrusive landforms
1. Lava plateau
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Elevated flat-topped platform with steep slopes
Height ranges from hundreds to thousands of metres
Formed when basic lava escapes through long narrow
openings (fissures) in the earth’s crust and spreads out very
quickly
Over time, the lava accumulates in layers with successive
eruptions
Chapter 10: Vulcanicity
2. Volcano
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Formed when lava escapes through a vent
The lava and ash accumulate around the vent and cool to
form a dome-shaped feature
The volcano grows in size through successive eruptions
The top of the volcano through which lava is ejected is called
the crater
A violent eruption may blow the crater off to create a large
depression known as
a caldera
When water collects
in the caldera, a crater
lake is formed
A crater lake
Chapter 10: Vulcanicity
Structure of a volcano
Chapter 10: Vulcanicity
Classification of volcanoes
1. Active volcano
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Is showing signs of life by giving
off one or more of the following –
gases, steam, ash, rocks or lava
E.g. Soufrière Hills volcano in
Montserrat
2. Dormant volcano
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Currently inactive but may erupt
in the future
E.g. Mount Fuji in Japan
3. Extinct volcano
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Not expected to erupt again
E.g. Mount Kenya in Africa
Chapter 10: Vulcanicity
Types of volcanoes
• The shape and size of the volcano formed depend on
the nature of the lava and the way the lava erupted
• Types of volcanoes:
– Acid lava cone
– Basic lava cone
– Ash and cinder cone
– Composite volcano
Chapter 10: Vulcanicity
Acid lava cone
• Volcano formed from acid lava
• Acid lava is viscous and solidifies quickly
• The lava is unable to move far away from the crater
before it solidifies
• The volcano formed has steep sides and a narrow base
Grand Etang in Grenada is one of the many explosive volcanoes in the
Caribbean
Chapter 10: Vulcanicity
Acid Lava Cone
Chapter 10: Vulcanicity
• A volcanic plug may be formed when viscous magma
solidifies within the pipe of the volcano and is exposed
after years of denudation
Shape of
original cone
Volcanic plug
Remnants of
the cone
Petit Pitons is one of two volcanic plugs
Chapter 10: Vulcanicity
Basic lava cone
• Also called a shield volcano
• Basic lava is fluid and has high temperatures of
between 1,100C and 1,200C
• The lava spreads far from the crater before solidifying
• The resultant volcano has a broad base and gentle
slopes and is often
of low height
Mauna Loa in
Hawaii
Chapter 10: Vulcanicity
Basic Lava Cone
Chapter 10: Vulcanicity
Case study: Hawaiian Islands
• The Hawaiian Islands in the Pacific Ocean are the tops
of massive undersea shield volcanoes
• They were formed far away from the plate boundary as
the Pacific Plate has been moving slowly over a hot
spot
• Hot spots are fixed locations deep within the mantle
from which columns of magma rise to the surface
• The magma rises to the surface by melting through the
crust
• As the plate moves, the volcano over the hot spot is
slowly carried away and eventually becomes extinct
while a new one is formed in its place
Chapter 10: Vulcanicity
Formation of volcanoes in the Hawaii islands
Chapter 10: Vulcanicity
• The hot spot is believed to be currently under the island
of Hawaii, also known as the Big Island
• The entire Big Island is a shield volcano with three
active peaks – Mauna Kea, Mauna Loa and Kilauea
Chapter 10: Vulcanicity
Ash and cinder cone
• Acid lava tends to trap large amounts
of gases, resulting in violent eruptions
• Volcanic rock fragments, called
pyroclasts, are ejected with great
force
• Ash refers to pyroclasts smaller than
4mm while the larger ones are called
cinders
• Larger pyroclasts fall nearer the
summit and form steep slopes
• Finer pyroclasts get blown
farther and form gentle slopes
Mount Bromo in Indonesia
Chapter 10: Vulcanicity
Composite volcano
• Made up of alternate layers of viscous lava, and ash
and cinders
• The gases trapped in the acid magma eventually lead
to a violent eruption
• This ejects ash and cinders which settle around the
crater
• The eruption clears the vent, allowing lava to flow out
freely
• The lava cools and solidifies over the ash and cinders
deposited earlier
• Repeated eruptions of ash and cinders followed by lava
flows build the composite volcano
Chapter 10: Vulcanicity
• The violent eruptions may cause cracks along the sides
of the volcano
• Lava may escape through these cracks, resulting in
parasitic cones
Chapter 10: Vulcanicity
• Mount Fuji in Japan and Mount Mayon in the
Philippines are composite volcanoes
Chapter 10: Vulcanicity
Distribution of vulcanicity
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Most volcanoes are located near convergent plate boundaries and
coincide with earthquake zones

Plate boundary
Big earthquake
Direction of
plate movement
Active volcano
Destructive Plate
Constructive Plate
Tsunami
occurrences
Chapter 10: Vulcanicity
• There are also volcanoes in the Caribbean. Below are
two shuttle radar maps of Grenada and St. Lucia
showing the locations of volcanoes on the islands.
Chapter 10: Vulcanicity
Benefits of vulcanicity
1. Fertile soil
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Weathering breaks down the volcanic material into rich soil
Many farmers live on the slopes of volcanoes such as Mount
Soufrière in St Vincent
The soil supports a large variety of crops such as dasheen,
cassava and tree crops like bananas and coconuts
The banana is one of the crops
that grows well in the rich
volcanic soils of St Vincent
Chapter 10: Vulcanicity
2. Tourism
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Volcanoes, hot springs and geysers found in areas of
volcanic activity are often tourist attractions
Volcanoes can produce beautiful scenery
Examples are the Grand Etang crater lake in Grenada and
the Pitons in St Lucia
The latter have been
declared a United
Nations World Heritage
Site because of their
exceptional beauty
The Pitons are a
world heritage
site
Chapter 10: Vulcanicity
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Hot springs occur when heated groundwater seeps up to
the surface to form a pool of hot water
Examples are the Sulphur Springs of St Lucia and the
Soufriere hot springs in Dominica
Geysers occur when heated
water and steam underground
are periodically forced up into
the air through a vent
The eruptions can reach great
heights
One of the best-known geysers
in the world is Old Faithful in the
Yellowstone National Park, USA
Sulphur springs of St Lucia
Chapter 10: Vulcanicity
3. Geothermal energy
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When magma heats up
underground rocks, nearby
groundwater is heated up as
well
Steam generated from the
heated groundwater can be
harnessed to produce
electricity
4. Precious stones and minerals
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Minerals such as gold, silver and
copper and gemstones such as
sapphire and opal can be found in
igneous rocks
Mining can be carried out to extract
these
Krafla Geothermal Station in
northeast Iceland
Chapter 10: Vulcanicity
Hazards of vulcanicity
1. Lava flows
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Lava flows burn and bury everything in their path
They lead to loss of lives and property, especially when the
flow is through a densely populated area
Lava flow by the
side of the road
Chapter 10: Vulcanicity
2. Landslides and mudflows
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Violent eruptions may trigger landslides and mudflows that
can bury villages and farmlands
In 1985, 23,000 people in the town of Armero were killed
when the Nevado del Ruiz volcano erupted and triggered a
mudflow
Effect of the mudflow
in Armero
Chapter 10: Vulcanicity
3. Pyroclastic flows
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These clouds of superheated ash and gas from volcanic
eruptions can be deadly
The 1902 Mount Pelée eruption produced a pyroclastic flow
that killed 28,000 people in the town of St Pierre
Pyroclastic flows from Soufriere damaged many infrastructure like
roads; cars were damaged
Chapter 10: Vulcanicity
4. Volcanic blasts
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Occur when a large amount of energy is suddenly released
When the blast is directed sideways rather than into the
atmosphere, it can trigger huge floods and mudflows
This happened with the Mount St Helens eruption in 1980
which killed 34 people
5. Poisonous emissions
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The large amount of carbon dioxide released during a
volcanic eruption can suffocate humans and animals
Ash lingering in the air after an eruption can cause lung
problems
In 1986, a volcanic eruption caused one of the crater lakes in
Cameroon to release an invisible cloud of gases that killed
almost 1,800 people in a nearby valley
Chapter 10: Vulcanicity
6. Tsunamis
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Explosive eruptions in the sea can cause tsunamis
These giant waves can wipe out settlements near the coast
Underwater volcano Kick ’em Jenny off the north coast of
Grenada is a potential threat
 Gas bubbles from the volcano’s vent can cause ships to
lose their buoyancy and subsequently sink
7. Environmental effects
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Sulphur dioxide emitted from volcanic eruptions cause acid rain
which pollutes the air and water
Sulphur particles released from huge eruptions can remain in
the atmosphere for a long time and lead to the depletion of
ozone
Volcanic ash can cause engine failure of airplanes and
increase maintenance costs
Chapter 10: Vulcanicity
Case study: Montserrat eruptions
• Montserrat is one of the islands of the Lesser Antilles
• It is the result of the North American Plate subducting
beneath the Caribbean Plate
• The Soufrière Hills volcano erupted violently in 1995,
emitting ash and steam
• The eruption started a
series of larger eruptions
that lasted over two years
• 19 people were killed
• The capital town of
Plymouth was destroyed
Cork Hill is one of the ghost towns created by
the Montserrat eruptions