Shield volcanoes

Landforms at plate margins- volcanoes and
Learning Objectives:
1) To know why people continue to live near volcanoes
2) To know how volcanoes are formed and the difference between a shield
and composite volcano
STARTER: Why do people choose to live
near volcanoes?
60 million people continue to live near or on dangerous volcanoes despite volcanoes
often claiming many lives.
e.g. volcanic areas offer a variety of attractions for tourists, including bathing in the hot
springs and mud pools, watching geysers, volcano walking up to and around the
Why do people live in volcanic areas?
This lava is weathered
(broken down) to form
a fertile soil.
Can you think of any other reasons?
Tourists are attracted to
areas of volcanic activity.
Geothermal energy can be
produced in many volcanic areas.
Volcanoes have a wide range of effects on
humans. These can be problematic or beneficial.
However, many people rely on volcanoes for their
everyday survival. Today, many millions of people
live close to volcanoes for this very reason.
People live close to
volcanoes because
Geothermal energy can
be harnessed by using the
steam from underground
which has been heated by
the Earth's magma. This
steam is used to drive
turbines in geothermal
power stations to produce
electricity for domestic
and industrial use.
Countries such as Iceland
and New Zealand use this
method of generating
Why do people live
close to volcanoes?
Lava from deep within the
earth contains minerals
which can be mined once
the lava has cooled. These
include gold, silver,
diamonds, copper and zinc,
depending on their mineral
composition. Often, mining
towns develop around
Volcanoes attract millions of
visitors (tourists) around the
world every year. Apart from the
volcano itself, hot springs and
geysers can also bring in the
tourists. This creates many jobs
for people in the tourism industry.
This includes work in hotels,
restaurants and gift shops. Often
locals are also employed as tour
Volcanic areas often
contain some of the most
mineral rich soils in the
world. This is ideal for
farming. Lava and
material from pyroclastic
flows are weathered to
form nutrient rich soil
which can be cultivated to
produce healthy crops and
rich harvests. Remember
poor countries such as
this rely on farming as a
way of life.
Hot spots
dust, gas
and steam
What is a volcano?
A volcano is a coneshaped mountain
formed by surface
eruptions from a
magma chamber
inside the Earth.
The magma that
reaches the surface
in an eruption is
called lava.
How are volcanoes formed?
Volcanoes are formed where magma escapes through a vent, which is a fracture
or crack in the Earth’s crust. This happens most often at plate margins. Lava and
other products are thrown out from the circular hole at the top called the crater.
Each time an eruption takes place, a new layer of lava is added to the surface of
the volcano; since more lava accumulates closer to the crater during every
eruption, a cone-shaped mountain is formed.
Different types of volcano
Volcanoes are divided into two main types, depending upon the material thrown out
in an eruption and the form (height and shape) of the volcanic cone produced.
Along constructive margins the basic lava that has come from within the mantle has
a low silica content: it pours out easily, it is runny and flows long distances, building
up shield volcanoes.
Along destructive margins the acid lava has a high silica content, which makes it
more viscous so that it travels shorter distances before cooling; these are more
explosive volcanoes.
After an eruption the vent becomes blocked, which results in great pressure building
up before the next eruption. During explosive eruptions lava is shattered into pieces
so that bombs, ash and dust are showered over a wide area.
Composite are
constructed along
zones. Examples of
volcanoes include
Mount Fugi, and
Alternating layers
of lava and rock
Composite volcanoes usually erupt in an
explosive way. This is usually caused by viscous
magma. When very viscous magma rises to the
surface, it usually clogs the crater and gas in the
crater gets locked up. Therefore, the pressure will
increase resulting in an explosive eruption.
Composite Cone
Mt St Helens - USA
The volcano is built up
of alternate layers of
LAVA and ASH. They can
explode with great
Pyroclastic Flow
Ash layer
Lava Layer
Composite (Mt St Helens)
 Shield volcanoes are huge in size. They are built by
many layers of runny lava flows. Lava spills out of a central
vent or group of vents. A broad shaped, gently sloping
cone is formed. This is caused by the very fluid, basaltic
lava which can't be piled up into steep mounds.
 The eruptions of shield
volcanoes are not very
 Famous shield volcanoes
can be found for example in
Hawaii (e.g. Mauna Loa and
Composite Cone
Mt St Helens - USA
The volcano is built up
of alternate layers of
LAVA and ASH. They can
explode with great
Pyroclastic Flow
Ash layer
Lava Layer
Shield ( Basic) Cone
Mauna Loa Hawaii -USA
Shield Volcanoes are enormous features
built up only from layers of lava. They
produce lots of lava but they tent not to
erupt violently.
Layers of Lava
Lava Flow
250 miles
Composite volcano to scale
Shield volcano (Kilauea)
Internet links
Volcano World - a fun and informative web site
Virtual field visits
The Michigan Technological University Volcanoes Page
Global Volcanism Program
Fallout: Eye on the Volcano
Savage earth
What is a supervolcano?
Learning Objective:
1) To understand what a supervolcano is and how it differs from a volcano
2) To know the potential impact of a supervolcano eruption in contrast to a
volcanic eruption
Primary effects- the immediate impact
• people injured and killed by falling debris; suffocated by poisonous gases or
buried under mud flows after the heat of the eruption melts snow at the top of the
• buildings, homes, property, animals and farmland destroyed. It is many years
before the lava weathers into fertile soil that can be used for farming.
• communications and public services (roads and transport, electricity, telephones)
• livelihoods lost and people forced to migrate
Secondary effects- the medium and long-term impact
• shortages of drinking water, food and shelter
• spread of disease from contaminated water
• economic problems from the cost of rebuilding and the loss of farmland,
factories, tourism and other economic activities
• social problems from family losses and stress
Loss of life is always more likely in volcanic eruptions along destructive plate
margins. Living close to shield volcanoes is safer as lives are rarely threatened when
only lava is erupted, even from a composite cone as people have time to move out
of the way of the lava flows.
Supervolcanoes- are massive volcanoes
A supervolcano is a volcano that erupts with a massive volume of material,
much more than from a normal volcano- at least 1000km3 of magma. They are
very large scale/ mega (colossal) eruptions.
In 1980 the big eruption of Mount St Helens, USA produced 1km3.
A supervolcanic eruption alters the landscape over hundreds, if not thousands,
of kilometres. So much dust is circulating in the atmosphere that it can lead to
a ‘volcanic winter’- lower temperatures on Earth (i.e. global cooling) because
less sunlight reaches the surface. All the world would be affected. The last
known supervolcano eruption was Toba in Indonesia- 75000 years ago.
Supervolcanoes do not look like a volcano with its characteristic cones as they
have large depressions called calderas, often marked by a rim of higher land
around the edges.
Millions visit the National
Park for its surface hot
pools and geysers, but how
many of them know what
lies under the surface and
gives the heat?
Last eruption- about 2 million years ago. Volcanic ash from this eruption covered more than half of
North America. Probability of the next few thousand years is thought to be low.
Supervolcanoes are much bigger than standard volcanoes. They develop in a
few places around the globe- at destructive plate margins or over parts of the
mantle that are really hot (called hotspots) e.g. Yellowstone National Park in
the USA is on top of a supervolcano.
How do supervolcanoes form at hotspots?
Magma rises up through cracks in the crust to form a large magma basin
below the surface. The pressure of the magma causes a circular bulge on the
surface several kilometres wide.
The bulge eventually cracks, creating vents for the lava to escape through.
The lava erupts out of the vents causing earthquakes and sending up
gigantic plumes of ash and rock.
As the magma basin empties, the bulge is no longer supported so it
collapses-spewing up more lava.
When the eruption’s finished there’s a big crater (a caldera) left where the
bulge collapsed. Sometimes these get filled with water to form a large lake
e.g. Lake Toba in Indonesia.
Supervolcanoes are flat (unlike normal volcanoes which are mountains); cover a
large area (much bigger than normal volcanoes); have a caldera (normal
volcanoes have a crater).
Supervolcano: a mega colossal volcano, that erupts at least 1,000km3 of
Caldera: the depression of the supervolcano marking the collapsed magma
Fissures: extended opening along a line of weakness that allow magma to
Geothermal: water that is heated beneath the ground, which comes to the
surface in a variety of ways.
Geyser: a geothermal feature in which water erupts into the air under pressure.
Hot spot: a section of the earth’s crust where plumes of magma rise weakening
the crust. These are away from the plate boundaries.
When a supervolcano erupts there will be global consequences
There are only a few supervolcanoes and an eruption has not happened for many
years e.g. the last one was Lake Tobago supervolcano 74000 years ago. When
there is an eruption it is predicted an enormous area will be affected:
1) A supervolcanic eruption will throw out thousands of cubic kilometres of rock,
ash and lava (much more than normal volcanoes)
2) A thick cloud of super-heated gas and ash will flow at high speed from the
volcano, killing, burning and burying everything it touches. Everything within
tens of miles will be destroyed.
3) Ash will shoot kilometres into the air and block out almost all daylight over the
whole continents. This can trigger mini ice ages as less heat energy from the
sun gets to the Earth.
4) The ash will also settle over hundreds of square kilometres, burying fields and
buildings (ash from normal volcanoes covers a couple of square kilometres.
Describe the likely worldwide effects of a supervolcano eruption (6 marks)
Yellowstone Case study:
• In the immediate area after the eruptions many deaths are predicted totalling 87000.
One in three people will be killed within 1000km of an eruption.
• A large ash cloud rising 40-50km into the atmosphere
• Destruction of 10000 square km of land
• Ash 15cm thick covering building within 10000km causing them to collapse
• Flights suspended
• Live stock dies as it chokes in hot ash and farmland affected
• Planes grounded and road transport difficult
The UK would see
• the arrival of ash 5 days after the eruption. This circles the earth changing the climate.
• temperatures would fall between 12 and 15 degrees- difficult to grow food
• Parts of northern Europe and America and Asia would see constant snow cover for 3
• Crops would fail, monsoon rains would fail and 40% of the population could face
Millions of people live in places where volcanic eruptions can happen putting lives
and risk and so we must try to predict eruptions to minimise the damage.
Effective monitoring and
prediction allows authorities to
warn people living nearby to
evacuate the area reducing the
loss of human life.
Mapping paths of old lava
flows and mudflows can show
areas of risk
Electronic tiltmeters measure very
small changes in the profile of the
Engineeringminor lava flows
Prediction of volcanic eruptions
can be diverted by
bulldozing walls to
turn the flow away
from villages. In
A number if indicators help warn people of the impending
Iceland seawater
is sprayed on lava
• the frequency of earth tremors
flows to solidify
• gravity measurements of the movement of magma inside
the volcano/ bulging around the crater
• the build up of magma causing the ground temperature to
• emissions of steam and gases such as sulphur dioxide rise
Satellites are used to
monitor infra-red
radiation and changes in
heat activity.
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