Download Landforms at plate margins – Volcanoes and supervolcanoes

Landforms at plate margins –
Volcanoes and supervolcanoes
Where are active volcanoes found? Why are not all volcanoes the same?
What is different about supervolcanoes?
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Major volcanoes
Figure 1
World distribution of active volcanoes.
A volcano is a cone-shaped mountain formed by
surface eruptions from a magma chamber inside
the Earth. The magma that reaches the surface in
an eruption is called lava, and is one of the many
different products that can be thrown out, including
ash, cinders, pumice, dust, gases and steam. The
world distribution of active volcanoes (Figure 1)
shows an almost perfect fit with the locations of the
tectonic plate margins (see Figure 2 on page 6).
How are volcanoes formed?
Volcanoes form 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 accumulates closer
to the crater during every eruption, a mountain that
is cone-shaped is formed (Figure 2).
Different types of volcanoes
Volcanoes are divided into two main types,
depending upon the material thrown out in an
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Krakatoa
2000 Kilometres
eruption and the form (height and shape) of the
volcanic cone produced. These
differences are
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shown in Figure 3. Basically Job
theNo:
division
J6644 is between
Fig No: S351710_aw_201
volcanoes formed along constructive
plate margins
Artwork By Hl Studios
and along destructive margins,
because of the
different types of lava emitted. Along constructive
margins the basic lava that has come from within
the mantle has a low silica content: it pours out
easily, is runny and flows long distances, building up
shield volcanoes. However, 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.
Figure 2
The Osorno volcano
in Chile, an almost
perfect cone shape.
Last eruption 1869.
Plate margin
Constructive (Figure 4, page 7)
Destructive (Figure 3, page 7)
Formation
As the plates move apart, magma rises upwards from the
mantle to fill the gap. This adds new rock to the spreading
plates. Some of the magma may also be forced out to the
surface through a vent. Some volcanoes grow high enough
to form volcanic islands.
When the plates collide, the denser oceanic plate is pushed down
into the mantle. Here the plate melts and is destroyed in the
subduction zone. In the subduction zone the plateforms a pool of
magma. The great heat and pressure may force the magma along
a crack where it erupts at the surface to build up a volcano.
Form of volcano
Composite cone volcano
Shield volcano (basic lava)
Crater
Vent
Crater
Wide base
Vent
Magma
chamber
Magma
chamber
Characteristics
•
•
•
•
Examples
Hekla and Surtsey in Iceland
Mauna Loa and Kilauea in Hawaii
cone with wide base and gentle slopes
made of lava only
regular and frequent eruptions
lava pours out with little violence
•
•
•
•
tall cone with narrow base and steep sides
made of alternate layers of lava and ash
irregular with long dormant periods
violent explosions possible
Etna, Vesuvius and Stromboli in Italy
Krakatoa in Indonesia
How different plate margins affect volcanoes.
Supervolcanoes
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. To give you some idea of the great volume, the big eruption of
Mount St Helens in the USA in 1980 produced 1km3. A super-volcanic
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. Think of the likely effects on nature and people. What
about farming and food supply? The last known supervolcano eruption
was Toba in Indonesia, about 75 000 years ago.
Example: Yellowstone supervolcano
GradeStudio
1 a State two differences between
each of the following pairs:
(i) Composite
and shield
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Job No: J6644 (2 marks)
volcanoes
No: S351710_AW_202
(ii) AcidFig
and
basic lava
Artwork by HL Studios
(2 marks)
(iii)Volcano and supervolcano
(2 marks)
2 Give reasons why some volcanic
eruptions are more violent than
others.
(3 marks)
1
The restless earth
Figure 3
Layers of
ash and lava
Subsidiary cone
Gentle slopes
3 Is the Osorno volcano in Figure
2 a shield or composite volcano?
Explain your answer.
(3 marks)
Exam tip
When stating differences – make
sure that you mention both, not
just one of them.
Figure 4
Millions visit Yellowstone 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 it covered over
half of North America.
Next eruption? Probability during the next few thousand years is thought to
be low.
Further Research
Find out more about the
Yellowstone supervolcano at the
companion website.
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