Q How are volcanoes formed?
A Volcanoes are formed by tectonic plates
colliding with each other. The heavier plate
is usually forced down below the lighter one,
where part of it is melted by the heat of the
crash. The melting plate forms magma
or molten rock that collects below the surface
of the Earth in magma chambers. As the
amount of magma increases, the pressure
inside the chamber rises. This creates
a mountain, or volcano. The volcano has a
cone, from which gases and lava may trickle
out. When the pressure gets too high, the
whole chamber explodes, ejecting the
magma. This is a volcanic eruption.
Planet Earth
Volcanoes
A volcano is a mountain through which molten rock and gases
erupt from the Earth’s crust. Volcanoes are named after the
Roman god of fire, Vulcan.
Quick Q’s:
1. Which is the highest
volcano on Earth?
Mauna Loa in Hawaii is
the highest volcano on
Earth. It rises about 4
kilometres above sea
level; below that, it
extends to 5 kilometres
down before it reaches
the seabed. Its massive
weight has pushed the
volcano down a further
8 kilometres below the
seabed! So Mauna Loa
is 17 kilometres
(56,000 feet) from its
base to its summit.
2. Which is the most
active volcano?
Mount St Helens in
Washington State of
USA is the most active
volcano. It last erupted
in 1980.
3. What are geysers?
Geysers are jets of hot
water that erupt from
the Earth. When water
trickles down into the
hot molten rock under
the Earth’s crust, it is
heated up. As the water
becomes hotter, the
pressure builds up,
finally causing it to
spurt out.
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Q Do volcanoes erupt regularly?
A Volcanoes may be active, intermittent,
dormant or extinct depending on how often
they erupt. Active volcanoes erupt often.
Intermittent volcanoes erupt at regular
intervals. Dormant volcanoes have been
inactive for a long time. They are the most
dangerous because they are merely ‘sleeping’
and can erupt without warning. Extinct
volcanoes have not erupted for thousands of
years. It is difficult to distinguish between
dormant and extinct volcanoes because some
volcanoes may remain quiet for a long time
before suddenly becoming active again.
Movements of the Earth
The tectonic plates that cover the Earth can move in
various ways, and are constantly rubbing against one
another, sometimes violently.
Hot water
Old Faithful is a geyser of hot water in the Yellowstone
National Park, USA. It erupts from the Earth every
90 minutes, on average.
Active volcano
Mount St Helens is the most active volcano in the world.
It last erupted in 1980.
Volcanic rocks
Hardened lava from
volcanoes forms new rocks.
Oceanic Crust
Continental
Crust
Mantle
Q What are the various things that happen
during a volcanic eruption?
A Magma, or molten rock, erupts through
the surface of Earth during an eruption.
Magma that comes out is called lava. It can be
thick and slow moving or thin and fast. Pieces
of rocks and ash also erupt from the volcano.
Pumice stone, a light rock full of air bubbles,
is formed in an explosive volcanic eruption.
These volcanic materials are called pyroclasts.
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