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Summary of OCR A level Geography Chapter 14.2 pages 473-479
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Summary of Volcanoes, volcanic eruptions, types of eruptions and case studies
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Chapter 14.2 Notes, page 473-479Volcanic eruptions produce a range of landforms. The diversity of these landforms is related
to where the eruption occurs, the type of lava, the variety of the materials and gases
produced, and how the eruption takes place.
The differences in chemistry of the magma influences the type of eruption and shape of the
volcano. Once magma is ejected at the surface, its behaviour is determined by viscosity,
which is determined by chemical composition and temperature. Eruptions are divided into
explosive and effusive volcanoes, and can be classified according to the type of eruption.
Explosive Eruptions Effusive Eruptions
Location Convergent plate boundaries Divergent plate boundaries
Type of Rhyolite (more acid) and andesite Basalt
Lava (less acid)
Lava Acid (high % silica), high viscosity, Basic (low % silica), low viscosity,
Characteris lower temperature at eruption higher temperature at eruption
tics
Style of Violent bursting of gas bubbles Gas bubbles expand freely; limited
Eruption when magma reaches surface; explosive force
highly explosive; vent and top of
cone often shattered
Materials Gas, dust, ash, lava bombs, tephra Gas, lava flows
Erupted
Frequency Tend to have long periods with no Tend to be more frequent, an
of Eruption activity eruption can continue for many
months
Shape of Steep sided strato-volcanoes, Gently sloping sides, shield
Volcano caldera volcanoes, lava plateaux when
eruption from multiple fissures
Types of Volcanoes
Icelandic Lava Eruptions
Persistent fissure eruption, large quantities of basaltic lava build up vast horizontal plains.
On a large scale they have formed the Deccan Plateau and the Columbia Plateau
Hawaiian Eruptions
More noticeable central activity than Icelandic. Runny basaltic lava travels down the side
of the volcano in lava flows. Gases escape easily, occasional pyroclastic activity occurs but
this is less important than the lava eruption
Strombolian Eruptions
Frequent gas explosions which blast fragments of runny lava in the air to form cones. They
are very explosive eruptions with large quantities of pyroclastic rock thrown out. Eruptions
are commonly marked by a white cloud of steam emitted from the crater
Vulcanian Eruptions
Violent explosions blast out plugs of sticky or cooled lava. Fragments build up into cones
of ash and pumice. Vulcanian eruptions occur when there is very viscous lava which
solidifies rapidly after an explosion. Often the eruption clears a blocked vent and spews
large quantities of volcanic ash into the atmosphere
Vesuvian Eruptions
Very powerful blasts of gas pushing ash clouds high into the sky. They are more violent
than vulcanian eruptions. Lava flows also occur. Ash falls to cover surrounding areas
Plinian Eruptions
Gas rushes up through sticky lava and blasts ash and fragments into the sky in a huge
eruption. The violent eruptions create immense clouds of gas and volcanic debris several
kilometres thick. Gas clouds and lava can also rush down the slopes. Part of the volcano
may be blasted away during the eruption.
Phoebe Arnfield HPHI
, Products of Explosive Eruption
Strato-volcanoes are also known as composite volcanoes, made up of layers of ash and acid
lava. They have concave symmetrical profiles, and most of them contain complex internal
networks of lava flows which form minor igneous features such as sills and dykes.
Acid magma does not flow easily, so the vents of strato-volcanoes are often filled with a
mass of solidified magma. This prevents magma from rising freely from depth. As a result of
this, enormous pressures can build up inside of a volcano until
eventually it erupts explosively, sometimes it literally blows its
top off.
A caldera is a volcanic crater (often more than 2km diameter),
it develops when an explosive eruption destroys much of the
cone and the underlying magma chamber is mostly emptied.
Without the underground magma as support for the volcano,
the sides of the volcano collapse and form a caldera. In 1883,
the Krakatoa eruption in Indonesia left a 7km wide caldera.
Convergent plate boundaries and explosive eruptions often give rise to chains of strato-
volcanoes. Indonesia has 130 active strato-volanoes along its archipelago, while in the
Andes, 66 strato-volcanoes have erupted in recorded history.
Products of Effusive Eruption
Lava Basic magma erupts from multiple fissures, vast areas can be covered by free
Plateaux flowing lava. These events are called flood basalts. An example is the Deccan
Plateau in central India, which covers more than 500,000 km 2 , the Columbia
Plateau in the northwest USA covers 130,000 km2. When these plateaux are
first formed, they have a uniform slop of approx.. 1⁰. Millions of years of
denudation however have created a more varied relief, no large scale flood
basalt events have taken place in the past 50 million years.
Shield Effusive eruptions are found at divergent plate boundaries, these boundaries
Volcanoe coincide with mid ocean ridges, so most effusive eruptions occur unseen on the
s ocean floor. Iceland is an example of an effusive eruption, but is an exception
to this trend. Iceland was formed due to effusive volcanic activity, and is one of
the most volcanic regions in the world. Eruptions of basaltic lava result in
volcanoes with gently sloping sides, if successive flows accumulate for long
enough, then huge volcanoes can develop, they extend horizontally for tens of
kilometres, such as Skjaldbredier on Iceland.
Eruptions at Hot Spots
Hawaiian chain of islands lies at the centre of the Pacific plate, thousands of km from
nearest plate boundary. The formation is due to a hot spot.
A hot spot is a fixed area of intense volcanic activity, where magma from a rising
mantle plume reaches the Earth’s surface
As the Pacific plate has slowly moved northwest over the Hawaiian hot spot (at
average rate of 10cm/year), vast amounts of basalt have accumulated on the ocean
floor to produce the Hawaiian Islands. Active volcanic peaks of Mauna Loa and Mauna
Kea on the Big Island (Hawaii) reach over 4000m above sea level, and rise more than
9000m from ocean floor
Phoebe Arnfield HPHI
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