Landforms at plate margins – Volcanoes and · PDF filevolcanoes formed along constructive plate margins and along destructive margins, because of the different types of lava emitted.

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Major volcanoesKey

MountSt Helens

R o c k i e s

Krakatoa

HawaiiMontserrat

Iceland

MountEtna

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Landforms at plate margins – Volcanoes and supervolcanoesWhere are active volcanoes found? Why are not all volcanoes the same? What is different about supervolcanoes?

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 volcanoesVolcanoes 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. These differences are shown in Figure 3. Basically the division is between volcanoes formed along constructive plate margins 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.

World distribution of active volcanoes.Figure 1

The Osorno volcano in Chile, an almost perfect cone shape. Last eruption 1869.

Figure 2

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supervolcanoesA 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.

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Magmachamber

Composite cone volcano

Crater

Crater

Gentle slopes

Shield volcano (basic lava)

Magmachamber

Constructive (Figure 4, page 7)

As the plates move apart, magma rises upwards from themantle to fill the gap. This adds new rock to the spreadingplates. Some of the magma may also be forced out to thesurface through a vent. Some volcanoes grow high enoughto form volcanic islands.

• cone with wide base and gentle slopes• made of lava only• regular and frequent eruptions• lava pours out with little violence

Hekla and Surtsey in IcelandMauna Loa and Kilauea in Hawaii

Etna, Vesuvius and Stromboli in ItalyKrakatoa in Indonesia

• tall cone with narrow base and steep sides• made of alternate layers of lava and ash• irregular with long dormant periods• violent explosions possible

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.

Formation

Form of volcano

Characteristics

Examples

Destructive (Figure 3, page 7)

Subsidiary coneWide base

VentLayers ofash and lava

Vent

Plate margin

ExampLE: Yellowstone supervolcano

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.

How different plate margins affect volcanoes.Figure 3

Find out more about the Yellowstone supervolcano at the companion website.

Further research

Gradestudio1 a State two differences between

each of the following pairs:(i) Composite and shield

volcanoes (2 marks)(ii) Acid and basic lava

(2 marks)(iii) Volcano and supervolcano

(2 marks)

2 Give reasons why some volcanic eruptions are more violent than others. (3 marks)

3 Is the Osorno volcano in Figure 2 a shield or composite volcano? Explain your answer. (3 marks)

Exam tipWhen stating differences – make sure that you mention both, not just one of them.

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?

Figure 4

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