Volcanoes at the Edge of Tectonic Plates

This GCSE Geography quiz challenges you on volcanoes at the edge of tectonic plates. You should be well aware that volcanoes exist mainly in narrow belts around the world. As the theory of plate tectonics was developed and improved, it became possible to understand how volcanoes were formed, why there were several different types of volcano and why they existed mainly in certain areas of the world.

Before plate tectonics, no-one really understood how volcanoes were formed. It was known that there were magma chambers under volcanoes and that there were different types of volcano which depended on what lava was erupted, but that was about all.

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In 1912, a German meteorologist called Alfred Wegener had a theory that at least some of the continents had been joined together in the past. He came to this conculsion when he was comparing the coastlines of Africa and South America. They seemed to fit together reasonably well. When he looked at the edges of the continental shelves of the two continents, the fit was even better. He also spotted that there were rocks and fossils which matched up too.

But our understanding of volcanoes didn't really change for a long time. Geologists ignored his ideas because he was a meteorologist. Those scientists who seriously considered this idea of 'Continental Drift' could not see that there could be any mechanism of moving the continents around. But gradually, geologists found other pieces of evidence that suggested that Wegener was correct and in the middle of the 1960s, the discovery of magnetic stripes on the sea bed confirmed that it was slowly moving on a very long time scale. Wegener's theory became the theory of plate tectonics which is now widely accepted.

Where plates come together or are moving apart, volcanoes appear. At constructive plate boundaries, as the edges of the plates move apart, a type of magma which produces rocks like basalt pushes its way up from the mantle to the surface, forming volcanoes. This type of lava is not very viscous, so it is relatively mobile (runny like warm treacle). The volcanoes are wide and relatively flat.

At destructive plate boundaries, one plate is pushed down into the mantle where it melts. The molten rock from the plate mixes with other types of rock as it rises through the crust. The magma is more viscous and so it does not flow as far meaning that the volcanoes have a taller and narrower shape. Sometimes, the magma is so viscous that the neck of the volcano is blocked. When the pressure builds up enough, the volcano will explode, with disastrous consequences for the living world around the volcano.

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  1. Which of the following is a primary effect of a volcanic eruption?
    Primary effects are effects that have been caused directly by the eruption
  2. What is the name given to magma that flows out of a volcano onto the Earth's crust?
    The type of igneous rock that is formed depends on the chemical composition of the magma
  3. What type of magma usually occurs in volcanoes at destructive plate boundaries?
    The molten rock from the plate that is being destroyed is mixed with many other types of rock on its journey to the surface
  4. What type of magma occurs in volcanoes at constructive plate boundaries?
    Low viscosity means more it is runny. The magma from volcanoes found at this type of plate boundary comes directly from the mantle and can flow long distances before it cools and solidifies
  5. Where do volcanoes occur?
    These belts are where plates meet
  6. Volcanic eruptions are often seen as being negative, however, they can have some positive effects on an area. Which of the following is NOT a positive effect for a volcanic area?
    Geothermal energy is a renewable resource. We made up the answer about the fish!
  7. Which of the following is NOT a reason why more people might die when a volcano erupts in an LEDC?
    People with low literary skills will not be able to read leaflets and posters telling them what to do. Some people in LEDCs are too poor to be able to afford radios and TVs so they don't get the early warnings. Poor communications infrastructure means that warnings might not reach everyon in the danger zone and if it does, the poor road network makes evacuation harder
  8. What is a destructive plate boundary?
    As the plate is pushed down into the mantle it gradually melts - the mantle is very hot, at least 500°C where it meets the crust
  9. What else comes out of a volcano as well as lava?
    Even volcanoes with runny lava send ash clouds high into the air, a good example of this is the 2010 eruptions of Eyjafjallajökull in Iceland. There was so much ash sent high into the air that many aircraft flights were cancelled for safety
  10. What is a constructive plate boundary?
    It is also sometimes called a diverging plate boundary

Author: Kev Woodward

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