Activities
Sum Up
l At destructive plate boundaries, subduction leads to violent eruptions and to the formation of volcanoes.
l At constructive boundaries, gentle eruptions lead to the formation of volcanoes and volcanic plateaux.
l Plumes of molten magma rise from hot spots in the mantle to form volcanic mountains or islands. Most hot spots are far from plate boundaries. The Hawaiian islands were formed over a hot spot.
l The products of volcanic eruptions include gases (some of which form nuées ardentes), ash, pyroclasts, lava and lahars (volcanic mudflows).
l Volcanic mountains at destructive plate boundaries tend to be steep-sided composite cones. Typical features include craters, pipes, secondary pipes and conelets. A caldera is a huge crater-like depression that was formed when the top of a volcano exploded or collapsed inwards.
l Gently sloped shield volcanoes occur at constructive plate boundaries or at hot spots such as Hawaii.
l The Antrim Plateau formed when the American and Eurasian plates began to move apart and magma emerged gently through fissures in the ground. The magma cooled to form basalt.
l The most violent volcanic eruption in the USA was that of Mount St Helens. l Intrusive or plutonic volcanic features include batholiths (e.g. the Leinster Batholith), laccoliths, sills and dykes.
l The negative effects of volcanic activity include poisonous gases, nuées ardentes, pyroclastic ‘missiles’, lahars and flash floods.
l Positive effects include the creation of new land, fertile soils, valuable minerals and tourist attractions.
l Geothermal energy in Iceland generates electricity, provides central heating and heats soil in greenhouses. It is a renewable and clean source of energy.
l Vulcanologists try to predict the occurrence and dangers of volcanic eruptions. They study changes in ground level, the history of past volcanic eruptions, the frequency of small earthquakes and the volume and temperature of gases emerging from volcanic craters.
l Active volcanoes erupt regularly. Dormant volcanoes may erupt again. Extinct volcanoes have not erupted in historic times.
1. Name each of the volcanic features labelled 1–5 in Figure 15. (Based on LC Ordinary Level – 30 marks)
2. Explain how the study of plate tectonics has helped us to understand the global distribution of volcanoes. (LC Higher Level – 30 marks)
3. Discuss the positive impacts of volcanic activity. (LC Higher Level – 30 marks)
28 See Chapter 2 of your Workbook 15 3 4 2
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