Subduction zones Some volcanoes are formed when an oceanic plate is pulled underneath a continental plate (see lesson 14.2). As the crust is forced down, it heats up and becomes magma. It can then rise to the Earth’s surface through a magma chamber.
FIGURE2 A subduction zone occurs where an oceanic plate is pulled under a continental plate.
Volcanoes in rift zones The longest mountain range in the world is underwater, between the African and American continents, and is 16 000 kilometres long. It is called the Mid-Atlantic Ridge, and is made up of many volcanic mountains. The volcanoes are formed where two plates move away from each other in rift zones . The molten lava rises to the surface in the space between the plates, and the largest volcanoes appear above the water as islands. Examples of rift islands are Iceland, the Azores, Ascension Island, Gough Island and Bouvet Island. The rifting, or spreading apart, can occur on land or on the seabed.
SkillBuilder discussion Geographical inquiry 1. Look at FIGURE3 . Identify the two plates involved in the formation of Iceland.
FIGURE3 Rifting in Iceland
2. Describe what is
happening at this plate boundary (fault). 3. Which population centres in Iceland may be at risk as a result of volcanic activity?
TOPIC14 Geomorphological processes and hazards
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