Fact


How Plate Tectonics Leads to Volcanic Activity Most of the world’s great volcanoes occur at destructive plate boundaries (Figure 2a). Crustal plates converge at such boundaries, for example near the west coast of South America where the Nazca and South American plates collide. Subduction takes place there. The denser (oceanic) Nazca Plate is pushed under the lighter (mainly continental) South American Plate. As the subducted plate descends into the upper mantle it reaches temperatures of more than 1000°C. It then begins to melt and form molten magma. Molten magma begins to force its way upward towards the earth’s


surface. It often accumulates in huge underground masses or ‘magma chambers’ called batholiths, before erupting to the surface through holes called vents or pipes. Such eruptions are usually violent because:


Frequently asked


exam topic


The Leinster Batholith (described on


page 23) is a large Irish batholith.


Cone-shaped volcanic mountain


Vent Batholith Plate Mantle 2a Volcanic activity at a destructive plate boundary Violent Eruption


l The vents through which the magma erupts are usually quite small. This increases pressure on the moving magma.


l The magma usually contains explosive, super-heated steam and gases.


l Magma derived from crustal plates is usually viscous (thick and sticky), so gases often need to explode in order to escape from it.


Once the magma reaches the earth’s surface the gases in it escape and it is called lava. Layers upon layers of lava gradually build up to form cone-shaped mountains. These mountains are usually steep-sided because the thick, viscous lava cannot run very far from the mouth of the vent.


Gentle eruption Mid-ocean ridge Ocean Mantle 2b Volcanic activity at a constructive plate boundary 14


A great deal of volcanic activity also happens at constructive plate boundaries (Figure 2b). Crustal plates separate from each other at such boundaries, for example beneath the middle of the Atlantic Ocean. Where plates separate, magma emerges from the earth’s mantle through long fissures or cracks in the earth’s crust. These eruptions are often relatively gentle because:


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