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Volcanoes |
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What is a volcano?
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A
volcano is an opening or vent in the earth’s surface
through which molten material erupts and solidifies as lava.
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Cross section
of a volcano
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Why do they
happen at destructive plate boundaries ?
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A destructive
plate boundary is found where a continental plate meets an
oceanic plate.
The oceanic plate
descends under the continental plate because it is denser.
As the plate descends it starts to melt due to the friction
caused by the movement between the plates. This melted plate
is now hot, liquid rock (magma). The magma rises through the
gaps in the continental plate. If it reaches the surface,
the liquid rock forms a volcano.
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| Why do they happen at constructive plate boundaries ? | ||
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At
a constructive plate boundary, two plates move apart.
As
the two plates move apart, magma rises up to fill the gap. This
causes volcanoes. However, since the magma can escape easily at
the surface, the volcano does not erupt with much force.
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| A Shield Volcano | ||
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They occur at
constructive plate boundaries. At constructive plate boundaries the lava is runny and it can flow far away from the volcano's vent. This makes the volcano gentle-sided. Shield volcanoes do not erupt violently as the lava flows freely. |
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| A Composite Volcano. | ||
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They occur at destructive
plate boundaries. At destructive plate boundaries the lava is viscous (thick like treacle) and it cannot flow very far from the volcano's vent. This makes the volcano steep-sided. Such volcanoes erupt violently because the rapidly hardened rock blocks the vents and allows pressure to build up under the blockage. |
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| A Dome Volcano. | ||
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Very viscous lavas. Very explosive with pyroclastic flow. Large debris emitted from a central vent. |
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| A Cinder Volcano. | ||
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Explosive liquid lava. Small debris emitted from a central vent. If continued long enough may build up a shield volcano. |
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| A Caldera Volcano | ||
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Very large composite
volcano. Collapsed after an explosive period. Frequently associated with plug domes. |
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| A Fissure Volcano. | ||
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Very liquid lava. Flows very widespread. Emitted from fractures. |
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| Predicting Volcanic Eruptions. | ||
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Volcanologists (people who study volcanoes) are skilled
at predicting the likelihood of an eruption.
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However, it's very difficult to pinpoint exactly when an
eruption will happen. Often, moving magma doesn't result in an
eruption, but instead cools below the surface. Monitoring
potential eruptions is expensive. With many volcanoes erupting
only every few hundred years, it's not possible to monitor every
site.
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