What is a volcano?

     
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an erupting volcano

Lava gushes from a spatter cone at Kilauea, Hawaii, 2004. Hawaii Volcano Observatory (United States Geological Survey) image.

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A volcano is an opening in the earth’s crust through which lava, volcanic ash, and gases escape. Volcanic eruptions are partly driven by pressure from dissolved gas, much as escaping gases force the cork out of a bottle of champagne. Beneath a volcano, liquid magma containing dissolved gases rises through cracks in the Earth’s crust. As the magma rises, pressure decreases, allowing the gases to lớn size bubbles. How the magma (lava) behaves when it reaches the surface depends on both its gas content and chemical composition. Lavas with low silica contents have sầu low viscosities và flow freely, allowing any gas bubbles lớn escape readily, while lavas with high silica contents are more viscous (resistant lớn flow), so that any trapped gases cannot escape gradually.

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Mount Garibaldi

Mount Garibaldi, a potentially active volcano in southwest British Columbia, located within 30 km of the towns of Whistler và Squamish.

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Nazko cone
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Images of volcanic eruptive sầu behavior

How the silica và gas content of lava affect volcano eruptive sầu behaviour. Images courtesy of (a) and (c) Hawaii Volcano Observatory (HVO), (b) United States Geological Survey (USGS) /Alaska Volcano Observatory (AVO) (Clucas), and (d) USGS/AVO (McGimsey).


A low viscosity (runny) lava, like basalt that contains lots of gas, forms fire-fountains, spewing spectacularly inkhổng lồ the air and breaking inkhổng lồ globs that solidify as they fall khổng lồ the ground. Small fire-fountain eruptions produce cinder cones (lượt thích Eve sầu Cone in northern British Columbia). When runny lava contains less gas, however, it erupts in outpouring lava flows. Repeated fire-fountain và lava flow eruptions over long time periods form gently sloping shield volcanoes like Anahyên Peak in central British Columbia, & the volcanoes of the Hawaiian Islands. Andesite, dacite, and rhyolite lavas are progressively higher in silica & more viscous, so gases cannot escape gradually. If high-silica lavas contain little trapped gas, they may ooze slowly onto lớn the surface khổng lồ pile up as steep-sided lava domes. When high-silica lavas contain lots of trapped gas, the pressure builds up & is released in explosive sầu eruptions that produce volcanic ash. Some volcanoes experience both explosive sầu and non-explosive activity, alternating explosive eruptions with periods of dome-building, forming stratovolcanoes lượt thích Mount St. Helens và Mount Garibaldi.

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