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How volcanoes are formed
Volcanoes form when magma works its way up through the mantle and eventually all the way through the surface of the Earth where it erupts outward. An active volcano is currently erupting or has erupted recently. Dormant volcanoes have not erupted recently or in a long time but there is still some activity going on underground. Extinct volcanoes have not erupted in a long time and there is no activity going on underground.
Source: slipperysnake.co.uk
Source: slipperysnake.co.uk
Different types of volcanoes
There are different main types of volcanoes; Cinder Cones, Shield Volcanoes, Lava Domes, and Composite Volcanoes. Cinder Cones are made of small fragments of ash and lava that have fallen and created a circular or ovular base to the volcano. Shield volcanoes are volcanoes shaped like a shield. They spit out lava composing of mostly basalt. Composite Volcanoes are steep sided volcanoes and their lava has a high viscosity. Lava Domes form when the lava is too thick to erupt out of the opening and so it builds up and erupts out of another area of the volcano. Grimsvotn is Iceland's most active volcano and it is a sub glacial caledra lake which means that there is a lake under a glacier and it is warmed by geothermal heat. Iceland has mostly composite volcanoes. These form when lava escapes through a fissure and then runs a long span of land. One example of a composite volcano is Hekla. Hekla's last eruption was on February 28, 2000.
How was Grimsvotn volcano formed and how did it erupt?
Since Grimsvotn is an Icelandic volcano, it has formed on a divergent plate boundary. Most volcanoes form on convergent plate boundaries where one plate goes under another plate. This means that the plates are moving apart, creating new crust material. There is a hotspot under Iceland which causes the magma to come up and also caused Grimsvotn to erupt because of the divergent plate boundaries spreading apart.
http://www.youtube.com/watch?v=2j1UzJulWBk