A few active volcanoes do exist in locations well away from any plate margin. These are volcanic hot spots . They may be in the ocean, which is where the best known example, the Hawaiian islands , lies. Others are on land, for example the volcanic region around Rotorua in the North Island of New Zealand.
Iceland is unique in that it is a hot spot but is located on a mid ocean ridge. As their name suggests, these are points where molten material from the mantle breaks through the rocks of the Earth's crust to reach surface. They are caused by convectional plumes of very hot magma in the mantle which burn through the crustal rocks to reach the surface.
The Hawaiian islands are a perfect illustration of this. All the Hawaiian islands are volcanic. The nearest land is over 3,000 km away. The islands lie on top of the Pacific plate which is moving north westwards by about l0cm a year.
Volcanic activity today only takes place in the south-east corner of the island chain on the big island, called Hawaii. Mauna Loa is a huge volcano It rises 9000 m from the floor of the Pacific Ocean to its summit, which itself is 4,170m above sea level. At its ocean base it is over 90 km wide
Example
The magma comes from the mantle, it is very hot and as it is basalt it has a low viscosity. The activity now seems to be focused more and more on the south east part of the island where Kilauea has been active for over 150 years. Here we see the lava fountains and lava fountains. More recent eruptions have occurred on the coast.
Subsidence due to the pressure &/or erosion by waves the islands can transform into a flat toped sea mount called a guyot.
http://www.youtube.com/watch?v=Hec9yK-QQ4o
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Cheers Big Ears
