From: Andy Soos, ENN
Published June 17, 2010 07:10 AM

Oceanic Volcanos

Though unseen the ocean floor is a volcanic hot bed where the tectonic plates collide and spread apart. New research reveals that when two parts of the Earth's crust break apart, this does not always cause massive volcanic eruptions. The study, published today in the journal Nature, explains why some parts of the world saw massive volcanic eruptions millions of years ago and others did not. The Earth's crust is broken into plates that are in constant motion over timescales of millions of years. Plates occasionally collide and fuse, or they can break apart to form new ones. When the latter plates break apart, a plume of hot rock can rise from deep within the Earth's interior, which can cause massive volcanic activity on the surface (sort of like blood from a skin cut).

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A volcano is an opening, or rupture, in a planet's surface or crust, which allows hot magma, ash and gases to escape from below the surface. The word volcano is derived from the name of Vulcano island off Sicily which in turn, was named after Vulcan, the Roman god of fire.

Volcanoes are generally found where tectonic plates are diverging or converging. A mid-oceanic ridge, for example the Mid-Atlantic Ridge, has examples of volcanoes caused by divergent tectonic plates pulling apart; the Pacific Ring of Fire has examples of volcanoes caused by convergent tectonic plates coming together. By contrast, volcanoes are usually not created where two tectonic plates slide past one another.

When the present-day continent of North America broke apart from what is now Europe, 54 million years ago, this caused massive volcanic activity along the rift between the two. This is similar to the present day Great Rift Valley in Africa.

Historically scientists had thought that such activity always occurred along the rifts that form when continents break apart.

However, new research shows that comparatively little volcanic activity occurred when the present day sub-continent of India broke away from what is now the Seychelles, 63 million years ago.

Researchers had previously believed that the temperature of the mantle beneath a plate was the key to determining the level of volcanic activity when a rift occurred. The new study reveals that the prior history of a rift also strongly influences whether or not volcanic activity will occur along it.

In the case of the break up of America from Europe, massive volcanic activity occurred along the rift because a previous geological event had thinned the plate. This provided a focal point where the mantle underneath the plate could rapidly melt, forming magma that erupted easily through the thinned plate and onto the surface, in massive outbursts of volcanic activity.

In comparison, when India broke away from the Seychelles very little volcanic activity occurred along the North Indian Ocean floor, because the region had experienced volcanic activity in a neighboring area called the Gop Rift 6 million years earlier. This exhausted the local supply of magma and cooled the mantle, so that when a new rift occurred, very little magma was left to erupt. So there is volcanic activity but sometimes of a diminished type.

The team reached their conclusions after carrying out deep sea surveys of the North Indian Ocean to determine the type of rock below the ocean floor. They discovered only small amounts of basalt rock, which is an indicator of earlier volcanic activity .The team also used new computer models that they had developed to simulate what had happened along the ocean floor in the lead up to India and the Seychelles splitting apart.

Dr John Armitage, lead author of the paper from the Department of Earth Science and Engineering at Imperial College London, adds: "Our study is helping us to see that the history of the rift is really important for determining the level of volcanic activity when plates break apart. We now know that this rift history is just as important as mantle temperature in controlling the level of volcanic activity on the Earth's surface."

So the Great Rift Valley, as an example, may be mild as it splits further apart or more violent.

For further information: http://www.eurekalert.org/pub_releases/2010-06/icl-nii061510.php

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