Article Posted on : - Oct 24, 2008

Planet Earth

MELBOURNE (PTI): Scientists have discovered new evidence that the Earth's interior has cooled considerably over the last three billion years, a finding they claim leads to fresh questions about whether plate tectonic movement is only a relatively recent phenomenon.

An international team, led by the Australian National University, used synchrotron technology to study the chemical composition of komatiites -- ancient volcanic rocks thrown up from the planet's mantle -- to come to the conclusion.

“Because the rocks are so old, they have been subject to alteration from erosion, weathering and metamorphism. But there are tiny drops of ancient magma trapped inside crystals in the komatiites that are protected from alteration, and by studying these we've been able to get a sense of what was going on inside the planet in its infancy,” lead researcher Dr Hugh O'Neill said.

For years, researchers have argued about whether or not komatiites formed at a time when the planet's mantle was up to 500 degrees hotter than at present, or if the mantle was only marginally hotter than it is currently, and the unusual composition of komatiites was due to the presence of water.

Now, Dr O'Neill and colleagues from Imperial College in London, University of Tasmania and University of Chicago found that there was no evidence of oxidation inside the melt inclusions, which means that the small amount of water trapped in the inclusions was likely all that ever existed at the time of the rocks' formation, the 'Nature' journal reported.

The lack of water means that the mantle must have been much hotter billions of years ago than it is today, leading the team to conclude that the planet has cooled markedly.

“This might mean that we can't assume that plate tectonics and continental drift occurred in the distant past, because there could have been a different mechanism needed to cool a hotter Earth,” Dr O'Neill said.