Scientists uncover evidence to show earth has cooledOctober 24th, 2008 - 3:08 pm ICT by IANS
Sydney, Oct 24 (IANS) Scientists have uncovered evidence that the earth’s core has cooled considerably over the last three billion years, sparking questions about whether plate tectonic movement is only a relatively recent phenomenon. The international research team used synchrotron technology to study the chemical composition of komatiites - ancient volcanic rocks thrown up from the planet’s mantle. Synchrotrons are giant rings around which electrons are fired to produce intense x-rays.
Komatiites have long intrigued geologists because these magmas must have been created by unusually high degrees of melting of the mantle. The examples studied by the team are 2.7 billion years old, and were found in present-day Zimbabwe. Similar magmas are not produced on the planet today.
“Because the rocks are so old, they have been subject to alteration from erosion, weathering and metamorphism,” explained Hugh O’Neill from the Research School of Earth Sciences at Australian National University (ANU), said in its press release.
“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.”
For years, scientists 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. Water lowers the temperature at which rocks melt by many hundreds of degrees.
O’Neill and his 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 lack of water means that the mantle must have been much hotter billions of years ago than it is today, leading the researchers to conclude that the planet has cooled markedly.
The findings were published in Nature.