New research results from a team led by geologist Rajdeep Dasgupta of Rice University have shown that magma forms much deeper than geologists had previously thought.
The scientists put minute samples of peridotite – a rock derived from Earth’s mantle – under very high pressures in a laboratory and found that the rock can and does liquify at pressures equivalent to those found as deep as 250 kilometres down in the mantle beneath the ocean floor.
Dasgupta said that this answers several questions about Earth’s inner workings.
“The results show that in some parts of the Earth, melting, or magma formation, happens very deep beneath Earth’s surface,” said geologist Jennifer Wade, a program director in NSF’s Division of Earth Sciences, which funded the research.
“It also means that some carbon dioxide and water could come from different sources – and deeper within the Earth – than we believed.”
To take a momentary diversion, the mantle is “a highly viscous layer between the crust and the outer core” according to Wikipedia, and illustrated in the image to the right. It is a slowly churning mass of materials that slides materials from deep within the planet to the surface.
Scientists have for some time been perplexed by their understanding of the mantle an its depth. For many years the starting point for the melting in the mantle was 70 kilometres beneath the seafloor. However when scientists measured the seismic shockwaves from earthquakes they noticed that the waves slowed through parts of the Earth’s outer layers that suggested a liquid layer rather than a solid one.
“Seismologists have observed anomalies in velocity data as deep as 200 kilometers beneath the ocean floor,” Dasgupta said. “It turns out that trace amounts of magma are generated at this depth, which would potentially explain that” slower velocity.
Unable to test directly that far down, scientists have had to make do with determining how rocks that have been carried up from those depths react. So they expose these rock particles to high pressures believed to be representative of what these rocks experience at those depths.
“We have all the necessary tools to simulate very high pressures–to nearly 750,000 pounds per square inch–and temperatures,” said Dasgupta. “We can subject small amounts of rock to these conditions to see what happens.”
Given all that Dasgupta and his team have found, they now believe that melting in the mantle can take place at 200 kilometres and 250 kilometres beneath the seafloor.
Source: The National Science Foundation
Image Source: Wikimedia Commons and Dasgupta Group/Rice University