Rock can and does liquify, at least in small amounts, as deep as 250 kilometers in the mantle beneath the ocean floor. The starting point for melting has long thought to be 70 kilometers. (Credit: Qfl247 via Wikimedia Commons)

EARTH & ENVIRONMENT – Posted by David Ruth-Rice on Friday, January 11, 2013 10:25

To find where rock melts, go deeper

RICE (US) — Magma melts hotter and deeper in the Earth than previously thought, a finding that scientists say explains several long-standing puzzles.

For a new study published in the journal Nature, researchers put very small samples of peridotite under very large pressures to discover that rock can and does liquify, at least in small amounts, as deep as 250 kilometers in the mantle beneath the ocean floor.

The mantle is the planet’s middle layer, a buffer of rock between the crust—the top 5 miles or so—and the core. If one could compress millions of years of observation down to minutes, the mantle would look like a rolling mass of rising and falling material. This slow but constant convection brings materials from deep within the planet to the surface—and occasionally higher through volcanoes.

A team of scientists focused on the mantle beneath the ocean because that’s where the crust is created and “the connection between the interior and surface world is established,” says Rajdeep Dasgupta, assistant professor of Earth science at Rice University. Silicate melts—aka magma—rise with the convective currents, cool and spread out to form the ocean crust. The starting point for melting has long been thought to be at 70 kilometers beneath the seafloor.

That has confounded geologists who suspected but could not demonstrate the existence of deeper silicate magma, Dasgupta says.

Scientists determine the mantle’s density by measuring the speed of a seismic wave after an earthquake, from its origin to other points on the planet. These waves travel faster through solids than liquids, and geologists have been surprised to detect waves slowing down through what should be the mantle’s express lane.

“Seismologists have observed anomalies in their velocity data as deep as 200 kilometers beneath the ocean floor,” Dasgupta says. “Based on our work, we show that trace amounts of magma are generated at this depth, which would potentially explain that.”

The research also offers clues to the bulk electrical conductivity of the oceanic mantle.

“The magma at such depths has a high enough amount of dissolved carbon dioxide that its conductivity is very high,” Dasgupta says. “As a consequence, we can explain the conductivity of the mantle, which we knew was very high but always struggled to explain.”

Read more: Futurity – To find where rock melts, go deeper.

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