Magazine article Oceanus

Taking Earths Inner Temperature: WHY THE MANTLE MAY BE HOTTER THAN WE THOUGHT

Magazine article Oceanus

Taking Earths Inner Temperature: WHY THE MANTLE MAY BE HOTTER THAN WE THOUGHT

Article excerpt

Woods Hole Oceanographic Institution wasn't an obvious fit for Emily Sarafian.

"I always felt a little out of place here, because I don't study the ocean, really," said Sarafian, a recent graduate student of the MIT-WHOI Joint Program.

At WHOI, Sarafian studied geology--specifically, Earth's mantle, the mostly solid part of our planet's interior that lies between its superheated core and its outer crust. One good place to do that is under the seafloor, at mid-ocean ridges.

"At mid-ocean ridges, the tectonic plates that form the seafloor gradually spread apart," said Sarafian. "Rock from the upper mantle slowly rises to fill the void between the plates, melting as the pressure decreases, then cooling and re-solidifying to form new crust along the ocean bottom."

To better understand how that process works, Sarafian needed to know the temperature at which rising mantle rock starts to melt. But how do you take the temperature of the planet, thousands of feet below the ocean surface and tens of miles below the ocean floor?

It's not possible to do directly, so geologists have to estimate it through laboratory experiments. Sarafian used a piston-cylinder apparatus: a massive, old-school, intimidating-looking machine that combines electrical current, heavy metal plates, and stacks of powerful pistons to simulate the high pressures and temperatures found deep inside the Earth: more than 200,000 pounds per square inch and 2,500[degrees] F--hot enough to melt rock.

Sarafian created a synthetic mantle rock to test in the machine. Then, following standard methods of experimental petrology, she dried it in an oven to remove water that would contaminate her results.

"Water--or more specifically, the hydrogen in it--changes the reaction," said Glenn Gaetani, a WHOI petrologist and Sarafian's advisor. "The more water in rock, the lower its melting point."

The problem is that water in the air gets into the powders used to create mantle samples. "So, whether you added water or not, there's water in your experiment," Sarafian said.

To get around that problem, Sarafian added a mantle mineral called olivine in spheres about 300 micrometers in diameter--the size of fine sand grains. …

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