Walking On Water

HOST INTRO: Geologists have found evidence that there’s a massive amount of water trapped deep inside the earth… more than we see in the oceans around us. Pierre Bienaimé has more on what that means for our planet, and others that may be similar to it.

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Steve Jacobsen/Northwestern University.
Steve Jacobsen/Northwestern
University.

Scientists didn’t discover oceans underground. Instead, the water they found is trapped in a mineral called ringwoodite. It only exists hundreds of miles below ground, in a part of the earth that scientists call the mantle. An international team of geologists worked with a sample of the stuff that spewed sizzling hot out of a volcano in Brazil, according to a paper they published in the journal Nature. It was a lucky find. If ringwoodite isn’t kept under the massive amount of pressure that exists in its usual setting deep underground, it turns into something else. And it wouldn’t have been a boon to scientists if it had. So James van Orman of Case Western Reserve University says it all happened pretty quickly.

VAN ORMAN: It had to have come up very fast, so that it cools off quickly enough that it basically quenches in that structure. (0:07)

The tiny sample of the mineral also stayed in its original form because it was trapped inside a diamond.

VAN ORMAN: Diamond is so strong that it actually keeps it under some pressure, it’s basically acting as a little pressure vessel. (0:07) 

That allowed geologists to analyze the ringwoodite. They’ve known from lab tests that the mineral could hold water. Nick Cowan, who teaches geology at Northwestern University, says he and other geologists believed that the mineral held plenty of it.

COWAN: Of course at the time we didn’t know exactly how much water that was, and of course that’s changed now with this nice sample of ringwoodite. (0:09)

Scientists knew roughly how much ringwoodite is underground because of how it affects seismic waves—the kind generated by earthquakes. Those results are consistent with tests on samples of ringwoodite created in laboratories. Based on the amount, and tests on the sample, Cowan estimates there’s actually three times more water deep underground than there is in Earth’s oceans. Ringwoodite holds water because its imperfect structure allows molecules of H20 to get inside. Cowan says they’re forced there by the intense pressure of the mantle.

COWAN: If you think of like a sponge, you have exactly the wrong idea, right? If you squeeze it, that would squeeze out the water. And it turns out that’s exactly the backward picture for how the mantle works. Under these tremendous pressures, there’s actually more opportunity for water to kind of hide in there. (0:12)

All of that ringwoodite holding all of that water is crucial to how the Earth’s geology works. For one, it helps explain a mystery, according to Steve Jacobsen, a colleague of Cowan’s at Northwestern.

JACOBSEN: If you look at the melting temperature of rocks in the deep mantle, the melting temperatures are higher than we believe the actual temperature in the Earth to be. (0:11)

In other words, the mantle shouldn’t be melting. But geologists know that it is because the continents slide around, and occasionally bump into each other, occasionally causing earthquakes.

JACOBSEN: So there has to be some way of generating melt in the upper mantle, and water is probably the key player there. (0:09)

Plate tectonics is responsible for much of the earth’s landscape, like mountain chains. It also explains the way water got deep underground in the first place. Think of the tectonic plates as conveyor belts. Over a massive time scale—hundreds of millions of years—they carry old rocks into the earth through seams on the ocean floors. Some water goes along for the ride, and stays underground. And Jacobsen says it’s a good thing the water doesn’t just sit on the surface— because if it did…

JACOBSEN: I suppose you could argue that most of the Earth’s surface would be submerged and maybe the Alps and the Himalayas would be island chains. (0:09) 

Planetary scientists think that scenarios like that one probably exist elsewhere. They call them waterworlds, since they’re covered in one continuous ocean. But Nick Cowan of Northwestern University says this new study makes waterworlds less likely. Planets like Earth might avoid Biblical-scale flooding the same way it does: by storing some of its water deep inside. Pierre Bienaimé, Columbia Radio News.

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