Part of Earth's Mantle Is Melting. Here's What That Means.
Thanks to an alien-looking mineral, scientists have determined that part of Earth's mantle -- which constitutes about 84% of our planet's volume -- is melting, indicating the presence of vast reservoir of water that dwarfs all of Earth's oceans.
Chances are, you've never seen ringwoodite in person, but don't feel too badly. The mineral, a combination of Magnesium, Iron, Silicon, and Oxygen, only forms under the most extreme temperatures and pressures, like those found deep within Earth's interior around 525 and 660 kilometers below. We only first identified ringwoodite in 1969 from the remains of a meteorite. Since then, sightings outside the laboratory were nonexistent until earlier this year, when scientists reported the discovery of a new sample that originated not from space, but from the belly of Earth. The odd mineral had been belched from a volcano -- Earth's upchuck.
The ringwoodite sample held about 1.5% water by weight, bound in the form of hydroxide ions. The finding demonstrated the presence of water in Earth's mantle, which had been theorized for decades.
Today, the result has been confirmed in a new report published in the journal Science. Brandon Schmandt and a team of researchers first observed the seismic signals from earthquakes to glean that melting is taking place in the mantle's transition zone, a region approximately 410km and 660km below the surface. The signals undergo telltale changes when they encounter regions of melt.
"The fastest wave type, the P-wave, converts a little bit of its energy to the S wave type when it encounters sharp boundaries like partial melt. So we process the seismic waveforms to isolate the small amount of converted S-wave energy and figure out where it came from in the Earth," Schmandt told RCS.
The melt appears to extend far and wide.
"We see evidence of it beneath a large fraction of North America. The process could occur globally but just at the lower boundary of the transition zone."
How much melting are we talking about? Percentage-wise, not much. Approximately one percent of the mantle rocks are changing from solid to liquid. But it's not the amount of melt that matters, it's what that melt means.
"This melt exists gives us an indication of how much water is stored in Earth's deep interior. If there were no water in the mantle transition zone then this melting process would not occur," Schmandt told RCS.
And so, the story as science currently knows it goes something like this: As ringwoodite layers in the transition zone shuffle down slightly to the lower mantle, higher temperatures prompt the water contained inside to evaporate as steam, which suffuses in the lower transition zone and melts some of the crystals. Since we observe the melt, we know there's water. And given that ringwoodite is common within the transition zone, there's probably a massive amount of water.
Source: Brandon Schmandt, Steven D. Jacobsen, Thorsten W. Becker, Zhenxian Liu, Kenneth G. Dueker. "Dehydration melting at the top of the lower mantle." Science 13 JUNE 2014 • VOL 344 ISSUE 6189