Earth's Interior May Contain Oceans Of Water, Geologist Says

Ten years ago, Professor Joseph Smyth discovered that a mineral called wadsleyite, located 250 miles to 350 miles below the earth's surface, could contain water. The wadsleyite does not contain liquid water, but the elements needed to make water bound up in crystals in solid form.

As a rule, rocks on earth are quite dry -- much drier than meteorites, for example, which also contain wadsleyite. Earth rocks generally contain only a small fraction of 1 percent of water. Wadsleyite is about 3.3 percent water.

That may not seem like much, but given the amount of wadsleyite scientists think is in the earth-- it could be three to five times the amount of all the surface water on the planet, Smyth said.

"It's possible the earth has this way of regulating the amount of water on the surface," Smyth said.

The part of the earth where wadsleyite is located, the mantle, convects and occasionally breaks through to the surface in the volcanic vents of mid-ocean ridges where the continental plates are spreading.

Water may be carried down into the upper mantle through subduction zones where plates converge, and it is possible that wadsleyite could store large amounts of water in this region, he said. With rising convection currents, the wadsleyite may melt and release water vapor into the oceans as the molten rock cools.

The wadsleyite in the mantle is at a pressure of about 3 million pounds per square inch and a temperature of about 3,000 degrees Fahrenheit. It is located in the top half of the transition zone between the upper and lower mantle.

The amount of wadsleyite in the earth has been calculated. The big question is how much water the wadsleyite contains, and Smyth is conducting laboratory tests to try to find the answer.

His tests consist of making wadsleyite in the lab by squeezing it between two diamonds in a vice -- which creates 3 million pounds per square inch of pressure -- and then analyzing the sample with X-rays to determine water content and other physical properties.

Determining the amount of water in wadsleyite would allow scientists to know the speed at which wadsleyite transmits seismic waves. If seismic waves sent through the earth matched that speed, scientists would know how much the water is held within the earth.

Smyth chaired a panel of 13 scientific presentations on "Water in the Mantle" Dec. 12 at the fall meeting of the American Geophysical Union in San Francisco.

The earth's oceans have existed for at least four billion years, and have been fairly constant in volume over the last 500 million years. These "inner oceans" may play a role in regulating that supply, Smyth said.

In 1996 Smyth also discovered wadsleyite II, which may store water under even greater pressures at a lower portion of the transition zone. Two undergraduate students and four graduate students are assisting Smyth on his research.

More information on wadsleyite structures can be found on the Internet at http://ruby.colorado.edu/~smyth/Home.html.