in what could provide vital clues into the evolution of continents in the present form, scientists now have evidence that the earth's mantle contains a water reservoir the size of the Arctic Ocean.
Studying seismograms, a group of geoscientists recently uncovered the first evidence of water in the earth's deep mantle--within 700 to 1,400 km below the surface.
The finding was published in October 2006 in Earth's Deep Water Cycle, a volume published by the American Geophysical Union.
"This explains the reason we have such active movement of the crust with continents zipping around the surface. This is because there is a lot of water in the mantle, and this water softens the rock and allows it to flow more easily. The mantle is entirely solid, but solids can still flow, and water helps in that process," said Michael Wysession, one of the authors.
The scientists said they have been investigating the internal structure, composition, temperature and dynamics of Earth for more than 20 years. The object was to find what Earth is made of and how it works.
"Seismic waves from earthquakes are our best means of seeing into the planet, the way a doctor would use an x-ray or ct scan to see inside a patient," the paper explained.
Wysession analysed the way waves damp out from their source, to come to this conclusion. If you take a hammer and pound it hard on a desk, waves will go from the source to the end of the table with the mass of the table lessening, or attenuating, the power of waves. A picture near the striking point might topple, but a stapler two feet away might not even budge.
Attenuation data tells seismologists how stiff a region is, which depends on how hot it is and how much water it contains. Looking at the seismic wave speeds and attenuation at the same time can tell whether a change in the character of the waves is due to temperature or water.
"Water slows down the waves a little. Lots of damping (flatter waves) and a little slowing match the predictions for water very well," Wysession added.
This factor was most distinctly observed in the top of the lower mantle (700 to 1,400 km below the surface of the earth) beneath eastern Asia. Seismic velocities decreased only slightly in this region and water content could best explain this.
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