The Water Cycle, Revisited

We do lessons on the “water cycle.” But is our version of the water cycle complete? New information suggests that our version of the water cycle may not be complete.

Our story begins with the work of two investigators, one a mineralogist from Northwestern University, the other a geophysicist at the University of New Mexico and continues to include a large scale NSF-funded project called Earthscope. Earthscope is a multi-disciplinary project to explore the Earth in depth through the lens of geoscience in order to advance our understanding of the materials that compose it, its systems, and how the whole functions.

First, the mineralogist, Dr. Jacobson has done research on the minerals that are found in transition zone of Earth’s mantle, 300-400 miles deep. In his lab he attempts to recreate the temperatures and pressures in order to synthesize and study the minerals that exist deep in the Earth. In particular he has recreated in his lab a naturally ocurring mineral called ringwoodite, a kind of olivine but with the remarkable ability to hold water in the form of hydroxide ions. It is thought to be one of the most abundant minerals in the mantle. In the ringwoodite created in Dr. Jacobson’s lab, the mineral contains water at the rate of 2.6 percent of the mineral’s weight. The implication is that there is a huge amount of water locked up in the Earth’s mantle. But this is a finding in the lab not in the mantle.

Enter Dr. Brandon Schmandt, a geophysicist at the University of New Mexico who has done extensive analysis of data developed by the USArray project, part of the Earthscope. The USArray consists of 400 mobile seismographs that have been used to measure energy waves resulting from Earth motions. It has previously been observed that where tectonic plates meet, one plate will be pushed down and the increased pressures will cause the minerals to lose their water.  It has also been demonstrated that the loss of water will drastically lower the melting temperature of the rock. This is known as “dehydration melting.” So the minerals pushed downward at the edge of the tectonic plates, lose their water, lowering the melting temperature and creating the magma “hotspots” that account for the vulcanism where tectonic plates meet.

What had not been observed before was was evidence of melting much farther down in the mantle’s transition zone. The seismic data from USArray indicated a sudden reduction in the velocity of in the downwelling of minerals caused by the convection in the mantle. The loss of velocity is a sign of melting in the rock.  

Drs Jacobson, Schmandt did additional laboratory experiments as well as developing mathematical models that support the conclusion that “suggest hydration of a large region of the transition zone and that dehydration melting may act to trap H 2 O in the transition zone.

So what about the water cycle?

It may no longer be accurate to view the water cycle only in its circulation between the oceans, lakes, and other surface water and the atmosphere. It appears that there is another circulation; that between the water locked into minerals like ringwoodite and the surface. In this circulation, evaporation is replaced by dehydration melting at the top of the mantle.

Science is about explaining natural phenomena. What is so interesting about the science enterprise is that the more one asks questions, the more that is found out. If you think you understand it all, you probably haven’t asked the right questions or looked hard enough. 

 

 

References

 

How did water come to Earth:It took an out-of-this-world arrival to get that perfect chemical combination for water to fill our planet. Brian Greene, Smithsonian, May 2013

 

The Earth’s Hidden Ocean, NYTimes, June 16, 2014.

 

Dehydration melting at the top of the lower mantle in Science 13 June 2014: 

Vol. 344 no. 6189 pp. 1265-1268 

Brandon Schmandt,*, Steven D. Jacobsen,*, Thorsten W. Becker, Zhenxian Liu, Kenneth G. Dueker

 

Melting in the mantle: Joint MIT, Harvard and WHOI seminar “Mantle Convection”
Spring 1998  Lecture by Timothy L. Grove http://quake.mit.edu/hilstgroup/MantleConvection/170298_notes.html

 

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