New theory explains mystery of
Yale geophysicists may have unraveled one of the great, unsolved mysteries about the Earth's interior -- why the mantle appears both well mixed and unmixed at the same time.
The Earth's mantle is the 1,800-mile thick layer of rock between the crust and core. Although it is solid rock, the mantle flows quite vigorously due to convection -- heated material rises, and cold material sinks.
"With all of this convective stirring, one would expect the mantle to be well mixed, however lavas collected over the two main types of mantle upwellings that breach the surface have distinct chemical signatures," David Bercovici and Shun-ichiro Karato, professors of geology and geophysics at Yale, say in their study published Sept. 4 in the journal Nature.
Heavy elements, such as uranium and thorium, are depleted in lavas from mid-ocean ridges where tectonic plates pull apart. In contrast, there seems to be enrichment of these elements in lavas from ocean-islands like Hawaii, generated by hot jets of material rising from deep in the mantle.
This finding suggests that these two types of upwellings come from different layers in the mantle that remain unmixed. However, seismological images of the mantle suggest that cold downwellings, known as subducting slabs, sink across the entire mantle. This action is expected to stir the mantle and destroy any layering.
"These pieces of contradictory evidence have been a thorn in the side of the earth science community for decades," Bercovici says. "Many theories have been put forward, most on how to keep the mantle layered or poorly mixed even in the face of all this convective stirring."
Bercovici and Karato proposed an alternative hypothesis: Rather than being divided into unmixed layers, the mantle melts slightly, deep within, and in this process, it is filtered and cleaned. The theory proposes that as most mantle material slowly upwells through a region called the "transition zone" at several hundred kilometers depth, it is hydrated; transition-zone minerals are able to retain relatively large quantities of water.
As this material passes out of the transi-tion zone, it can hold less water, note the scientists. A modest or small amount of water for the material in the transition zone causes the same material above the transition zone to be water-saturated.
Since water-saturated minerals have a lowered melting point, and the upwelling mantle materials are hot, they partially melt. The slight melt readily absorbs and extracts heavy elements from the remaining solid minerals. The resulting "cleaned and filtered" solid continues to rise slowly and generates the depleted lavas of mid-ocean ridges, explain the Yale geologists, while the melt remains behind like a dirty filter, pooled above the transition zone, until it is dragged down by cold, subducting slabs and the "dirt" is stirred into the deeper mantle.
In contrast, contend Bercovici and Karato, when upwelling plume material rises from this deeper enriched mantle, it does not undergo cleaning: It is too hot, and moves too fast to absorb much water in the transition zone. Since it is not water-saturated when leaving the transition zone, it does not melt, and remains unfiltered, delivering enriched "dirty" mantle to the surface at ocean islands, they explain.
Given its radical rethinking of mantle convection, Bercovici and Karato's new theory of mantle filtering, with its testable predictions, is expected to keep geoscientists busy for years to come.
-- By Jacqueline Weaver
T H I S
Earth mantle's mixed-up composition
W E E K ' SS T O R I E S
Yale, unions forge 8-year agreements
Team Lux car to compete internationally
Camp stamp to be dedicated at Cornell game
Scully honored for shaping he vision of urban planners
Once-misunderstood tree is now a state champ
ENDOWED PROFESSORSHIPSAlumnus Robert Post is named the first David Boies Professor
Dan Kahan appointed the Elizabeth Dollard Professor of Law
Stathis Kalyvas comes to Yale as the Arnold Wolfers Professor
Grant supports SOM partnership with non-profits
Episcopal Church at Yale launches new initiatives with recent gift
Events honor theologian Jonathan Edwards' legacy
Jazz luminary, virtuoso pianist to play at Sprague
Texas native is winner of Yale poetry prize
Celebration will feature winners of prestigious Italian literary award
In Focus: Geology and GeophysicsRenovated facilities
Strengthened faculty
Team traces Utah desert sand to Appalachian mountains
Yale geologist leading international study of Italian mountain range
New theory explains mystery of Earth mantle's mixed-up composition
Fall workshop series will focus on gallery's collections and treasures
Symposium honors the contributions of late sociologist Roger Gould
Symposium will showcase the research of graduate students . . .
Open house
Volunteer helps others 'feel at home'
Campus Notes
Bulletin Home|Visiting on Campus|Calendar of Events|In the News
Bulletin Board|Classified Ads|Search Archives|Deadlines
Bulletin Staff|Public Affairs|News Releases|
E-Mail Us|Yale Home