A new method for mapping Earth's interior shows broad regions underthe Pacific Ocean and Africa where magma hotter than its surroundings isfloating through the upper mantle and then traveling horizontally underthe Earth's lithosphere. Some seismologists call these slow upwelling regionssuperplumes, but primarily relegated the phenomena to the lower mantle.
"Superplumes refer to these two features at the bottom of the mantle,not the hot spot surface expression of narrow plumes," says Barbara Romanowiczof the University of California at Berkeley. "Seismologists don't knowyet if these superplumes are large features from one plume or a collectionof much more narrow plumes in the sense that geodynamicists think of them."
In this week's issue of Science, Romanowicz suggests that thecontinuation of the superplumes through the mantle may require reevaluatingEarth's heat budget from the core and the role upwelling from the mantleplays in the driving forces of plate tectonics.
In these broad regionsof extra hot magma, seismic waves traveling through the mantle slow down.While previous studies indicated these superplumes traveled high abovethe core-mantle boundary, "other models of the upper mantle don't showas clearly the relationships between these two superplumes in the lowerand upper mantle," Romanowicz says.
Rather than using the changing velocity of seismic waves to determinethe temperature patterns of the upper mantle, Romanowicz and graduate studentYuancheng Gung studied the amplitudes of these waves. "Seismic waves travelfaster through cold and rigid slabs, but travel time - or velocity - issensitive to chemical composition and in the upper mantle the effects arecompeting," Romanowicz says. "Temperature distribution blurs travel timesignals making it more difficult to interpret."
At left, the top map showsseismic velocity in the lower mantle, while the bottom map shows the attenutionof seismic waves (a decrease in amplitude or energy) in the upper mantle.High attenuation occurs in areas of high temperature. Image courtesy ofRomanowicz and Gung.
They produced 3-D maps that showed decreasing amplitudes of the waves,or their attenuation through hotter-than-average material in the uppermantle. "We're using attenuation measurements that are more sensitive totemperature. It measures how seismic waves lose energy as they propagatethrough hot, soft absorbing material," Romanowicz explains.
Linking their maps to previous velocity maps of the lower mantle createda close match. "They show the hotter bits in the lower mantle are beneaththe hotter bits in the upper mantle. One is above the other," says geologistKevin Burke of the University of Houston in Texas. But at some points theflow has shifted in the upper mantle, indicating a change in directionof the upwelling flow. "This is an innovative, comprehensive review onthe way in which heat moves in the mantle driving forces for plate tectonics,"he says.
But the role these hot areas play in perpetuating the cycle of platetectonics is itself a contentious sticking point for modelers. "Geodynamicistssay the most important feature in mantle driving convection pattern isthe cold slabs falling into the mantle and displacing material," Romanowiczsays. "They consider the return flow that rises a passive return flow.We think because these features are strong and continue through the mantlethat they must be putting energy into the system. Some heat from the coreis basically driving the upwellings."
Gerald Schubert of the University of California in Los Angeles is skepticalof Romanowicz's findings, calling the comparison between the attenuationof the upper mantle and the velocity of the lower mantle a tenuous connection.Because seismic velocity measurements come with differing interpretations,a better comparison would be "attenuation throughout the mantle and showthe connection between the upper and lower mantle this way," he says. "Thenyou could significantly say when you measure attenuation you have a strongercase to interpret measurement in terms of temperature."
Christina Reed
 |
Members | Publications |