The asthenosphere—the weak, ductile layer that underlies the Earth’s crust—may play a larger role in tectonic activities, including those that cause midplate volcanism known as hot spots, than previously thought.
A new paper published in the 4 December issue of Science (doi:10.1126/science.1261831) examines the prevailing theory about hot spots, namely, that they are fueled by narrow plumes of magma originating at the core-mantle boundary. The new paper discusses an alternative theory—that the asthenosphere plays a larger role.
“What’s going on in the asthenosphere is probably more important toward our understanding of hot spots than continuing to search for conduits to the deep mantle,” Scott King, professor of geophysics at Virginia Polytechnic Institute and State University and coauthor on the paper, told Eos.
There are some inconsistencies with the plume theory, King explained. For instance, when a weak, low-viscosity layer like the asthenosphere is added to three-dimensional spherical models of the Earth, what researchers see is not the many narrow plumes rising from the core-mantle boundary but, instead, one or two large upwellings of hot material.
“[This] causes you to rethink the sort of mental picture of the Earth,” King said, “and whatever it is that causes hot spots doesn’t come from core-boundary regions in terms of plumes.”
King also explained that lava from hot spots does not exhibit the same extensive mixing as lava from mid-ocean ridges because it interacts with the asthenosphere as it rises to the surface.
“We think that has more to say [about] mixing in the asthenosphere than it does about a different source coming from a significantly different part of the Earth,” King said.
Some scientists disagree with this alternative theory.
Barbara Romanowicz, professor of geophysics and director of the Seismological Laboratory at the University of California, Berkeley, said that although it is important to consider the role of the asthenosphere, it may not be the answer to what fuels hot spots.
“I don’t think that the fact that the asthenosphere is active [and] is producing the melt that is needed for hot spots necessarily rules out the existence of deep mantle plumes,” Romanowicz told Eos, “I think more work should be done at the fine scale to elucidate what’s going on.”
“The community needs to kind of take a step back and recognize that understanding the processes in the asthenosphere is incredibly important to understanding what’s going on at hot spots,” King said. “[The asthenosphere] has been particularly underappreciated in terms of its importance.”
Editor’s note: Don L. Anderson, the paper’s lead author, passed away a few days before its publication. See “In Memoriam,” Eos, 95(49), 457, doi:10.1002/2014EO490005.
—JoAnna Wendel, Staff Writer
Citation: Wendel, J. (2014), Research shines light on asthenosphere’s contribution to hot spots, Eos Trans. AGU, 95(50), 477, doi:10.1002/2014EO500004.