Deep down below us is a tug of war moving at less than the speed of growing fingernails. Keeping your balance is not a concern, but how the movement happens has been debated among geologists. New findings from under the Pacific Northwest Coast by University of Oregon and University of Washington scientists now suggest a solution to a mystery that surfaced when the theory of plate tectonics arose: Do the plates move the mantle, or does the mantle move the plates.
The separation of tectonic plates, the researchers proposed in a paper online ahead of print in the journal Nature Geoscience, is not simply dictating the flow of the gooey, lubricating molten material of the mantle. The mantle, they argue, is actually fighting back, flowing in a manner that drives a reorientation of the direction of the plates.
The new idea is based on seismic imaging of the Endeavor segment of the Juan de Fuca Plate in the Pacific Ocean off Washington and on data from previous research on similar ridges in the mid-Pacific and mid-Atlantic oceans.
"Comparing seismic measurements of the present mantle flow direction to the recent movements of tectonic plates, we find that the mantle is flowing in a direction that is ahead of recent changes in plate motion," said UO doctoral student Brandon P. VanderBeek, the paper's lead author. "This contradicts the traditional view that plates move the mantle."
While the new conclusion is based on a fraction of such sites under the world's oceans, a consistent pattern was present, VanderBeek said. At the three sites, the mantle's flow is rotated clockwise or counterclockwise rather than in the directions of the separating plates. The mantle's flow, the researchers concluded, may be responsible for past and possibly current changes in plate motion.
The research -- funded through National Science Foundation grants to the two institutions - also explored how the supply of magma varies under mid-ocean ridge volcanoes. The researchers conducted a seismic experiment to see how seismic waves moved through the shallow mantle below the Endeavor segment.