Long before geologists worked out the theory of plate tectonics, there was discussion about whether Earth’s continents had moved about. The most detailed, and most famous, case was made by Alfred Wegener after the turn of the 20th century. The best objection to his idea was that he couldn’t provide a plausible mechanism that could drive this “continental drift."

In a 1928 volume, Arthur Holmes proposed a possible answer: convection of rock in the mantle could drag the plates around. This ended up being the dominant explanation when plate tectonics was accepted. But there have since been some challengers. One alternative that could move plates is the density-driven downward sinking of oceanic plates at subduction zones, which people recognized would exert a force that pulled on the portion of the plate that was still at the surface—what’s known as “slab pull.” Once the plates are moving, they'd simply drag nearby mantle along with them.

Now, thanks to some finely detailed imaging, researchers have come up with evidence that, in at least one location, the mantle drove plate motion, rather than being swept up by it. The results will have to be confirmed at other plate boundaries, but it's a good start toward settling one of the oldest arguments in plate tectonics.

A debate has persisted over the relative roles of these factors. To focus on one piece of the system, we can look at the rising, hot mantle rock that produces volcanic activity at mid-ocean ridges. Does the mantle make this happen, or does the motion of the oceanic plate create a gap that the hot rock rises to fill?

We've not come up with an answer in part because the mantle is inherently a tricky thing to study. But with technological advances, our ability to probe its structure with seismic imaging—basically sonar that can penetrate below the surface of rocks—has steadily improved. A group of researchers from Japan's Agency for Marine-Earth Science and Technology, led by Shuichi Kodaira, used that technology to get a very detailed picture of a very old section of crust off the Japanese coast.

The researchers made measurements along two lines—one parallel to a mid-ocean ridge (now subducted beneath Japan) where the crust formed, and one perpendicular. The measurements allowed them to see the locations of prominent layers in the rock, which bounced some of the seismic energy back. But the measurements also let the researchers see how quickly seismic waves moved through different regions of rock. Changes in that velocity can indicate changes in composition, structure, or temperature.

The resulting images revealed a series of evenly spaced surfaces slanted toward the old ridge, like books tilted on a spacious bookshelf. They start at the boundary of the mantle and extend upward into the oceanic plate. The uppermost portion of the mantle exhibited a large amount of “seismic anisotropy”—meaning that seismic waves were able to travel faster through it in one direction than in the other.

That anisotropy is thought to result from an alignment of the mineral crystals that make up the rock of the mantle. Why would they end up aligned? Imagine you added sprinkles to some pie crust dough (for some reason). If you rolled and stretched that dough out in one direction, the sprinkles would tend to align themselves in the same direction. The same thing would happen when mantle rocks get smeared by shear stress.

That would mean that the mantle and crust haven't been traveling in the same direction at the same speed at this location. If they were both moving away from the mid-ocean ridge, one would have to be moving faster than the other to produce the smearing. But which one was faster?

This is where the slanted surfaces come in. They’ve been seen before, and several possible explanations have been offered up to explain them: they could be faults, layers of basalt, or expressions of that smearing. Because of the detail in the new seismic images, the researchers argue for the last of those options. The surfaces resemble a familiar type of deformation pattern produced by shear stress, one that indicates the direction of smearing.

In this case, they point to the mantle moving faster than the oceanic plate. That implies that mantle motion was dragging the plate along rather than plate motion dragging the mantle. That would put upwelling mantle in the driver’s seat at the mid-ocean ridge, rather than it being a passive response to the motion of the plates above.

The researchers point out that this sort of study will have to be repeated in other regions to see if the details are consistent. This doesn’t prove or disprove anything, but it does give geologists something substantive to chew on.

Nature Geoscience, 2014. DOI: 10.1038/ngeo2121 (About DOIs).