Year in review: Pacific Plate slides over slick layer - Mantle might not drive movements after all

With 6,000 kilograms of dynamite and an ear to the ground, a team of geologists shook the understanding of plate tectonics this year.

Ricocheting vibrations from the dynamite blasts, intentionally set off over two nights in New Zealand, gave geologists their first clear glimpse of the underside of a tectonic plate. The work revealed an underlying layer of partially melted rock, 100 kilometers belowground and 10 kilometers thick, that lubricates the motion of the Pacific Plate (SN: 3/7/15, p. 6).

The finding is “remarkable,” says geophysicist Simon Klemperer of Stanford University. “Explaining how these plates move is one of the things that held back the identification of plate tectonics for 50 years.”

The layer contains an estimated 2 percent molten rock, enough to drastically reduce the strength of the rock and essentially grease the overlying plate, like a layer of melted water beneath an ice skater’s blades. Because it is sandwiched between the plate and the mantle, the layer also forms a barrier between the two. That separation challenges the prevailing view that flowing material in the mantle drives plate tectonics, says the geophysicist who led the study, Tim Stern of Victoria University of Wellington in New Zealand. Instead, forces at the edges of tectonic plates, such as the pull of a sinking plate, probably move the rocky slabs across Earth’s surface.

Such a sideways yank is what broke apart the Pangaea supercontinent around 200 million years ago, earth scientist Fraser Keppie of Nova Scotia’s Department of Energy in Halifax proposed in February (SN: 4/4/15, p. 13). Previous explanations held that a rising plume of magma from the mantle wedged the supercontinent apart. Instead, Keppie contends, as the ancient forerunner to the Indian Ocean shrank, Pangaea was pulled from two sides, ripping the continent apart between Africa and North America.

A separate team cruising across the Atlantic Ocean in March and April fired air guns that sent vibrations downward through the seawater and into the ocean crust. That work should reveal whether slick layers are ubiquitous beneath tectonic plates, and further explain how the Earth moves under our feet.