Gravity lows mark burial sites of ancient tectonic plates

Dips in Earth's gravitational field tied to 'slab graveyards'

Scientists have unearthed a new explanation for several low-gravity spots detected around the world. They’re blaming the anomalies on vast “slab graveyards” that lie buried deep near the planet’s core.

RESTING IN PIECES Earth’s gravitational field is much lower than average in some places (dark blue areas circled in red) because of water-rich material that lies over the dehydrated remnants of long-subducted tectonic slabs. Spasojevic et al., Nature Geoscience

When these slabs of rock were buried long ago, they released water that reduced the density of overlying rock, Caltech geophysicist Michael Gurnis and his colleagues reported online May 9 in Nature Geoscience. Low-density rock has less mass, and so less gravitational pull.

Scientists had previously noticed that gravity’s tug is smaller where tectonic plates, or large sections of Earth’s crust, once plunged below the surface, Gurnis says. The team’s new findings, he notes, provide insight into the causes of super-low values measured in four regions — in particular, areas south of Asia, along the coast of Antarctica south of New Zealand, in the northeastern Pacific and in the western Atlantic.

Beneath those four areas, seismic waves travel slower than normal at shallow depths —1,000 kilometers or less — but faster than normal deeper down, says Gurnis. Because sound waves travel faster through dense material (for example, water compared with air) the difference in speed with depth told the team that lighter, less dense material is lying over a layer of very dense material. Most likely, the top layer’s relative buoyancy stems from its water-rich composition, the researchers contend.

The water could have been injected in the mantle as a result of slow-motion tectonic collisions between lighter and heavier plates. As dense slabs slipped downward through the mantle, water in those rocks — as well as water in the seafloor sediments that had accumulated above those slabs — was released. “When that water was added to the surrounding rocks, it lowered their melting point and reduced their density,” buoying them upward, Gurnis explains.

The dehydrated remains of the dense tectonic plates now lie deep in the mantle, where they boost the speed of seismic waves, says Gurnis.

The notion that upwelling deep in the mantle could be linked to water carried downward in long-buried tectonic slabs is new, says Carolina Lithgow-Bertelloni, a geophysicist at University College London. A similar upwelling of water-rich material near subduction zones has long been associated with volcanism at Earth’s surface, she notes.

Previously, studies suggested that hot spots along the core-mantle boundary create buoyant plumes of rock that rise within Earth’s mantle (SN: 7/9/05, p. 24). In the new study, however, lowered density arises from presumed changes in the composition of rock, not its temperature.

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