The Earth Churns

Deep within our planet, halfway to its center in an area where Earth’s core meets its mantle, lays a massive folded slab of rock that once was the ocean floor.

The slab sank beneath North America some 50 million years ago. Geologists say that it holds important clues as to the behavior and composition of the deep interior of Earth. It also could help explain how surface features such as volcanoes and earthquakes form.

Arizona State University’s Edward Garnero was part of research team led by University of California, Santa Cruz seismologists Alexander Hutko and Thorne Lay. Justin Revenaugh of the University of Minnesota was also part of the group. The geologists detected the slab by analyzing seismic waves reflected from the deepest layer of the mantle beneath an area off the west coast of Central America.

“In this one location we see quite strong evidence for whole mantle circulation,” says Garnero, an ASU seismologist. “Slabs descending deep into the mantle are thought to drive the convective system found within Earth. They are dense and fall into the mantle. But they are connected to the outer shell that includes the oceanic crust.”

“It’s like a carpet sliding off the dining room table,” Garnero explains. “If it is more than halfway off, it just goes, taking everything with it.” The discovery sheds new light on the processes that drive the movement of Earth’s tectonic plates. Earth’s outermost layer is called the lithosphere. This layer is broken into large, rigid plates composed of the crust and the outer layer of the mantle.

New plate material is created at mid-oceanic ridges, where the ocean floor spreads apart. Old plate material is consumed in subduction zones, where one plate dives beneath another. Garnero says that the fate of subducted lithosphere has been uncertain, at least until this slab was detected.

The ASU geologist says there is an ongoing debate over whether subducted slabs sink all the way down to the base of the mantle or get trapped in the upper mantle. The new evidence favors the presence of subducted slabs in the deep mantle. If this is the case, he adds, then finding this slab could have significant ramifications for our understanding of the inner workings of Earth.

“Earth’s interior is rich in complexity,” Garnero says. “Earthquakes, volcanoes, and large pieces of Earth’s outermost layer, or ‘plates,’ slowly move, grinding and shifting. This discovery could shed light on large scale circulation of rock in Earth’s interior. This circulation in turn shifts the tectonic plates, and the nature of the chemistry of material deep in Earth’s interior.”

The mantle extends to a depth of about 1,800 miles (2,900 kilometers). Within the mantle, cold rock sinks while hot plumes rise toward the surface. This slow circulation of mantle rock is thought to drive the movement of Earth’s tectonic plates.

The base of the mantle absorbs heat from the core. A temperature difference exists between the relatively cool slab and the hotter mantle rock surrounding it. As a result, the researchers were able to image the buckling and folding of the subducted oceanic slab at the base of the mantle.

The scientists used seismic data from earthquakes in South America that were recorded at seismographic stations in the western United States. They analyzed the data with techniques adapted from the oil exploration industry that were devised to study complex structures in Earth’s crust.

“Alex Hutko used a method that takes hundreds of recordings that all sample the same volume in the deep mantle. He then reconstructs an image of reflective surfaces that give rise to the specific bumps and wiggles on the seismograms in a technique called ‘migration,’” Garnero explains. “This is the most accurate deep mantle imaging effort to date.”

Using the method, the researchers found the subducted slab is composed of essentially the same minerals as the surrounding mantle. But its temperature is about 700 degrees Celsius cooler. This temperature difference affects the location of a “phase transition,” Garnero says. This is where the crystal structure of the mantle rock compresses to a more compact form due to increasing pressure and temperature with depth.

Seismic energy reflected by this phase transition revealed an abrupt step in the phase boundary about 60 miles (100 kilometers) high.

The researchers also saw evidence of hot plume-like structures at the edge of the slab. Garnero says this indicates possible upwelling of hot material from the base of the mantle as the spreading slab pushes into it.

“There is a conservation of mass in the mantle. As a result, something must return as the slab sinks into the Earth,” Garnero explains. This return flow can include plumes of hot material that can generate volcanic activity on the surface.

“We are very excited about employing our migration approach to other regions in the mantle,” Garnero adds. “This study is just a starting point for bringing once blurry or obscured structures into sharper focus.” —Skip Derra