May 22, 2006
Seismologists detect a sunken slab of ocean floor deep in the Earth
By Tim Stephens
Halfway to the center of the Earth, at the boundary between the core and the mantle, lies a massive folded slab of rock that once formed the ocean floor and sank beneath North America some 50 million years ago.
A cold buckled subducted slab (blue) in the lowermost mantle may account for the thermal structure that results in the "step" detected by seismic imaging. The green plane represents a vertical cross-section through the image volume. (See also image below)
Image: Hutko et al., Nature (May 18, 2006)
A team of seismologists led by UCSC scientists 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.
"If you imagine cold honey pouring onto a plate, you would see ripples and folds as it piles up and spreads out, and that's what we think we are seeing at the base of the mantle," said Alex Hutko, a graduate student in Earth sciences and first author of a paper describing the new findings in the May 18 issue of the journal Nature.
The discovery sheds new light on the processes that drive the movement of Earth's tectonic plates. The planet's outermost layer, or lithosphere, 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, and old plate material is consumed in subduction zones, where one plate dives beneath another. But the fate of subducted lithosphere has been uncertain.
"There is a big debate over whether subducted slabs sink all the way down to the base of the mantle or get trapped in the upper mantle. This is one line of evidence favoring the presence of subducted slabs in the deep mantle," said Thorne Lay, professor of Earth sciences and coauthor of the Nature paper.
"It's the first evidence from direct imaging to support the idea that ancient seafloor makes its way down to the bottom of the mantle," Hutko added.
Within the mantle, which extends to a depth of about 1,740 miles, cold rock sinks while hot plumes rise toward the surface, and this slow circulation of mantle rock is thought to drive the movement of plates in the lithosphere. The base of the mantle absorbs heat from the core. The researchers were able to image the buckling and folding of subducted oceanic lithosphere at the base of the mantle because of the temperature difference between the relatively cool subducted slab and the hotter mantle rock surrounding it.
The subducted slab is composed of essentially the same minerals as the surrounding mantle, but its temperature is about 700 degrees Celsius cooler, Hutko said. This temperature difference affects the location of a "phase transition," where the crystal structure of the mantle rock changes 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.
"That's more than the thickness of the crust," Lay said. "It's a huge geological structure and it requires some large-scale dynamic process to produce it. A subducted slab piling up and spreading out is the only mechanism we know of that could give such an abrupt step."
The researchers also saw evidence of hot plume-like structures at the edge of the slab, indicating possible upwelling of hot material from the base of the mantle as the spreading slab pushes into it.
"We think there is a kind of pushing and bulldozing away of a hot basal layer of the mantle, giving rise to small plumes at the edges," Hutko said.
The study used seismic data from earthquakes in South America that were recorded at seismographic stations in the western United States. The researchers analyzed the data using imaging techniques adapted from those used in oil exploration to study complex structures in the crust.
"The oil industry has been using these techniques for decades, but only recently have we been able to exploit them for the deep Earth because of new data available from the seismographic network," Hutko said.
In addition to Hutko and Lay, the other authors of the Nature
paper are Edward Garnero of Arizona State University and Justin Revenaugh of the University of Minnesota.
A cold buckled subducted slab (blue) in the lowermost mantle may account for the thermal structure that results in the "step" detected by seismic imaging. Hot plume-like structures on the edges of the slab are shown in red. The green plane represents a vertical cross-section through the image volume. The image in the lower panel shows the postulated slab structure in cross-section, with seismic imaging data represented in blue (negative-amplitude reflectivity) and red (positive-amplitude reflectivity). Image: Hutko et al., Nature (May 18, 2006)