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Looking deep into earth, researchers see possible ‘root’ of volcanic islands

Deep within our planet, researchers are finding hints of exotic materials – and behaviors unrivaled anywhere else on the planet. Now a team of researchers is making connections between the dynamic activities deep inside Earth and geologic features at its surface.

The researchers, which include two ASU seismologists, have detected a relatively small and isolated patch of exotic material – known as an ultra low velocity zone (ULVZ) – that could be a “root” for mantle plumes that connect Earth’s hot and tumultuous core and its surface. Specifically, the researchers have found a spot at Earth’s core-mantle boundary, 3,000 kilometers (1,900 miles) deep, that could play a pivotal role in the formation and existence of volcanic islands and island chains such as Hawaii.

“This is a small and very isolated region of possibly molten mantle material that is sitting at Earth’s core mantle boundary,” says Sebastian Rost, an ASU faculty research associate.

Rost and fellow researchers – seismologist Edward Garnero of ASU, Quentin Williams of the University of California-Santa Cruz and Michael Manga of University of California-Berkeley – recently detected an ultra low velocity zone, a region where seismic waves propagate extremely slowly, under the southwest Pacific Ocean. They reported their findings in the June 2 issue of Nature.

In “Seismological Constraints on a Possible Plume Root at the Core Mantle Boundary,” the researchers describe a small and highly active region of inner Earth that is peculiar on several levels.

“We have identified a little bubble of partially molten rock at the bottom of Earth’s solid mantle, which we relate to a plume of hot material,” Garnero says. “Such plumes may give rise to surface volcanoes, like Hawaii or Iceland. Now we might know what feeds such plumes.”

The size of the “bubble” is about 50 kilometers (30 miles) across, smaller than the metropolitan Phoenix area, and about 8 kilometers (5 miles) deep. The density of the material in the bubble is significantly higher than the density of the area that surrounds it.

“It might be that every plume might have one of these at its source,” Rost explains. “Geodynamic modeling shows that these dense blobs of material don’t move around a lot in the mantle. So while the mantle is convecting and material is moving around, these dense piles of material do not get pushed around that much. They may actually give a stable root to long-lived mantle plumes, and that might be the reason why we have island chains like Hawaii.”

The team studied a portion of Earth’s core-mantle boundary layer (CMB), where Earth’s molten iron core meets the silicate mantle rock, east of Australia and slightly to the south of New Caledonia. Traditionally, this area inside Earth had been thought to be a fairly well-defined and predictable region. Researchers now are finding that the core mantle boundary is a complex and dynamic area that churns and chugs as the liquid iron core roils at the bottom of the rock-like mantle, Garnero said.

Within this environment lies the peculiar bubble of ULVZ material.

“It is very isolated and very dense, and it is partially molten,” Garnero says. “Therein lies the enigma.”

The existence of dense and partially molten material poses a dynamical problem for keeping the material in a neat pile. It would be expected that the material spread out along the CMB – something that is not observed in the seismic data, Rost says.

The researchers instead are proposing a model that resembles a sponge, where the molten material fills the holes of the sponge and is kept in place by surrounding crystals of the Earth’s mantle. The material’s high density also might indicate the existence of core material in this region, although leaking of the molten iron of the core in the mantle is not expected from current geophysical Earth models. The recent finding might change this view by allowing material to flow through the core-mantle boundary.

The new findings were made possible by clever use of a seismic array – an instrument consisting of several distributed seismic monitoring stations in Australia. The array allowed high enough resolution to detect the relatively small bubble using seismic waves that are reflected from the CMB. The monitoring stations first were installed in the 1960s to detect nuclear detonations and can be used like a sensitive antenna to look deep inside Earth.

In this study, the team sampled a region measuring 100 kilometers by 250 kilometers using newly assembled data sets of 305 Tonga-Fiji earthquakes recorded at the small-scale (20-kilometer diameter) Warramunga seismic array.

In a sense, the team used the array data as a way to perform ultrasound scans of Earth’s interior.

Rost said one of the tasks of the team in subsequent studies is to determine “if this is just a tiny bubble or a paradigm for what is down there.”

The work, Garnero says, is helping seismologists see the inner workings of Earth in a new light. It will help fill out their view of the relationship between Earth’s interior and its surface, he says.

“Other researchers have suggested that, by using seismic tomography, they can see where mantle plumes appear to be connected from Earth’s surface down to the core mantle boundary,” Garnero says. “This work enables us to go to those areas and study them in great detail to see if the hot spots at the surface are connected at the core mantle boundary. It gives us a chance to connect the dots between the surface and the core.”

Skip Derra. Derra, with Marketing and Strategic Communication, can be reached at (480) 965-4823 or (skip.derra@asu.edu).
June 2, 2005