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Seismologists get handle on heat flow deep in earth

November 28, 2006

Earth's interior is not a benign world that only stores the geologic history of our planet. Geologists see the normally assumed placid inner Earth as a dynamic environment filled with exotic materials and substances roiling under intense heat and pressures. It is an environment that continues to evolve in interesting ways – and one that has an impact on what happens on our planet's surface.

The latest evidence of this dynamic inner Earth is revealed in a recent series of measurements that peered deep within Earth, halfway to its center. The new experiments have yielded important results that help determine temperature halfway to the center of Earth. It also has implications for the age of Earth's solid inner core and how its magnetic field may be generated.

“We have found unexpected rock layering in Earth's deepest mantle,” says Edward Garnero of ASU's School of Earth and Space Exploration, and one of the researchers on the team. “The implications of the layering are far-reaching, with intimate connections to the rock chemistry, temperature and convective flow, all of which have been previously inaccessible.

“Understanding of Earth's core-mantle boundary environment puts us in the position of answering a host of important questions, such as how much heat from the molten outer core cooks the overlying mantle. While this might seem distant and esoteric, it actually relates to the vigor of convective mantle flow that ultimately jostles Earth's surface with volcanic and earthquake processes.”

Garnero, and his fellow researchers (Thorne Lay of the University of California-Santa Cruz; John Hernlund of the Institut de Physique du Globe de Paris; and Michael Thorne of the University of Alaska-Fairbanks ) report their findings in the Nov. 24 issue of Science magazine.

Going with the ‘heat' flow

In “A post-perovskite lens and D” heat flux beneath the central Pacific,” the researchers discuss measurements that have led them to determine temperatures at different levels deep within the earth. The researchers, for the first time, have been able to measure the flow of heat emanating from Earth's core into the base of its mantle, which can help determine the age of the core and help understand how Earth's magnetic field is generated.

Earth is made up of several layers. The crust, which includes the surface of Earth, extends 40 kilometers (about 25 miles) deep. Below the crust is the mantle area that extends to about 2,900 kilometers (1,800 miles) into Earth. The D” layer of the mantle is the deepest (200 to 300 kilometers), and beneath that is the outer core, which extends to 5,150 kilometers (3,200 miles), and then the inner core, extending to about 6,400 km (4,000 miles). The researchers probed the D” layer, which lies at the bottom of the mantle.

The boundary between the Earth's core and mantle lies halfway to the center of Earth, to a depth of 2,900 km. The seismologists were able to probe the structure of this region by studying its effects on seismic waves generated by large earthquakes.

“What we have found are various layers deep within Earth under the central Pacific Ocean , near the edge of what appears to be a pile of hot, chemically distinct material,” Garnero says. “In this ‘thermochemical pile,' the layering is consistent with a new, high-pressure phase of a compound called perovskite, a material that exists specifically under high pressures that cause new arrangements of atoms to be formed.”

Riding the seismic waves

Using seismic waves generated by earthquakes in the Tonga-Fiji region of the southwest Pacific Ocean, the seismologists were able to probe the structure of the D” region inside Earth by studying the patterns of waves reflecting from any distinct objects in the deep Earth. They detected the waves with a new array of highly sensitive instruments deployed by the EarthScope project (a National Science Foundation initiative), located throughout the Western United States.

What they detected was a novel material alternating between two distinct forms, Garnero says. The material they detected is called post-perovskite, a modified version of perovskite. Separate laboratory mineral physics tests set the temperatures and pressures that would be required to change perovskite material to post-perovskite material.

Temperature measurements in Earth were obtained by relating seismic observations to the mineral transformations of perovskite/post-perovskite material, which occurs under extremely high pressures and temperatures that the researchers say prevail near the core-mantle boundary.

“Perovskite refers to a specific arrangement of the silicon-iron-oxygen-magnesium atoms,” Garnero says. “Post-perovskite happens at the highest pressures in the deep mantle of Earth, so if we can pinpoint a depth where that occurs, it will allow us to determine the temperature (as determined in laboratory tests) required to make this phase transition from perovskite to post-perovskite.

“We not only found a boundary marking an entrance into this material, but an additional boundary showing an exit from the material back to its original structure as well. It is like a lens or a cloud that is hovering in the lower most mantle above Earth's core.”

Search for the ‘Holy Grail'

Because the research team was able to determine the temperature at two different depths, one right above the other, it gave them a temperature gradient, “which tells us the amount of heat flowing from the core into the base of the mantle,” says UC-Santa Cruz's Thorne Lay, the lead author of the Science paper.

“Heat flow is the ‘Holy Grail,' because it tells us how much energy powers the geodynamo, and it tells us how much the mantle is being heated from below,” Lay says.

The geodynamo is the convective motion in Earth's fluid outer core that generates the magnetic field we observe at Earth's surface.

As heat flows from the outer core into the mantle, it drives important processes in the mantle and the core. The mantle is a thick layer of silicate rock and surrounds a dense, predominantly iron core.

The high heat flow found within Earth supports the idea that mantle convection, the slow turnover of mantle material that moves Earth's tectonic plates at its surface, is strongly controlled by this intense degree of heating at the mantle's base by the upwelling of hotter material from near the core mantle boundary.

Rocks and ages

“The implication from this study is that the flow of heat from the core to the mantle suggests that the inner core of Earth is not as old as the Earth itself,” Garnero says.

“The core must have been pretty hot in the past for this much heat to still be coming out – and the inner core, which is slowly solidifying from the inside out as it cools, may be only 1 billion years old,” Lay adds.

The age of Earth itself is generally regarded as 4.6 billion years old.

The researchers suspect that an upwelling of hot mantle material may be taking place near the edges of the lens of the post-perovskite material. They detected the lens in the lowermost mantle southeast of Hawaii, an area where previous studies have suggested there is a root of a hot upwelling plume from near the core mantle boundary that ultimately is responsible for volcanism that created and continues to create the chain of the Hawaiian Islands .

Garnero says that this new finding supplies an important missing piece to the puzzle of what is going on deep in Earth's interior, adding to the larger picture emerging of a dynamic interior that affects what happens at the surface.

“These glimpses into Earth are revealing key pieces of Earth as a system,” Garnero says. “They beckon for a larger understanding of larger-scale cycles of our planet.”