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Hotspot in the hot seat

New seismic imaging alters the picture beneath Hawaii.
Image: The National Geophysical Data Center

The Hawaiian archipelago, and its chain of active and extinct volcanoes, has long been viewed as a geological curiosity. While most volcanoes arise at the boundaries of shifting tectonic plates, the Hawaiian chain lies smack in the middle of the Pacific plate, nowhere near its borders.

Now a study by researchers at MIT and Purdue University, published this week in Science, paints an unexpected picture of what’s beneath Hawaii. Using a new imaging technique adapted from uses in oil and gas exploration, MIT’s Robert van der Hilst and colleagues produced high-resolution images that peek hundreds of kilometers below the Earth’s surface.

They found a hotspot — but not where many scientists had thought it would be. Instead, the MIT team found evidence of hot mantle activity some 600 kilometers deep and 2,000 kilometers wide, in an area far west of the “Big Island” of Hawaii.

Many geologists had thought the Hawaiian Islands resulted from a stationary plume of white-hot material rising from the Earth’s lower mantle, spewing out masses of magma in fits of volcanic eruption. This theory held that the massive Pacific plate, moving slowly northwestward, carries newly formed volcanoes away from the hotspot, forming the Hawaiian island chain seen today.

According to the theory, the Big Island, the newest formation in the chain, sits directly over the blistering plume. Scientists have attempted to characterize this hotspot for decades, believing that if a plume exists, it may be a window into the Earth’s deep processes that could help quantify how the Earth loses heat from its core.

“The implication [of this new work] is that there is no simple, deep plume directly beneath Hawaii,” says Van der Hilst, the Cecil and Ida Green Professor of Earth and Planetary Sciences at MIT, and director of the Earth Resources Laboratory. “So the textbooks on Hawaii will have to be rewritten.”

Heat wave

The team developed a new deep-Earth imaging technique using seismic- and mineral-physics data to determine the temperature of the Earth at various depths. Extreme temperature profiles, they reasoned, might suggest plumes or hotspots.

Seismic waves travel through the Earth’s interior at speeds that are primarily influenced by temperature: The higher the temperature, the slower the waves. For years, seismologists have used seismic wave speeds to create — much like CAT scans — 3-D views of the Earth’s internal structure. This tomographic technique works well near earthquake sites or below vast networks of seismographic sensors. But Hawaii, as Van der Hilst observes, is in a no-man’s land of seismic data, far from any tectonic upheaval and adequate seismograph arrays.

Van der Hilst  — along with co-authors Qin Cao, an MIT graduate student; mineral physicist Dan Shim, associate professor of earth, atmospheric and planetary sciences at MIT; and Maarten de Hoop of Purdue University — came up with a new technique, combining seismic data and mineral physics to map temperatures in the Earth’s mantle. The team first collected all available seismic data from the Incorporated Research Institutions for Seismology Data Management Center, based in Seattle, which collects and distributes seismic information to the research community. This amounted to more than 100,000 records of seismic waves from more than 5,000 earthquakes in the last 20 years.  Much of the data came from the so-called “Ring of Fire,” a massive horseshoe of seismic and volcanic activity surrounding the entirety of the Pacific Ocean.

The team then modified a technique used in the oil and gas industry. Typically, companies such as Shell and Exxon Mobil create seismic shocks, and then listen to the echoes that bounce back. The seismic reflection creates a map of the underlying rock compositions, and clues to where oil and gas might lie.

Instead of creating shocks, Van der Hilst’s team took advantage of Earth’s natural shocks — earthquakes — and analyzed seismic waves as they reflected off the rocks underneath Hawaii. By analyzing seismic reflections, the team determined mineral compositions at various depths, noting the boundaries at which minerals changed. Knowing at which pressures and temperatures such boundaries occur in laboratory simulations, they were able to map out the temperatures deep beneath Hawaii.

Seismic shift

Cao, the lead author of the study, developed an algorithm that worked the massive amount of seismic data into deep-Earth temperature maps, revealing the newfound hotspot west of Hawaii. Van der Hilst says the discovery of this 2,000-kilometer-wide anomaly refutes the popular theory of a narrow, pipe-like plume rising straight up to Hawaii from the core-mantle boundary — a finding he anticipates will shake up the geodynamical and geochemical communities studying mantle convection.

Yang Shen, a professor of seismology and marine geophysics at the University of Rhode Island, says the new imaging technique provides much higher-resolution images of the Earth’s mantle than previous techniques, and may change the conventional wisdom on Hawaii’s hotspots.

“The observation is intriguing because it does not fit nicely within the current plume model,” Shen says. “So I think the paper will force us to rethink … mantle plumes and convection.”

Cao is now refining the mapping algorithm, and plans to make it accessible to other researchers in the next few months. As countries set up more earthquake monitors in the coming years, Van der Hilst says the new imaging technique will allow seismologists to draw up higher-resolution images of deep-Earth processes.

“I think this could be the technique of the future,” Van der Hilst says. “The receiver networks are exploding, and in the next five to 10 years we can probably do even more spectacular things.”

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