Scientific Method / Science & Exploration

Hawaii’s volcanic hotspot appears badly off-center

A new method for imaging the Earth’s mantle shows a much different structure …

Mantle hotspots are believed to explain unique geological phenomena—like the formation of the Hawaiian island chain, and the history of volcanism that has culminated in the Yellowstone caldera. At hotspots, stationary plumes of molten rock periodically punch through the crust as tectonic plates slide over, creating a linear chain of eruptions on the surface. It’s an idea taught in introductory geology courses, but the details of the process are actually a source of significant scientific controversy. There’s an excellent reason for that—if you’re standing on the big island of Hawaii, it’s difficult to get a good look at the rocks 100 miles below your feet. But, as always, the scientific process rolls on, and geophysicists have recently brought new tools to bear on the problem.

The results so far have been a bit confusing, challenging researchers to come up with some fresh ideas. It’s true that the most important discoveries often come from the least satisfying results, but sometimes they’re confusing simply because they’re wrong. Figuring out which category a study will end up in is a tricky business.

Back in April, Ars reported on some fantastic imaging of the mantle structure below Yellowstone, which produced a slightly different picture of the hotspot than had been seen previously. Last week in Science, a group reported using another innovative technique (adapted from the petroleum industry) to study the Hawaiian hotspot, and they also report a surprising departure from earlier results.

Analysis of seismic waves produced by earthquakes is the primary tool that geophysicists use to probe the Earth’s interior. Using data from the vast network of seismographs around the world, subtle changes in the direction or speed of seismic waves can be detected. After a great many earthquakes, differences in material properties (like temperature and density) can be spatially resolved. The whole process works somewhat analogously to a CT scanner in a hospital.

Hotter material in the Earth slows down seismic waves more than cooler material does, and this method is commonly used to generate three-dimensional temperature maps of the Earth’s interior. In this new study, the seismic data was used to identify the location of important mineral stability horizons. If you were to travel downwards towards the core (logistical issues and lousy Hollywood portrayals aside), the increasing pressure would cause minerals to morph into variants that are stable at different conditions. These changes happen over relatively short distances at predictable depths, and some portion of the seismic waves passing through will be reflected at the boundaries between mineral variants.

The researchers used variation in the depths of these mineral horizons to infer temperature—a higher temperature will slightly deepen that transition boundary, and vice versa. This in itself is not new, but the team managed to create a three-dimensional image with much higher resolution than was previously possible. Their method picked up a hot region right below the Hawaiian Islands (so far, so good), but whereas earlier attempts at seismic imaging indicated that this hot region continues more or less straight downward, they saw something very different. They found evidence of a large body of hot material roughly 1,000 km to the west of Hawaii. The structure below this body was beyond the lower limits of their imaging (about 1,000 km depth), leaving the ultimate nature of the feature unsolved.

Now, the reigning explanation of the Hawaiian hotspot has been that a plume of hot material is rising more or less vertically from the deep mantle all the way to the surface. Imaging seemed to indicate that it might be rising from slightly southeast of the big island of Hawaii. The authors of this paper instead see evidence for a large body of hot material far west of Hawaii that is essentially trapped in the upper mantle. Magma rising from the eastern edge of the body would feed the Hawaiian volcanoes. The authors state that this large body must be fueled from below, but they can’t say anything about where in the mantle that material is sourced.

This will certainly set off an effort to improve imaging in the region and investigate the predictions of this paper, but it will also inform the geochemists who learn about the mantle by studying the lava erupting from these hotspots. For instance, if you assume the magma comes straight up from the deep mantle, you may interpret your data differently than if you’re thinking about shallow mantle processes. Insights could come from either field, but for now, the solution to the puzzle remains elusive.

Science, 2011. DOI: 10.1126/science.1202731  (About DOIs).

Listing image by Image courtesy of the USGS

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