New & Now: Feeling the Heat

by Charles E. Reineke

Just beneath our planet's rocky skin lie dozens of super-heated "hotspots," hubs of sustained volcanic activity that have spawned some of the world's most spectacular scenery -- places like Iceland's sublime Fjallabak region and America's own Hawai'i Volcanoes National Park -- as well as some of history's most terrifying natural disasters.

In recent years, hotspots have also generated some sizzling geological debate, discussions that are sure to become more lively thanks to the findings of two MU researchers, Mian Liu, a professor of geological sciences, and Shimin Wang, a postdoctoral fellow, who are helping to redefine scientists' understanding of these pillars of plate tectonics.

Hotspots were first described by J. Tuzo Wilson, the Canadian geophysicist who coined the term in 1963, as essentially stationary features pumping intense heat up from the depths of the earth. Wilson's definition was taken a step further, in 1968, by the renowned geoscientist William Jason Morgan of Princeton University. Morgan theorized that hotspots are surface expressions of so-called mantle plumes, hot upwelling flows that originate far below the Earth's surface.

Based on geological data interpreted in light of his thermal plume insight, Morgan was able to draw two conclusions that have come to dominate contemporary thinking about plate tectonics: namely, that hotspots remained fixed relative to each other, and that the direction and rate of movement of the plates above them hasn't changed the past 40 million years. His idea of fixity was particularly important because it made hotspots a convenient reference framework for studies of plate motion relative to the Earth's deep interior.

Unfortunately, says Liu, geologists like himself had never been able to fully explain exactly how hotspots might remain fixed in the mantle. In fact, he says, studies related to convective flows suggested hotspots were actually moving relative to one another.

For their own reexamination of Morgan's propositions, Liu and Wang subjected previously recorded international hotspot data to an analysis using a more up-to-date plate motion model. Their findings, published in the June issue of the journal Geology, added an interesting twist to the hotspot debate. While Liu and Wang found evidence supporting Morgan's assumption about plate motion, they also confirmed what other scientists had already come to suspect about hotspots; that is, that they are indeed on the move.

"Reexamining the hotspot data ...we find that [Morgan's] two propositions cannot be simultaneously tenable: either hotspots have been moving relative to each other, or a major reorganization of plate motion occurred in the past 40 million years," Liu and Wang wrote. "Statistical compatibility tests show that hotspot rate data are incompatible with trend data, implying that hotspots have moved."

The finding does not mean that Morgan's ideas are no longer relevant, the researchers caution, only that geophysicists need to take a more nuanced approach to understanding the complicated dynamics of plate movement. Wang and Liu's data show, for example, that hotspots were moving in the opposite direction of the continental plates -- a conclusion with implications for understanding mantle dynamics. Their description of hotspots' systematic motion, meanwhile, suggests that hotspots can provide a useful reference for determining long-term plate motion relative to the deep mantle, albeit with necessary corrections.

"Now we have the confidence to say that 40 million years ago the plates were moving in approximately the same way they are today," Liu explains. "Direct evidence for constructing a plate motion model is available only for the past 3 million years. Our study allows us to extend the history of plate motions with greater certainty."

And such knowledge ensures greater appreciation, adds Wang. "The surface of our Earth moves in a simple and beautiful way. Our research answers some confusing questions and clarifies the relationship between plate tectonics and hotspots."