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Scientists have long understood that solid rock—400 miles below Earth’s surface and subject to intensely high temperatures and lower-mantle pressures—can deform. That allows it to then ooze like chunky peanut butter with its lubricating oil and hard, unchanging nuts. 

But an international team of researchers, including James Van Orman, PhD, professor of geochemistry and mineral physics in the College of Arts and Sciences, has published findings in the journal Nature that upend previous understandings.

They posit that scientists may have confused which of the two key minerals in Earth’s mantle is the oil and which is the nut. Understanding what makes the mantle ooze and flow is especially important, given that this flow of rock produces most of Earth’s volcanoes and earthquakes.

Van Orman and colleagues from Université de Lille in France and the California Institute of Technology used a modeling system to better understand how the lower mantle’s two minerals—bridgmanite and periclase—deform over time. Scientists had believed periclase was like the oil and could make the mantle softer or harder depending on how it was distributed. But instead, Van Orman said, periclase is like the nuts. No matter how you distribute it, it doesn’t really change the “viscosity,” or consistency of the mantle itself.

“Understanding the viscosity of the lower mantle at such a granular level means we are in a better position to predict how Earth’s mantle convects,” or transfers its internal heat, Van Orman said.