Alana Semple

Earth, Environmental and Planetary Sciences
Research Advisor: Adrian Lenardic

As her adviser likes to say, Rice graduate student Alana Semple is changing textbooks. Working with Adrian Lenardic, professor of earth, environmental and planetary sciences, Semple is enhancing our understanding of how the movement of the mantle affects what happens to us on the surface of the Earth.

The mantle’s thick layer of rock makes up 84% of the total volume of the Earth and lies just beneath the thin crust on which we stand. The mantle circulates and flows in different directions over thou-sands of years and influences many things on the surface — from chemical cycling to volcanic activity to the movement of the continents.

It was previously thought that tectonic plates move independently of the mantle beneath them and that the mantle only moves because the plates drag it along. “The textbooks will tell you that the major driving force behind why the tectonic plates move is that its heavy on one side and it starts to sink, which drags the rest of the plate along,” Semple said. However, detailed observations of the asthenosphere —the upper layer of the mantle — from a Lamont research group returned a more nuanced picture suggesting that the asthenosphere speed does not continually decrease with depth, but can peak at the center and decrease toward its top and base and that it sometimes appears to flow in a different direction than the tectonic plates.

Using 3D computer models, Semple provided a new look at the relationship between the mantle and the tectonic plates that has the potential to change geophysics textbooks. With elaborate fluid dynamic models, which include a more accurate representation of the mantle’s viscosity, she looks at the movement of the mantle and the tectonic plates that sit atop it. Her simulations revealed that the flow in the mantle is a huge driving force behind tectonic plate movement. She discovered that the mantle actually flows faster than the plates above it, dragging them along as it moves. The models also show that flow in the mantle can be offset from the movement of the tectonic plates — in agreement with the Lamont observations.

“Our findings suggest that pressure-driven flow in the asthenosphere can contrib-ute to the motion of tectonic plates by dragging plates along with it,” she said. “A notable contribution does come from ‘slab-pull,’ a gravity-driven process that pulls plates toward subduction zones. Slab-pull can still be the dominant process that moves plates, but our models show that asthenosphere flow provides a more significant contribution to plate move-ment than previously thought.”