Mantle circulation may be driven by uneven forces and torques in the Earth-Moon-Sun system, according to a new theory on Earth’s interior dynamics.
According to a study led by Washington University geophysicist Anne M. Hofmeister, uneven forces and torques in the Earth-moon-sun system promote circulation of the entire mantle.
The new research challenges the theory that tectonic plate movement is linked to mantle convection currents. Hofmeister and her colleagues believe that convection does not apply to solid rocks because hot fluids rise buoyantly. They argue that huge things are moved by force rather than heat. The new findings were presented in a special publication by the Geological Society of America as part of a collection honoring geologist Warren B. Hamilton that will be released soon.
The inside workings of the Earth are commonly depicted as dissipating heat generated by internal radioactivity and leftover energy released during our planet’s formation events. Even proponents of mantle convection acknowledge that the amount of internal heat energy required to drive large-scale tectonics is insufficient. Furthermore, employing convection to explain observed plate motions has significant drawbacks.
Instead, Earth’s plates may be shifting as a result of the sun’s strong gravitational attraction on the moon, which has led the moon’s orbit around Earth to lengthen.
According to Hofmeister, the barycenter — the center of mass between the Earth and the moon’s orbital bodies — has moved closer to the Earth’s surface through time and presently oscillates 600 kilometers every month relative to the geocenter. As the Earth continues to spin, this puts pressure on the planet’s interior systems.
“Because the oscillating barycenter lies ~4600 km from the geocenter, Earth’s tangential orbital acceleration and solar pull are imbalanced except at the barycenter,” Hofmeister said.
“The planet’s warm, thick and strong interior layers can withstand these stresses, but its thin, cold, brittle lithosphere responds by fracturing.”
The Earth’s daily spin flattens it from a perfect spherical shape, contributing to the lithosphere’s brittle breakdown. According to the authors, these two distinct stresses are responsible for the mosaic of plates seen in the outer shell.
The variety of plate motions comes from the changes in size and direction of the imbalanced gravitational forces with time.
But how to test this alternative? Hofmeister suggested: “One test would be a detailed examination of the tectonics of Pluto, which is too small and cold to convect, but has a giant moon and a surprisingly young surface.”
The study contains a comparison of rocky planets that demonstrates that the presence and longevity of volcanism and tectonism are dependent on a unique combination of moon size, orbital orientation, proximity to the sun, and body spin and cooling rates.
According to Hofmeister, Earth is the only rocky planet with all of the necessary ingredients for plate tectonics.
“Our uniquely large moon and particular distance from the sun are essential,” she said.
Image Credit: NASA