The study, led by geophysicist Anne M. Hofmeister, was published in a special paper of the Geological Society of America as part of a forthcoming collection assembled in honor of geologist Warren B. Hamilton. Hofmeister is a geophysicist in the arts and sciences at Washington University in St. Louis.
The new analysis provided an alternative to the hypothesis that the motion of tectonic plates is related to convection currents in Earth’s mantle. Convection involved the buoyancy of hot liquids, which Hofmeister and his colleagues argued did not apply to solid rocks. He argued that force, not heat, moves large objects.
Earth’s inner workings are popularly modeled as the heat generated by internal radioactivity and the energy left over during collisions as our planet formed. But proponents of mantle convection also recognized that the amount of internal heat energy was insufficient to drive large-scale tectonics. And there were other problems with using convection to explain the observed plate motions.
Instead, Earth’s plates may move because the Sun exerts such a strong gravitational pull on the Moon that it has increased the Moon’s orbit around Earth.
Over time, the position of the barycenter – the center of mass between the objects orbiting Earth and the Moon – has moved closer to Earth’s surface and now oscillates 600 km per month relative to the geocenter, Hofmeister said. This sets up internal tension, as the Earth continues to rotate.
“Since the oscillating barycenter is ~4600 km from the earth’s center, Earth’s tangential orbital acceleration and solar pull are unbalanced,” Hofmeister said. “The planet’s hot, thick and strong inner layers can withstand these stresses, but its thin, cold, brittle lithosphere reacts by fracturing.”
The daily spin flattens the Earth to a perfect spherical shape, which contributes to this brittle failure of the lithosphere. The authors suggest that these two independent stresses create the mosaic of plates seen in the outer shell. The variety of plate motions comes from changes in the shape and direction of unbalanced gravitational forces over time.
But how to test this option? Hofmeister suggested: “One test would be a detailed examination of the tectonics of Pluto, which is too small and cold for convection, but has a massive moon and a surprisingly young surface.”
The study included a comparison of rocky planets that showed that the presence and longevity of volcanism and diffraction depend on a particular combination of the Moon’s size, the Moon’s orbital orientation, proximity to the Sun, and the rates of body rotation and cooling.
Hofmeister noted that Earth is the only rocky planet with all the factors necessary for plate tectonics. “Our uniquely large Moon and special distance from the Sun are essential,” she said. (ANI)
First Published:January 22, 2022, 6:13 pm