A research team led by Anne M. Hofmeister, geophysicist in Arts and Sciences at Washington University, proposes that imbalanced forces and torques in the Earth-Moon-Sun system drives the circulation of the Earth’s mantle.
How has the Earth’s mantle been previously proposed to function?
The internal workings of the Earth’s mantle are traditionally modelled as dissipating heat generated by internal radioactivity from leftover energy created during collisions when our planet formed.
However, even mantle convection proponents recognise that the amount of internal heat-energy is insufficient to drive large-scale tectonics. And there are other problems with using convection to explain observed plate motions.
How is the new research different?
Hofmeister’s research provides an alternative to the hypothesis that the movement of tectonic plates is related to convection currents in the Earth’s mantle. Convection involves a buoyant rise of heated fluids, which, according to researchers, does not apply to solid rocks.
They argue that force, not heat, moves large objects; Earth’s plates might be shifting because the Sun exerts such a strong gravitational pull on the Moon that it has caused the Moon’s orbit around Earth to become elongated.
Hofmeister explained: “Over time, the position of the barycentre — the centre of mass between the orbiting bodies of the Earth and the Moon — has moved closer to Earth’s surface and now oscillates 600 km per month relative to the geocenter.
“Because the oscillating barycentre lies ~4600 km from the geocenter, Earth’s tangential orbital acceleration and solar pull are imbalanced except at the barycentre. The planet’s warm, thick and strong interior layers can withstand these stresses, but its thin, cold, brittle lithosphere responds by fracturing.”
The daily spin compresses the Earth from a perfect spherical shape, which contributes to this brittle failure of the lithosphere. Researchers suggest that these two independent stresses create the mosaic of plates observed in the outer shell. The variety of plate motions comes from the changes in size and direction of the imbalanced gravitational forces with time.
How do scientists plan to further investigate this topic?
“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,” commented Hofmeister.
This study would include a comparison of rocky planets that shows that the presence and longevity of volcanism and tectonism depend on a particular combination of moon size, moon orbital orientation, proximity to the sun and rates of body spin and cooling.
“Earth is the only rocky planet with all the factors needed for plate tectonics. Our uniquely large Moon and particular distance from the Sun are essential,” she concluded.
