Tug of sun, moon could be driving plate motions on `imbalanced' Earth


Date:
January 21, 2022
Source:
Washington University in St. Louis
Summary:
A study proposes that imbalanced forces and torques in the
Earth-moon-sun system drive circulation of the whole mantle. The
new analysis provides an alternative to the hypothesis that the
movement of tectonic plates is related to convection currents in
the Earth's mantle.



FULL STORY ==========================================================================
A study led by geophysicist Anne M. Hofmeister in Arts & Sciences at
Washington University in St. Louis proposes that imbalanced forces and
torques in the Earth-moon-sun system drive circulation of the whole
mantle.


==========================================================================
The new analysis provides an alternative to the hypothesis that the
movement of tectonic plates is related to convection currents in the
Earth's mantle.

Convection involves buoyant rise of heated fluids, which Hofmeister and
her colleagues argue does not apply to solid rocks. They argue that
force, not heat, moves large objects. The new research is 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.

Earth's internal workings are popularly modeled as dissipating heat
generated by internal radioactivity and from leftover energy created
during collisions when our planet formed. But even mantle convection
proponents recognize that that 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.

Instead, 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.

Over time, the position of the barycenter -- the center 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, Hofmeister said. This sets up internal stresses, as the Earth continues to spin.

"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." Daily spin flattens the
Earth from a perfect spherical shape, which contributes to this brittle
failure of the lithosphere. These two independent stresses create the
mosaic of plates observed in the outer shell, the authors suggest.

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 includes a comparison of rocky planets that
shows that the presence and longevity of volcanism and tectonism depend
on the 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, Hofmeister noted.

"Our uniquely large moon and particular distance from the sun are
essential," she said.

========================================================================== Story Source: Materials provided by
Washington_University_in_St._Louis. Original written by Talia
Ogliore. Note: Content may be edited for style and length.


========================================================================== Journal Reference:
1. Anne M. Hofmeister, Robert E. Criss, Everett M. Criss. Links
of planetary
energetics to moon size, orbit, and planet spin: A new mechanism for
plate tectonics. GSA Special Papers, 2022 DOI: 10.1130/2021.2553(18) ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2022/01/220121165552.htm

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