(ORDO NEWS) — The earth is far from being a solid mass of stone. Our planet’s outer layer – known as the lithosphere – is made up of over 20 tectonic plates; as these giant plates slide across the face of the planet, we get the movement of the continents and the interaction at the boundaries of tectonic plates, where entire mountain ranges and oceanic troughs rise and fall.
However, there is some debate about what causes these giant slabs of rock to move.
Among the many hypotheses put forward over the centuries, convection currents generated by the hot core of the planet have been discussed as an explanation, but it is doubtful that this effect can provide enough energy.
A recently published study looks to the skies for an explanation. Noting that force, rather than heat, is most often used to move large objects, the authors suggest that the interaction of the gravitational forces of the Sun, Moon and Earth may be responsible for the movement of the earth’s tectonic plates.
The key point of the hypothesis is the barycenter – the center of mass of the orbital system of bodies, in this case the Earth and the Moon. This is the point around which our Moon actually orbits, and it is not located directly at the center of mass of our planet, which we call the geocenter.
Instead, the location of the barycenter inside the Earth changes during the course of a month by as much as 600 kilometers (373 miles) as the Moon’s orbit around the Earth is elliptical due to the Sun’s gravitational pull.
“Because the oscillating barycenter is about 4,600 kilometers [2,858 miles] away from the geocenter, the Earth’s tangential orbital acceleration and solar attraction are unbalanced except for the barycenter,” says geophysicist Ann Hofmeister of Washington University in St. Louis.
“Warm, thick, and strong inner layers of the planet can withstand these loads, but the thin, cold, and fragile lithosphere responds with cracks.”
Additional stress is added as the Earth rotates on its axis, flattening slightly from its ideal spherical shape – and these three stresses from the Moon, the Sun, and the Earth itself combine to cause the tectonic plates to shift and break apart.
“Differences in the alignment and magnitude of the centrifugal force accompanying solar attraction as the Earth moves in its complex orbit around the Sun impose highly asymmetric, time-varying forces on an Earth that is already under rotational stress,” the researchers write.
What’s happening beneath the Earth’s surface is that the solid lithosphere and the solid upper mantle rotate at different rates due to these stresses and strains, the researchers report – all thanks to our particular Earth-Moon-Sun configuration.
“Our uniquely large Moon and particular distance from the Sun play an important role,” says Hofmeister.
Without the Moon and the shifts it causes between the barycenter and the geocenter, we would not see the kind of plate tectonic activity that is observed on the Earth’s surface, the researchers say. Because the Sun’s gravitational pull on the Moon is 2.2 times greater than Earth’s, over the next billion years or so it will be moving away from our planet.
However, for the gravitational forces to take effect, the hot interior of the Earth is needed, the researchers say.
“We propose that plate tectonics is the result of two distinct but interacting gravitational processes,” they write. “We emphasize that the Earth’s internal heat is necessary to create the thermal and physical boundary layer known as the lithosphere, its basal melt, and the underlying low-velocity zone.”
To further support the hypothesis outlined in their paper, the researchers applied their analysis to several rocky planets and moons in the solar system, none of which has so far had confirmed tectonic activity.
Comparison of the Earth and other large celestial bodies in the solar system offers a potential explanation for why we have not yet detected tectonic activity on any of the large moons or rocky planets. However, Pluto is closest to the Earth in all necessary parameters.
“One of the tests could be a detailed study of the tectonics of Pluto, which is too small and cold for convection, but has a giant moon and a remarkably young surface,” says Hofmeister.
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