New wild paper suggests Earth’s tectonic activity has an unseen source
The Earth is far from being a solid mass of rock. Our planet’s outer layer – known as the lithosphere – is made up of more than 20 tectonic plates; as these gargantuan slates slide across the face of the planet, we get the movement of continents and the interaction at boundaries, not the least of which is the rise and fall of entire mountain ranges and ocean trenches.
Still, there’s some debate over what makes these giant boulders move in the first place.
Among the many hypotheses highlighted over the centuriesconvection currents generated by the planet’s hot core have been discussed as an explanation, but it is doubtful that this effect produces enough energy.
A recently published study looks to the skies for an explanation. Noting that force rather than heat is more commonly used to move large objects, the authors suggest that the interaction of gravitational forces from the Sun, Moon and Earth may be responsible for the movement of Earth’s tectonic plates.
The key to the hypothesis is the centroid – the center of mass of a system of bodies in orbit, in this case that of the Earth and the Moon. This is the point around which our Moon actually orbits, and it is not directly at the center of mass of our planet, what we call the geocenter.
Instead, the location of the barycenter on Earth changes during the month by 600 kilometers (373 miles) as the Moon’s orbit around Earth is elliptical due to the gravitational pull of our Sun.
“Because the oscillating barycenter is about 4,600 kilometers [2,858 miles] from the geocenter, the Earth’s tangential orbital acceleration and solar attraction are unbalanced except at the barycenter”, says geophysicist Anne Hofmeisterfrom Washington University in St. Louis.
“The planet’s warm, thick, and solid inner layers can withstand these stresses, but its thin, cold, and brittle lithosphere responds by fracturing.”
Additional tension is added when the Earth rotates on its axis, flatten slightly of a perfect spherical shape – and these three stresses of the Moon, the Sun and the Earth itself combine to cause tectonic plates to move and split.
“Differences in the alignment and magnitude of the centrifugal force accompanying solar pull as the Earth undulates in its complex orbit around the Sun superimposes highly asymmetric and time-varying forces on the Earth, which is already stressed by the rotation”, researchers write.
What’s happening below the surface is that the solid lithosphere and the solid upper mantle are spinning at different speeds due to these stresses and strains, the researchers report – all due to our particular Earth-Moon-Sun configuration. .
“Our exceptionally large Moon and our particular distance from the Sun are essential,” says Hofmeister.
Without the Moon and the shifts it causes between the barycenter and geocenter, we wouldn’t see the plate tectonic activity we get on Earth’s surface, the researchers say. Since the gravitational pull of the Sun on the Moon is 2.2 times greater than that of the Earth, it will pull away from our planet over the next billion years or so.
That said, the gravitational forces at play still need the Earth’s warm interior for this to all work, the researchers say.
“We propose that plate tectonics results from two different, but interacting gravitational processes,” they write. “We emphasize that the Earth’s interior heat is essential for creating the thermal and physical boundary layer known as the lithosphere, its basal melting and underlying low-velocity zone.”
To further validate the hypothesis described in their study, the researchers apply their analysis to several rocky planets and moons in the solar system, none of which have had confirmed tectonic activity to date.
Their comparison between Earth and other major celestial bodies in the solar system reveals a potential explanation for why we have so far not detected any tectonic activity on any of the major moons or rocky planets. Closest to Earth in all necessary parameters, however, is Pluto.
“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,” says Hofmeister.
The research has been published in GSA Special Documents.