Dynamic ‘Recycling’ Is Responsible for Earth’s Thin Skin, Study Says

Why is the Earth so thin-skinned?

That question has troubled scientists since unmanned spacecraft revealed in the 1960s and ‘70s that the moon and the planet Mars have a much thicker crust than the Earth’s.

Now, a team of geophysicists led by Don L. Anderson of Caltech believe they have the answer. The Earth’s interior, Anderson said, may simply be devouring chunks of its crust, only to send them back up to the surface eons later as part of the dynamic processes that continually push the great slabs of the Earth’s crust around the globe.

“In both Mars and the moon, the crust constitutes at least 10% of planetary mass,” Anderson said in a presentation Monday to the Geological Society of America in Denver. “But the Earth’s crust constitutes no more than four-tenths of 1% of terrestrial mass.”

Anderson, professor of geophysics and director of Caltech’s seismological laboratory, has used a recently developed technique to create a profile of the planet’s interior. He and two colleagues, Jay Bass of the University of Illinois at Urbana-Champaign and Tom Duffy, a graduate student at Caltech, used seismic waves generated by earthquakes to study parts of the Earth that no one has been able to see.

Since seismic waves are affected by the medium through which they travel, variations in the waves as received at ground stations around the globe have told the scientists much about the interior of the planet, according to Anderson. Various materials, for example, will slow seismic waves, causing them to arrive at some stations later than at others, and the extent of the tardiness tells the scientists what kind of materials the waves have passed through.

By compiling data such as that, Anderson concluded that the “crustal material” that forms the outermost skin of the planet is not limited to the planet’s surface. In fact, the scientists found evidence of crustal material more than 250 miles below the thin crust, which averages only about 12.5 miles in thickness.

That material, buried in a region known as the Earth’s middle mantle, consists largely of calcium-rich majorite, which is similar to garnet. That is particularly significant, Anderson said, because it is exactly what one would expect to find if crustal material were exposed to the high pressures of the molten mantle.

That, in turn, suggests that giant chunks of the Earth’s crust are constantly being thrust into the planet’s interior.

The idea that the Earth’s crust dives into the middle mantle, rather than just under the surface, has gained popularity in recent years with the advent of the seismic wave technique. But this appears to be the strongest evidence yet to support that idea.

“The middle mantle appears to be a dumping ground for the huge slabs of crust that dive into the Earth’s interior at subduction zones, such as the ‘ring of fire’ around the Pacific,” Anderson said. Subduction zones are regions of the Earth where giant pieces of crust--called tectonic plates--are being forced underneath other plates.

That process created the chain of volcanoes along the coast of the Pacific Northwest, where the Pacific plate, which lies under the Pacific Ocean, is being driven northward and under the North American plate, which lies under the North American continent.

The difference between the Earth’s crust and the crust of the moon and Mars is due primarily to the fact that only the Earth has a crust made up of many tectonic plates, Anderson has concluded.

“Mars and the moon don’t have plate tectonics the way the Earth does,” he said. The Earth is believed to have at least a dozen tectonic plates ranging in thickness from about 4 miles under the oceans to about 18 miles under the continents.

“They (the moon and Mars) seem to be single-plate planets,” Anderson said.

Because of that difference, the Earth’s crust is constantly being recycled, with large chunks broken off and forced down into the middle mantle as the plates are driven around the planet. In time, much of that same material will be regurgitated back to the surface at mid-ocean ridges called “spreading centers.” That constant upwelling is what is believed to be the driving force behind the movement of the tectonic plates, which are also pulled forward as their leading edges plunge beneath adjoining plates.

Anderson believes so much of the crust is now in the middle mantle that if it could be brought up to the surface, the Earth’s crust would be about 125 miles thick.