Towering structures in Earth’s depths may be billions of years old

Two giant masses inside the Earth may have been stable for billions of years, surviving the powerful internal tumult, according to a study of seismic waves ringing across the planet.

“When there’s a big earthquake, the whole Earth will expand and contract like a bell,” he says Arwen nothing at the University of Utrecht in the Netherlands. “The earth becomes a musical instrument.”

Decades ago, measurements of such seismic waves identified two strange continent-sized structures, one under the Pacific Ocean and the other under Africa. Extending almost 1000 kilometers from the outer core are the lower mantle, a slowly moving layer between the Earth’s crust and core.

Because seismic waves travel more slowly through these objects, they are called “large low shear velocity provinces,” or LLSVPs. But not much else is known about their composition or origin.

For more information, Deuss and his colleagues looked at how these regions dampened the energy of the seismic waves, in addition to changing the speed of the waves. Such measurements can reveal information about the temperature and makeup of LLSVPs, as well as their shape and size.

The researchers hoped to find that the structures – believed to be hot relative to the surrounding areas – would significantly dampen seismic waves. “Look, we found the opposite,” says Deuss.

To explain the lack of hydration, even at high temperatures, the researchers propose that LLSVPs are composed of minerals with large crystals that can be stable in heat. This would also suggest that the provinces are very viscous and can maintain stability as the mantle moves around them.

This stability also means that these objects are very old, dating back at least half a billion years and possibly more than 4 billion years ago, says Deuss. They can serve as deposits of the first material that reaches the earth’s surface through volcanoes – unchanged since the Earth took shape.