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

Two huge masses deep within Earth may have remained stable for billions of years, surviving the powerful churn of the interior, according to an analysis of seismic waves ringing throughout the planet.

“When there is a big earthquake, the whole Earth will expand and contract like a bell,” says Arwen Deuss at Utrecht University in the Netherlands. “Earth becomes a musical instrument.”

Decades ago, measurements of such seismic waves identified two strange continent-sized structures, one beneath the Pacific Ocean and one beneath Africa. They extend nearly 1000 kilometres up from the outer core into the lower mantle, a slowly-moving layer between Earth’s crust and core.

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

To gain more information, Deuss and her colleagues analysed how these regions dampened the energy of seismic waves, in addition to changing the velocity of the waves. Such measurements can reveal information about the temperature and makeup of the LLSVPs, as well as their shape and size.

The researchers expected to find that the structures – which are thought to be hot relative to surrounding areas – would significantly dampen seismic waves. “Lo and behold, we found the opposite,” says Deuss.

To explain the lack of dampening, even at high temperatures, the researchers propose the LLSVPs must be made up of minerals with large crystals that could remain stable in the heat. This would also suggest the provinces are highly viscous and could maintain stability even as the mantle moves around them.

That stability may also mean these objects are extremely old, with origins going back at least half a billion years and possibly even to the formation of the planet more than 4 billion years ago, says Deuss. They may serve as reservoirs for primordial material – unchanged since Earth took shape – that sometimes reaches the surface via volcanoes.

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