There could be billions of years old towering structures deep within the Earth

Two giant blobs deep within the Earth may remain stable for billions of years despite powerful internal fluctuations, according to an analysis of seismic waves reverberating across the planet.

“When a major earthquake occurs, the entire earth expands and contracts like a bell.” arwen Deus At Utrecht University in the Netherlands. “The earth becomes an instrument.”

Decades ago, measurements of such seismic waves identified two strange continent-sized structures, one under the Pacific Ocean and one under Africa. They extend for almost 1000 kilometers from the outer core to the lower mantle, the slowly moving layer between the Earth's crust and core.

Because seismic waves pass through these objects more slowly, they are called “large low shear velocity regions” or LLSVPs. However, little is known about its composition and origin.

To gain further information, Deus and her colleagues analyzed how these regions attenuate the energy of seismic waves, in addition to changes in wave speed. Such measurements reveal information about the temperature, composition, shape, and size of the LLSVP.

The researchers expected that the structure, which would be hotter than the surrounding area, would significantly attenuate seismic waves. “Lo and behold, we found the opposite,” says Deus.

To explain the lack of decay at high temperatures, the researchers propose that LLSVP must be composed of minerals with large crystals that are stable in heat. This also suggests that these regions are highly viscous and can maintain stability as the mantle moves around them.

This stability could mean that these objects are very old, dating back to the formation of planets at least 500 million years ago, and possibly more than 4 billion years ago, Deus said. They may act as repositories of primordial material, unchanged since the Earth formed, that sometimes reaches the surface via volcanoes.