Strange continent-sized structures (in red) lurk beneath the planet’s surface
Edward Garnero; SW French, BA Romanowicz, Geophys. J. Int. 199, 1303, 2014.
According to analysis of seismic waves reverberating across the planet, two huge masses within the Earth may have remained stable for billions of years, escaping the powerful churning of the interior.
“When a major earthquake occurs, the entire Earth will expand and contract like a bell,” says arwen deuce at Utrecht University in the Netherlands. “The 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 about 1000 kilometers from the outer core to the lower mantle, a slow-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 there is not much information about their creation or origin.
To learn more, Dess and his colleagues analyzed how these zones attenuated the energy of seismic waves, in addition to changing the velocity of the waves. Such measurements could provide information about the temperature and composition of LLSVPs, as well as their size and shape.
The researchers hope to find that the structures – which are known to be warm relative to surrounding areas – will significantly dampen seismic waves. “Lo and behold, we found the opposite,” says Des.
To explain the lack of moisture even at high temperatures, the researchers propose that LLSVPs must be composed of minerals with large crystals that can remain stable in the heat. This would also show that the provinces are highly viscous and can maintain stability even as the mantle rotates around them.
Dess says that stability could also mean that these objects are extremely old, having originated at least half a billion years ago and possibly more than 4 billion years before the planet formed. They may serve as reservoirs for primordial material – unchanged since the Earth took shape – which sometimes reaches the surface through volcanoes.
Subject:
(tagstotranslate)geophysics
