Imma Perfetto
Cosmos science journalist
Deep inside Earth there are mysterious and fascinating regions the size of continents at the boundary between the core and mantle, and earth scientists have just cracked the mystery of what they’re made of.
Known as “Large Low Seismic Velocity Provinces”, or LLSVPs, the regions cause seismic waves to slow down.
“The waves slow down because the LLSVPs are hot,” says Arwen Deuss, seismologist and professor of structure and composition of Earth’s deep interior at Utrecht University in the Netherlands.
“Nobody knew what they are, and whether they are only a temporary phenomenon, or if they have been sitting there for millions or perhaps even billions of years.”
One is found under Africa, the other under the Pacific Ocean.
“These 2 large islands are surrounded by a graveyard of tectonic plates which have been transported there by a process called ‘subduction’, where one tectonic plate dives below another plate and sinks all the way from the Earth’s surface down to a depth of almost 3,000 kilometres,” says Deuss.
In addition to the speed of the seismic waves, the researchers studied a new characteristic: how loud they were.
Deuss, a senior author of a new Nature study looked at whole-Earth oscillations – seismic waves caused by very large earthquakes, which make the planet ring like a bell for weeks or even months – to learn more about LLSVPs.
“We added new information, the so-called ‘damping’ of seismic waves, which is the amount of energy that waves lose when they travel through the Earth,” says study lead author Sujania Talavera-Soza, also from Utrecht University.
In the hot upper mantle, the region closer to the Earth’s crust, they found exactly what they expected – the waves were damped.
“Against our expectations, we found little damping in the LLSVPs, which made the tones sound very loud there,” says Talavera-Soza.
“But we did find a lot of damping in the cold slab graveyard, where the tones sounded very soft.”
They found that a factor other than temperature must explain the lack of seismic wave damping seen in the LLSVPs: grain size.
“Subducting tectonic plates that end up in the slab graveyard consist of small grains because they recrystallise on their journey deep into the Earth,” explains Deuss.
“A small grain size means a larger number of grains and therefore also a larger number of boundaries between the grains.
“Waves lose energy at each boundary they cross.
“The fact that the LLSVPs show very little damping, means that they must consist of much larger grains.”
The findings provide an unprecedented insight into the properties of LLSVPs, which must be much older than the surrounding slab graveyards and, with their much larger building blocks, are very rigid.
Therefore, the researchers think LLSVPs do not take part in mantle convection – the flow of mantle which causes tectonic phenomena such as volcanic eruptions and earthquakes at the Earth’s surface.
The findings suggest there is less flow in Earth’s mantle than is commonly thought.
