As we take our daily walks along the Earth’s surface, there are secrets hidden under our feet. Just this week, research revealed that two ancient, massive islands are buried in the planet’s mantle.
That’s a layer under the crust where temperatures are so hot that material has the ability to flow with the consistency of asphalt, per Oregon State University. Each of the islands found tucked away in this dynamic section of the Earth is the size of a continent, according to the researchers from Utrecht University in The Netherlands who published their findings in the Nature journal.
These islands also appear to be at least half a billion years old. Since they have survived that long in the hot conditions of the mantle (it is 1,600 to 4,000 degrees down there), the study authors said the discovery indicates that there is less flow in that region that previously thought.
Surrounding the islands are what the researchers called a “graveyard of cold sunken tectonic plates,” that have much lower temperatures than the islands themselves. Arwen Duess, a seismologist, professor of structure and composition of Earth’s deep interior and one of the study authors, explained that these plates traveled to their location in the mantle through a process called “subduction” that involves tectonic plates diving below one another and sinking.
To figure out that all of this is hanging out so deep under the Earth’s surface, Duess and her research team used tones created by large earthquakes. Utrecht University said in a press release that these quakes “make the whole Earth ring like a bell,” or a musical instrument. Using tones created by the quakes, scientists can figure out which areas are “out of tone” and thus learn more about the features buried deep inside the planet.
“At the end of the last century, an analysis of these oscillations showed the existence of two subsurface ‘super-continents’: one under Africa and the other one under the Pacific Ocean, both hidden more than two thousand kilometres (more than 1,200 miles) below the Earth’s surface,” near where the core meets the mantle, Utrect University said. For some perspective on how deep that really is, Mount Everest – the tallest mountain in the world – is around 5.5 miles above sea level.
Duess said that, for a long time, nobody knew what the subsurface continents were or if they were only a temporary phenomenon. Researchers have determined that seismic waves are slow near where the islands are located, and the regions are therefore called “Large Low Seismic Velocity Provinces” or LLSVPs.
“Deuss and her colleague Sujania Talavera-Soza were keen to find out if they could discover more about these regions,” said Utrecht University. So, they added new information about “damping” of the seismic waves – that’s the amount of energy the waves lose as they travel through Earth. In the LLSVP, they found little damping, according to Talavera-Soza. To study how old the islands were, the researchers looked at mineral grains.
Another interesting discovery from the team leads us to Hawaii, an area known for its volcanic activity. Vulcanism there and other similar regions includes mantle plumes, or large bubbles of hot materials that rise from the interior of the Earth. Duess and her team think that these lava lamp-like plumes actually originate at the edges of LLSVPs.