Some of the Earth ’s most “extreme features” have been discovered deep, deep underground, and scientists can’t work out what they are.
The mysterious zones have the power to slow down seismic waves by up to 50 per cent, yet experts don’t know what they’re made of or what role they play.
These strange black holes (figuratively speaking) are located within the Earth’s lower mantle – near the core – and are known as ultra-low velocity zones (ULVZs).
They have traditionally been detected near hotspots – regions of the mantle where hot rock moves upwards, forming volcanic island chains like Hawaii – but new research suggests they’re much more widespread than previously thought.
"Here's one of the most extreme features that we see anywhere inside the planet," geophysicist Michael Thorne, of the University of Utah, told Live Science .
"And we don't know what they are, where they're coming from, what they're made of, [or] what role they play inside the Earth."
Some researchers believe that ULVZs may be the remains of giant impactors from the time when Earth was relentlessly bombarded by meteors .
However, Thorne posits that if they are more ubiquitous – as they now appear to be – this suggests that they’re actively being generated today.
He suspects that these wave-sucking structures may be made up of the volcanic rock basalt, which forms at mid-ocean ridges where the seafloor spreads apart.
When this basalt eventually gets pulled into the mantle by subduction, it quickly melts and could form pockets where seismic waves slow.
Thorne and his colleagues were struck by the seeming ubiquity of ULVZs when conducting analysis of the waves generated by large, powerful earthquakes, known as PKP waves.
His findings suggested that something was dramatically slowing the earthquake waves to scatter their energy, Thorne told Live Science.
The two likely culprits were valleys and ridges along the core-mantle boundary where the waves travel or ULVZs, he said.
Thorne explained that ha and his team found signatures of ULVZs across the world, including under North America, North Africa, East Asia, Papua New Guinea, the Pacific Northwest, and the western Pacific.
Understanding exactly what these ULVZs are and how they work could help geologists better understand volcanic hotspots and how the mantle moves.
Still, Thorne conceded: "There are a lot of open questions we don't have answers to yet.”
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