Enormous 'mega-blob' under Hawaii is solid rock and iron, not gooey — and it may fuel a hotspot

A massive blob deep under Hawaii seems to be solid and iron-rich, new research finds.

This blob — scientifically known as a mega-ultralow velocity zone — may anchor the Hawaii hotspot, an area where hot material rises through the mantle and drives the volcanic activity that created the Hawaiian Islands.

"Because it's iron-rich material, it is going to be electrically more conductive, and that will actually promote thermal conduction — so it will actually help localize the plume to last longer," said Doyeon Kim, a seismologist at Imperial College London and the first author of the new study, published Jan. 28 in the journal Science Advances.

Ultralow velocity zones (ULVZs) are giant hunks of the planet that sit near the boundary of the mantle and the core, at about 1,800 miles (2,900 kilometers) below the Earth's surface. They get their name from the fact that seismic waves from earthquakes slow down dramatically in these regions. Mega-ultralow velocity zones are the largest of these regions, which often span hundreds of kilometers. They're often found near volcanic hotspots, such as Hawaii, Iceland and the Marquesas Islands of the South Pacific.

"It actually makes them one of our few direct windows into deep-Earth composition and dynamics," Kim told Live Science.

Because these blobs are so deep, scientists typically study them using compressional waves generated by earthquakes. But these pressure waves, or P waves, provide limited information. So Kim and his colleagues used a method they developed in 2020 that could also incorporate S waves, or shear waves, which create vertical motion. By combining data from both types of waves and then modeling rocks and minerals that could match those data, the researchers could get a clearer picture of why the waves slow down in those zones.

They found that the mega-ULVZ under Hawaii is likely rich in iron and solid rock. That largely rules out a competing hypothesis that suggested the area might be extra-melty.

With this information, "we can think about where it is coming from," Kim said. "It could be coming from the relics of Earth's earliest evolution, particularly from the crystallization of a basal magma ocean or recrystallized melt from past mantle melting."

Not every mega-ULVZ may be created equally, Kim added. Some might form from the subduction of water-rich oceanic crust deep into the mantle. Perhaps others involve material from the core itself. The approach in the new paper can help differentiate these types of ULVZs worldwide, he said, as well as shedding light on how planets form in the first place.

"We have to first clearly understand what's happening on Earth to understand fully what's happening on other planets," he said.