Enormous ‘mega-blob’ under Hawaii is solid rock and iron, not gooey — and it may fuel a hotspot
A massive blob at the bottom of Hawaii appears to be solid and rich in iron, new research shows.
This blob – scientifically known as the mega-ultra-low velocity zone – could anchor the Hawaiian hotspot, an area where hot material rises through the mantle and drives the volcanic activity that created the Hawaiian Islands.
“As it is an iron-rich material, it will be more electrically conductive, which will promote thermal conduction. So this will help localize the plume so it lasts longer,” said Doyeon Kimseismologist at Imperial College London and first author of the new study, published January 28 in the journal Scientific advances.
Ultra-low velocity zones (ULVZs) are giant chunks of the planet located near the boundary of the mantle and core, about 1,800 miles (2,900 kilometers) below Earth’s surface. They get their name from the fact that seismic waves coming from earthquakes slow down considerably in these regions. Mega-ultra-low speed zones are the largest of these regions, often extending for hundreds of kilometers. They are often found near volcanic hotspots, such as Hawaii, Iceland, and the Marquesas Islands of the South Pacific.
“This actually makes it one of our few direct windows into the composition and dynamics of Earth’s depths,” Kim told Live Science.
Because these blobs are so deep, scientists typically study them using compression 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, that create vertical motion. By combining data from both types of waves, then modeling rocks and minerals that might match that data, researchers could get a clearer idea of why waves slow down in these areas.
They found that the mega-ULVZ beneath Hawaii is likely rich in iron and solid rock. This largely rules out a competing hypothesis suggesting that the area could be extra melting.
With this information, “we can think about where it came from,” Kim said. “This could come from relics of Earth’s early evolution, particularly the crystallization of a basal magma ocean or recrystallized melt from past mantle melting.”
Not all mega-ULVZs can be created equally, Kim added. Some could form from subduction of water-rich oceanic crust deep in the mantle. Perhaps others involve hardware from the kernel itself. The approach presented in the new paper can help differentiate these types of ULVZs around the world, he said, as well as shed light on how planets form in the first place.
“We must first clearly understand what is happening on Earth to fully understand what is happening on other planets,” he said.
Kim, D., Song, J.-H., Dobrosavljevic, VV and Lekić, V. (2026). Seismic and mineralogical evidence for a mega-ultra-low-velocity iron-rich zone beneath Hawai’i. Scientific advances12(5). https://doi.org/10.1126/sciadv.adz1962
