Huge ‘megablob’ beneath Hawaii is solid rock and steel, not sticky – and that could irritate hot spots

A new study has found that a huge chunk deep underground in Hawaii appears to be solid and rich in iron.

This blob (scientifically known as the giant hyperslow zone) may be anchoring Hawaii’s hotspot, a region where hot material rises through the mantle and triggers the volcanic activity that formed the Hawaiian Islands.

“Because it’s an iron-rich material, it’s going to be more electrically conductive, and that’s actually going to promote heat transfer, so that could actually help localize the plume and make it last longer,” said Doyoung Kim, a seismologist at Imperial College London and lead author of the new study published in Science Advances on January 28.

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The ultra-low velocity zone (ULVZ) is a huge chunk of the planet located about 1,800 miles (2,900 kilometers) below Earth’s surface, near the mantle-core boundary. The name comes from the fact that seismic waves from earthquakes slow down dramatically in these areas. The ultra-ultra-low velocity zone is the largest of these areas, often spanning hundreds of kilometers. They are commonly found near volcanic hotspots such as Hawaii, Iceland, and the Marquesas Islands in the South Pacific.

“In fact, this is one of the few windows that gives us direct access to the composition and dynamics of the deep Earth,” Kim told Live Science.

These blobs are so deep that scientists typically study them using compression waves generated by earthquakes. However, these pressure waves, or P waves, provide limited information. Therefore, Kim and his colleagues used a method developed in 2020 that also incorporates S waves (shear waves) that produce vertical motion. By combining data from both types of waves and modeling rocks and minerals that might match those data, researchers were able to get a clearer picture of why waves slow in these zones.

They found that the giant ULVZ beneath Hawaii’s surface may be rich in iron and hard rock. This largely rules out competing hypotheses suggesting that the region may be specially melted.

With this information, “we can think about where it came from,” Kim said. “It may come from relics of Earth’s earliest evolution, particularly from crystallization in the basal magma ocean or from recrystallized melt from past mantle melting.”

Not all mega ULVZs are created equal, Kim added. Some may form when water-rich oceanic crust sinks deep into the mantle. Perhaps some of it has to do with the material of the core itself. The new paper’s approach could help differentiate these types of ULVZs around the world, as well as reveal how planets form in the first place, he said.

“To fully understand what’s happening on other planets, we first need to have a clear understanding of what’s happening on Earth,” he says.