When the sinuous structures first came into view on the computer screen, John Wilding’s jaw dropped. “I was jumping around the office,” said the graduate student of geophysics from the California Institute of Technology. “I was thinking that it’s a part of the Earth that, in this moment, I was the only person on the planet that knew these things were there.”
Scientists had suspected that somewhere below Hawaii, a secret was entombed in stone — something that plays a leading role in influencing the island chain’s famous volcanism. Now, with the help of almost 200,000 earthquakes and a machine learning program, Wilding and his colleagues have finally unearthed it.
In a study published Thursday in the journal Science, the team has revealed a previously hidden collection of magma caches that may act like the beating heart of the volcanoes above. The discovery offers a possible solution to a long-standing mystery — how does magma from the deep mantle travel to the Hawaiian surface? The work gives scientists a valuable new window into the behavior of some of the most capricious, and hazardous, volcanoes on Earth.
The shallow magma reservoirs that feed Hawaii’s eruptions have been known about for some time. This is partly thanks to seismic waves, which are closely monitored in Hawaii by an ever-expanding network of sensors. The waves act like an ultrasound for Earth; changes in their speed and trajectory during their subterranean voyages tell scientists what sorts of matter they have been traveling through, providing clues to its temperature, density and composition.
But to truly understand what drives these volcanic powerhouses, scientists need to know what is happening at the interface of the squishy mantle and the solid crust. That is what the new study at last reveals in unexpected detail.
The giant feature described in the paper is made up of several elongated chambers named sills. When eruptions drain magma from the shallow reservoirs above, these deep-seated sills seem to react. A cacophony of quakes signals when individual chambers begin to fill with molten rock at different times, a bit like “blood rushing into a heart,” Wilding said.
“We were just looking at it, and it was just mind-blowing, it really was,” said Zachary Ross, a geophysicist at Caltech. “Ever since then, I can’t get the image of it out of my head.”
Ken Rubin, a volcanologist at the University of Hawaii not involved with the study, said, “It is a very elegant study, and an immensely intriguing result.”
Suspicious seismic swarm
Like much of the planet, Hawaii would not exist without volcanism. Since time immemorial, a deeply rooted fountain of superheated rock known as a mantle plume has been torching the underside of the Pacific tectonic plate. As the plate has continued to drift, a succession of epic volcanoes has risen above the waves, creating the Hawaiian island chain.
A persistent seismic rumble from an area southwest of Kilauea and 20 miles below ground had previously suggested that a collection of faults may exist there, creating pathways for magma to travel from the hadean depths to near-surface reservoirs. And since the 1980s, special kinds of quakes suggestive of roaming fluids have hinted that magma has been churning about in the region. But until recently, the true nature of this underground labyrinth was based more on speculation than scientific truth.
“It’s been this mysterious box in the mantle,” said Wilding. “We really have very little idea what’s going on.”
What scientists needed was a sustained spike in quakes coming from that exact region, enough to strongly illuminate that shadowy zone. Things looked promising in 2015 when the region’s rumbling picked up a little.
But the team’s lucky break came in 2018 when, after Kilauea had been erupting more or less continuously for 35 years, a grand finale-style eruption sequence began at the volcano. The event produced 320,000 Olympic-size swimming pools’ worth of lava in just three months — and the speedy exsanguination of the volcano’s shallow magma reservoir caused its summit to collapse dramatically.
In an exciting plot twist, geologists recorded a shocking spike in deep seismic activity in 2019 way below the town of Pāhala, which sits roughly 25 miles southwest of Kilauea. Surely, scientists thought, this cannot be happenstance.
While the Pāhala quake storm was a chance to unearth the island’s buried magmatic treasure, scientists alone would not be able to identify many of the individual quakes in that cacophony, especially the more commonplace smaller ones that could be smothered by bigger bangs.
Not willing to miss a single beat of the geologic drum, the team from Caltech fed the entire recording of the seismic storm to a machine learning program — a technique Ross and his colleagues had previously used to identify millions of hidden quakes in California. The program quickly taught itself what was a real quake and what was extraneous noise, then identified and characterized thousands of temblors that would have been missed by conventional seismic signal detection programs and their human analysts.
From November 2018 to April 2022, the system logged around 192,000 quakes below Pāhala. Plotting these luminiferous points on a map, the team was stunned to discover a collection of pulsing magmatic structures — the beating volcanic heart of southern Hawaii.
Some of the quakes came from a region 28 to 32 miles deep: these long-period earthquakes are usually attributed to the vibrations made by the movement of fluids, including magma. The bulk of the seismicity came from an area 22 to 27 miles deep. These volcano-tectonic quakes — the sort produced when a fault moves and rocks break inside a volcanic region — delineated a number of near-horizontal sheetlike structures, some of them four miles long and three miles wide.
At different times, the scientists detected surges in seismic activity within separate sheets. The team surmised that these sheets were sills, magma pockets whose own grumbles tracked molten rock rushing up from the lower fluid-filled region close to the mantle plume’s peak.
In search of a deeper connection
This new 3D map of a key segment of the Hawaiian circulatory system “is extraordinary,” said Jackie Caplan-Auerbach, a volcano seismologist at Western Washington University who wasn’t involved with the new study. It’s “awfully cool,” she said, that scientists cannot only see this previously hidden heart, but also can perceive the convulsions of the ventricles within.
The Pāhala Sill Complex, as the heart is technically known, appears to have several arteries branching from it. One major pathway, marked by rock-breaking quakes, appears to lead right into one of Kilauea’s shallow magma reservoirs. It’s perhaps no coincidence, then, that the sill complex began to thunder relentlessly in 2019. During the 2018 eruption, Kilauea was drained of a significant portion of its shallow magma supply, causing a pressure drop. In response, magma was sucked into the sills to equalize the pressure. Similar events happened during Kilauea’s briefer 2020 eruption.
Further work may help resolve the controversial question of whether Kilauea and Mauna Loa, which are relatively close neighbors at the surface, are somehow connected at great depths. To date, little concrete evidence for this hypothesis exists, and experts generally agree that the two volcanoes are largely independent of one another.
The new study does not overturn that consensus just yet. It shows another major artery of the sill complex, again marked by rock-breaking quakes, streaking up toward Mauna Loa. But this one stops short at a large horizontal fault and does not appear to reach one of Mauna Loa’s shallow magma reservoirs.
It also isn’t certain that magma is moving through either of these pathways. That would change if future work detects long-period quakes coming from them — the sort signifying the presence of fluids, likely to be magma.
“The results are stunning,” said Diana Roman, a geophysicist at the Carnegie Institution for Science in D.C. who wasn’t involved with the study. But “it is still unclear whether the magma being intruded at Pāhala directly feeds the eruptions of Mauna Loa and Kilauea.”
Roman also has studied the Pāhala quakes. Her co-written 2021 paper concluded they were the result of magma intruding at depth, causing simultaneous unrest at Mauna Loa and Kilauea by squeezing both of their underlying plumbing networks. The new study supports this notion of an indirect connection. But even with this mapped-out magmatic web, a more explicit link is too early to call.
Still, much of the Hawaiian underworld remains unexplored, and more magmatic arteries may yet be located, Ross said.
“What else is still in there that has not been lighting up?” he said. Whenever Hawaii’s hellish subsurface furiously shakes again, the team from Caltech will be ready to shine a spotlight on it, hoping to reveal what for now remains concealed.