Scientists had suspected that somewhere below Hawaii, a secret was buried in the stone—something that plays a major role in influencing the island chain’s famous volcanic activity. Now, with the help of nearly 200,000 earthquakes and machine learning software, Wilding and his colleagues have finally discovered Earth.
In a study published Thursday in the journal Science, the team uncovered a previously hidden collection of magma caches that may act like the beating heart of the volcanoes above. The discovery offers a potential solution to a long-standing mystery – how does magma travel from the deep mantle to Hawaii’s surface? The work gives scientists a valuable new window into the behavior of some of Earth’s most volatile and dangerous volcanoes.
The Mauna Loa volcano in Hawaii has erupted for the first time in 38 years
Shallow reservoirs of magma feeding Hawaiian eruptions have been known for some time. This is due in part to seismic waves, which are closely monitored in Hawaii by an ever-expanding network of sensors. The waves act like Earth’s ultrasound; Changes in their speed and trajectory during their subterranean excursions tell scientists about the types of matter they were traveling through, providing clues to their temperature, density and composition.
But to truly understand what drives these volcanic forces, scientists need to know what happens at the interface of spongy mantle and solid crust. This is what the new study finally reveals, in unexpected detail.
The giant feature described in the paper consists of several rectangular chambers called lintels. As volcanic eruptions drain magma from the shallow reservoirs above, these deeper sills appear to interact. The cacophony of earthquakes, Wilding said, indicates that individual chambers begin to fill with molten rock at different times, like “blood rushes to the heart.”
“We were just looking at it, and it was amazing, it really was,” said Zachary Ross, a geophysicist at Caltech. “Since then, I can’t get the image out of my head.”
“It’s a very elegant study, and a very interesting result,” said Ken Rubin, a volcanologist at the University of Hawaii who was not involved in the study.
Like most parts of the planet, Hawaii would not exist without volcanoes. From time immemorial, a deep-seated geyser of superheated rock known as the mantle plume has been burning the underside of the tectonic plate in the Pacific Ocean. As the plate continued to drift, a series of epic volcanoes rose above the waves, forming the Hawaiian Islands chain.
Today, the chain hosts a small family of active volcanoes, including the mercurial Mauna Loa and the hyperactive Kilauea on the Big Island, both of which stopped erupting simultaneously this month.
A persistent seismic rumble from the Kilauea area southwest and 20 miles underground had previously indicated there may be a raft of faults there, creating pathways for magma to travel from the depths of the Pacific to near-surface reservoirs. And since the 1980s, special types of earthquakes indicative of wandering fluids have indicated that magma was churning in the area. But until recently, the true nature of this secret maze relied more on conjecture than on scientific fact.
“It was this mysterious box in the mantle,” Wilding said. “We really have very little idea what’s going on.”
What scientists need is a steady rise in earthquakes coming from exactly that region, enough to shed light on that dark region. Things looked promising in 2015 when the region’s roar picked up a bit.
But the team’s break came in 2018 when, after Kilauea has erupted more or less continuously for 35 years, it initiated an eruption sequence in a grand finale style at the volcano. The event produced 320,000 Olympic-sized pools of lava in just three months – and the rapid draining of the volcano’s shallow magma reservoir caused its summit to collapse dramatically.
In an exciting twist to the plot, geologists recorded a sudden spike in deep seismic activity in 2019 below the city of Pahala, located about 25 miles southwest of Kilauea. Surely, scientists believed that this could not be a coincidence.
While the Pahala quake storm was an opportunity to unearth buried treasure rock on the island, scientists alone won’t be able to identify many individual earthquakes in this cacophony, especially smaller quakes that are more common and can be smothered by larger explosions.
Not wanting to miss a single beat of the geological drum, the Caltech team fed the entire recording of the seismic storm to machine learning software—a technique Ross and his colleagues had previously used to identify millions of hidden earthquakes in California. The software quickly taught itself what was a real earthquake and what was a strange noise, then identified and characterized thousands of quakes that traditional seismic signal detection software and its human analyzers would have missed.
From November 2018 to April 2022, the system recorded about 192,000 earthquakes below Pahala. Plotting these bright spots on the map, the team was stunned to discover a collection of pulsating volcanic structures – the pulsating volcanic heart of southern Hawaii.
Some of the quakes came from an area 28 to 32 miles deep: These long-duration quakes are usually attributed to vibrations caused by the movement of fluids, including magma. The bulk of the earthquakes came from an area 22 to 27 miles deep. These volcanic tectonic earthquakes — the kind that occur when a fault moves and rocks break within a volcanic region — have identified a number of near-horizontal plate-like structures, some four miles long and three miles wide.
At different times, scientists have detected spikes in seismic activity within separate leaves. The team speculates that these plates are sills and pockets of magma trailing their molten rock from the lower, fluid-filled region near the apex of the mantle plume.
In search of a deeper connection
This new 3-D map of a key segment of the Hawaiian circulation system is “extraordinary,” said Jackie Kaplan Auerbach, a volcanic seismologist at Western Washington University who was not involved in the new study. “Absolutely remarkable,” she said, that scientists can not only see this previously hidden heart, but also perceive the spasms of the ventricles inside.
The Pāhala Sill Complex, as it is technically known, appears to have several arteries branching off from it. One of the main passes, marked by rock-breaking earthquakes, appears to lead directly to one of Kilauea’s shallow magma reservoirs. It was probably no coincidence, then, that the Thresholds complex began to thunder relentlessly in 2019. During the 2018 eruption, much of Kilauea’s shallow magma supply was drained, causing a pressure drop. In response, magma was sucked into the sills to equalize the pressure. Similar events occurred during the 2020 eruption of Kilauea.
Further work may help resolve the contentious question of whether Kilauea and Mauna Loa, two relatively close neighbors on the surface, are somehow connected at great depths. To date, there is little concrete evidence for this hypothesis, and experts generally agree that the two volcanoes exist so far independent of each other.
The new study doesn’t overturn that consensus yet. It shows another major artery of the sills complex, again marked by rock-shattering earthquakes, heading towards Mauna Loa. But that stops at a large horizontal fault and doesn’t seem to reach one of Mauna Loa’s shallow magma reservoirs.
Nor is it certain that magma is moving through either of these two pathways. That will change if future work detects long-term earthquakes coming from them – the kind that would indicate the presence of fluids, possibly magma.
“The results are amazing,” said Diana Roman, a geophysicist at the Carnegie Institution for Science in Washington who was not involved in the study. But “it remains unclear whether magma being intrusive at Pahala directly fuels the Mauna Loa and Kilauea eruptions.”
Roman also studied the Pahala earthquakes. The 2021 paper I co-authored concluded that it was the result of magma intruding at depth, causing simultaneous disturbances in Mauna Loa and Kilauea by squeezing both of their underlying plumbing networks. The new study supports the idea of indirect communication. But even with this molten web set, it’s too early to call the most obvious link.
However, much of Hawaii’s underworld remains unexplored, and more petroglyphs may exist, Ross said.
“What is still there that hasn’t been lit?” He said. Whenever the infernal underbelly of Hawaii gets violently shaken again, the Caltech team will be ready to shed light on it, hoping to reveal what is still hidden for now.
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