Back in May 2018, Hawaii’s Kīlauea volcano kickstarted a months-long eruption that would turn out to be the most destructive in its history. Its Halema’uma’u summit crater progressively collapsed and consumed its surroundings, while magma draining from below fed rivers and fountains of lava streaming out of its eastern flanks. Ultimately, the volcano expelled some 320,000 Olympic-sized swimming pools’ worth of lava.
This exuberant eruption didn’t end the way scientists were expecting. On August 1, it was still spewing lava like there was no tomorrow. But by August 4, all volcanic activity had suddenly slowed down. The summit fell into relative silence, and lava was reduced to a trickle from the eastern flanks. It was as if someone decided to turn off the tap.
Like similar eruptions around the world, those featuring a cauldron-like summit and plenty of lava, it was assumed the magma reservoir feeding Kīlauea’s mighty convulsion had run empty. But a new study published in Science this month has provided a baffling plot twist. Based on the changing dimensions of the volcano throughout the eruption, as little as 11 per cent and no more than 33 per cent of that magma reservoir was drained by the end of the eruption.
In other words, it only took a modicum of magma from Kīlauea’s stores to create all that destruction. And that raises a currently unanswerable question: “Why did the eruption stop then, if it still had plenty of magma to give?” Michael Poland, the scientist-in-charge at the U.S. Geological Survey’s Yellowstone Volcano Observatory and co-author of the study, told Gizmodo.
Solving this riddle matters. When it comes to forecasting a volcano’s shenanigans, a lot of attention is (understandably) paid to what triggers an eruption, Kyle Anderson, a geophysicist at the U.S. Geological Society’s Volcano Science Centre and lead author of the study, told Gizmodo.
Scientists do a good job at knowing when something troubling is afoot, even if it doesn’t always lead to an eruption. Sometimes, as Monday’s tragedy at White Island/Whakaari volcano in New Zealand grimly demonstrates, certain eruptions styles cannot yet be predicted. Perhaps one day they will be, but for now, these steam-driven blasts will continue to take people by surprise at several volcanoes across the world.
The end of an eruption gets far less attention, and Kīlauea’s fury last year demonstrates why that needs to change. That three-month-long eruption had a tremendous impact on those that lived in its shadow, said Anderson, and at the time it was difficult to say how long it would go on. Being able to forecast the termination of future eruptions would be of huge help to those most affected by them.
Although those near the top of the list are somewhat interchangeable, Kīlauea was recently ranked by scientists as America’s most hazardous volcano. Despite it being one of the best monitored fiery mountains in the world, much about it remains unknown. This includes the amount of magma it contains, which essentially determines how much life-threatening lava can appear when an eruption happens.
“It’s really remarkable how little we know about how much magma is stored beneath most volcanoes,” said Anderson. “It’s such a fundamental parameter of volcanoes. But we just don’t know.”
Previous estimates of the shallow Halema’uma’u magma pocket that sustained its now-gone lava lake and 2018’s eruption had varied by several orders of magnitude, from the fairly small to the colossal. The 320,000 Olympic-sized swimming pools of lava made during that eruption, said Poland, was within a sensible range of prior estimates for the volume of the magma reservoir, so it was reasonable to assume that the eruption stopped because the magma supply had simply run dry.
There was precedent to support this assumption. Iceland’s dramatic Holuhraun eruption that began in 2014 was forecast to end in early-2015. This prediction was based on the idea that magma reservoirs eventually peter out, leading to an exponential decrease in the amount of lava coming out until nothing remains. That forecast was bang on the money, so perhaps the same applied to Kīlauea.
But Kīlauea didn’t slow down at the end. It went from full steam ahead to pretty much seizing up within the space of two or three days. “It took us by surprise,” said Anderson. Naturally, scientists began to suspect something was up.
In October 2018, the summit was seen deflating for a few hours to days, then inflating again for a few hours to days. This cycle was also seen earlier that year, which meant something was still going on inside the volcano’s shallow magma reservoir. This suggested that there was magma still inside it and that it was already refilling right after the eruption ended.
It was impossible to comprehend all this without narrowing down how much magma had been originally stored within Kīlauea’s shallow magmatic cathedral. Fortunately, scientists were spoiled for data: The 2018 eruption was the most heavily documented geologic event in human history. The constantly recorded, ever-changing dimensions of the summit, as well as the drainage of its famed lava lake, allowed them to come up with a better estimate.
They found that somewhere between 0.6 and 1.7 cubic miles (2.5 and 7.09 cubic kilometres) of magma was kept beneath the summit before the eruption. About 0.2 cubic miles of this erupted in 2018—so why didn’t the rest of it join the party?
One possibility is that the remaining magma had a harder time getting out. After all, magma reservoirs aren’t 100 per cent molten ponds; much of them can be completely solid for most of their lifetimes, or in a mushy state, with liquid streams snaking through a more solid sponge-like magmatic maze. That stuff isn’t as easily eruptable as hot, molten, buoyant magma.
Another possibility relates to the collapsing summit. During the eruption, there was a tango between the crater collapse and the magma drainage through Kīlauea’s eastern flanks, with each collapse coinciding with an uptick in lava production. It appears that the piston-like collapse of the rock above the magma was helping to push it out elsewhere, a bit like squeezing a tube of toothpaste. Perhaps the eruption stopped because the collapsing block got stuck.
“The short answer is that we don’t really know,” said Anderson. “But at least we have a pretty strong indication now that it wasn’t because there was no more magma.” There is still plenty remaining, including magma kept within a deeper and potentially larger pocket at the summit. Perhaps other volcanoes around the world, too, have been deceiving us about their magmatic reserves.
If Kīlauea’s eruption didn’t hammer it home enough at the time, then this study serves as a reminder that we shouldn’t underestimate the complexity of volcanoes. They may appear to be straightforward lava-making machines, but “the volcano on the surface is just the tiny bit we can see,” Janine Krippner, a volcanologist at the Smithsonian Institution’s Global Volcanism Program not involved with the new research, told Gizmodo.
Figuring out what is happening below takes a huge amount of research, technology and experience, and even with over a century of all three, Kīlauea shows us just how little we know what lies below the surface.