Hawaii's Kilauea volcano erupted explosively early Thursday, tossing boulders hundreds of feet and sending a plume of ash about 30,000 feet into the predawn sky.
A webcam at the Hawaii Volcano Observatory caught the aftermath of the short-lived eruption on film: an onslaught of wet and dusty ash raining down on a darkened landscape. From the summit of Mauna Loa volcano, 20 miles away, cameras photographed an anvil-shaped plume billowing on the horizon.
In a news conference, USGS scientist Michelle Combs said that the activity at the vent could become more explosive again. "It's a real dynamic situation up there," she said of the summit.
Scientists had warned for days about a major eruption as the lava lake that once filled the crater at Kilauea's summit began draining back into the ground. Their concern was that the sinking molten rock would create steam as it interacted with the water table and that the steam would then jet upward, hurling rocks and ash into the sky in a phenomenon known as a phreatic eruption.
"This is the sort of explosive activity that was anticipated," Mike Poland, a geophysicist with the U.S. Geological Survey who was based at Kilauea from 2005 to 2015, said Thursday. "It's not going to be the only one. Very likely there will be additional events."
Though dramatic, Thursday's early eruption did not pose an immediate threat to any people in the vicinity, Poland said. Observatory staff had left their Kilauea location on Wednesday, for a facility at the University of Hawaii in Hilo, after concluding that wind could carry ashfall toward the station.
According to Poland, the greatest impact was to an area within a few hundred yards of the summit's eruptive vent. That's where the explosion would have sent hot gas and 1,000-pound rocks soaring. For this reason, the entirety of Hawaii Volcanoes National Park has been closed since last week.
But wind is carrying the plume from the eruption northeast, potentially raining ash into nearby communities, the Hawaii Volcano Observatory warned. Residents were instructed to shelter in place if they found themselves in the path of the ash plume. Depending on weather conditions, USGS said, ash might fall as far as Hilo, 30 miles to the northeast.
The observatory also warned that vog - a noxious smog formed when sulphur dioxide from eruptive vents interacts with water vapour and oxygen in the air - has been reported in the community of Pahala, southwest of the volcano.
Though disruptive and painful for people living near Kilauea - especially those who have already lost their homes - the eruption will not significantly affect life on the rest of the Big Island, Poland said. "And it is not likely to turn into some catastrophic event," he added.
Kilauea, a massive shield volcano on Hawaii's Big Island, is the site of the world's longest ongoing eruption, oozing lava since 1983. But in recent weeks the volcano has become particularly restless. Fissures opened in communities along the volcano's eastern slopes, prompting evacuations and engulfing dozens of homes in lava.
That activity along the East Rift Zone caused a dramatic depressurisations of the magma column below Kilauea's summit, slowly draining the lava lake in the summit crater.
As the molten rock dropped below the level of the water table, it's likely that water in the surrounding rock began pouring into the vacated chamber - much the way water rushes to fill a recently dug well, said Charlotte Rowe, a geophysicist at Los Alamos National Laboratory.
The water would then flash into steam, "and steam as we know is a very powerful source of energy, a very powerful propellant," Rowe said.
Kilauea has erupted in this manner before. In May 1924, the Hawaii Volcano Observatory reported more than 50 explosive events over the course of two-and-a-half weeks at the volcano's summit. The lava lake had drained from the summit crater several months earlier - the scenario now being repeated. The eventual eruptions generated ash clouds more than five miles high and threw blocks weighing as much as 28,000 pounds out of the crater. One person was killed by falling debris from the largest eruption.
Coincidentally, Thursday's explosive event comes one day before the 94th anniversary of that death and on the 38th anniversary of the 1980 eruption of Mount St. Helens in Washington state.
What is happening at Kilauea is fundamentally different from that 1980 eruption, experts said. Shield volcanoes like Kilauea produce a runny, basaltic lava that does not tend to erupt as dramatically as steep stratovolcanos like Mount St. Helens. Whereas St. Helens sits along the geologically active boundary of the Pacific Plate, Kilauea and the other Hawaiian volcanoes are powered by a hot spot deep within the Earth's mantle.
But phreatic eruptions can still pose a deadly threat to anyone near the eruptive vent - and they are extremely difficult to forecast, Poland said. Eruptions involving magma offer warning signs for a potential evacuation, including inflation of the surrounding ground, seismic activity caused as rocks break and changes in the gases being vented.
Phreatic eruptions are "much more random," Poland said. More than 30 people were killed when a 2014 phreatic eruption at Japan's Mount Ontake caught them unaware near the summit.
In addition to monitoring the volcano's current activity, researchers are scouring data from Kilauea's extensive monitoring network - which includes tilt meters, seismometers, and ground and aerial gas detectors - in search of any changes that preceded this explosive event. Their hope is to pinpoint warning signs that could be used to predict phreatic eruptions in Hawaii and elsewhere.