A 5.0-magnitude earthquake around 10:30 Thursday morning triggered a small collapse at Pu‘u Ō‘ō vent that sent a rose-colored plume billowing skyward. The plume was visible throughout Hawaii Volcanoes National Park and neighboring communities, including Kalapana Gardens, where this photo was taken/NPS, Janice Wei

Keep a bottle of soda tightly capped, and the gases are quiet. Rapidly uncap it, and you'll have a fountain of liquid spurting from the bottle. That, essentially, is what might happen to a coursing subterranean river of magma volcanologists are closely watching at Hawai'i Volcanoes National Park.

"It’s close enough to the surface that it can start to lose gases. And that’s what drives eruptions, gas," explained Mike Poland, the scientist-in-charge of the U.S. Geological Survey's Yellowstone Volcano Observatory. "Magma’s a soda, basically. Its got gases dissolved in it. And the more pressure you take off the magma, the more gases will start to come out. So it can become a runaway train.

"When magma starts to rise, more and more gas comes out. That provides more of a push, which causes it to go up more, which causes more gas to come out. Then pretty soon it’s rushing upwards and you get one of these (lava) fountains."

Thursday evening local time the "cap" seemed to be coming off the submerged river of magma, as spatters of lava were spit out of cracks that opened in an area just outside the park known as Leilani Estates in the lower Puna area. That led authorities to issue a mandatory evacuation order for the subdivision.

"White, hot vapor and blue fume emanated from an area of cracking in the eastern part of the subdivision. Spatter began erupting shortly before 5 p.m.," the Hawaii Volcano Observatory reported. A portion of the subdivision was being evacuated, it added.

Dr. Poland, who spent a decade with the USGS's Hawaii Volcano Observatory before moving to the mainland in 2015, has been keeping his eyes on both Hawai'i Volcanoes and Yellowstone national parks this week. While there has been some atypical behavior of Yellowstone's Steamboat Geyser (more on that Sunday on the Traveler), what's ongoing at Hawai'i Volcanoes has been captivating.

“What’s happening right now at Kīlauea is a real watershed moment," he said earlier Thursday during a phone call. "And when all is said and done, more will be learned about the volcano from this event than any other single event for a long time, since a long time ago."

Our interview came a few short hours after a 5 magnitude earthquake rumbled across the Pu'u 'Ō'ō cone on the East Rift Zone downslope of Kīlauea. The temblor cast rocks down into Pu'u 'Ō'ō and sent a short-lived ash cloud off to the southwest.

What could be occuring is the latest transformation of the Kīlauea Volcano. According to the National Park Service, the "main vent for Kilauea has shifted from the summit, where there was over 100 years of activity that ended in 1924, to the East Rift Zone, where most eruptions have been located since 1955. If this model is correct, activity on the East Rift Zone may continue for a century or more."

On Thursday, hours after that 5 magnitude earthquake, park officials cited increased seismic activity in expanding to nearly 32,000 acres a section of park near the rift that was off-limits due to the activity.

“Today’s activity further supports the continued instability in the East Rift Zone,” said park Superintendent Cindy Orlando. “Safety of visitors and staff is our highest priority.”

Since the third week of April there has been increased activity from both Kīlauea and Pu'u 'Ō'ō on the East Rift Zone, an area where the volcano is bursting a seam. "The rock in a rift zone has many cracks and is relatively weak, and thus it is easiest for magma to make its way to the surface through these rift zones," notes the University of Hawai'i at Hilo.

Several sets of new, small ground cracks observed on roads around Leilani Estates subdivision/USGS

Earlier this week cracks began to appear in roads around Leilani Estates subdivision in the lower Puna area, where residents have been told to stay vigilant.

"Whenever you see that pressure building, you know that at some point something’s going to give," Dr. Poland told me. "You never know where the weak link is. What is it that’s going to break? Well, apparently the little storage area beneath Puʻu ʻŌʻō broke. Usually, it breaks somewhere between the summit and Puʻu ʻŌʻō and you have a dike intrusion or fissure eruption. This time the break happened on the downhill, down the rift side of Puʻu ʻŌʻō, and so magma is moving underground down rift towards a populated area.

"... The question now becomes, 'Is that magma going to reach the surface?'" he wondered. "Most intrusions of magma actually stay underground. They just don’t erupt. They don’t have the umph to erupt. But a lot of them, more so than other places, tend to reach the surface in Hawaii. So, it’s unclear, really, what’s going to happen. If this erupts, it could be very damaging, just because you’re in the lower East Rift Zone, where thousands of people live right on the East Rift Zone.”

While the geologists aren't exactly sure how deep the plume of magma is, they suspect it's about 1-2 kilometers below the surface. While that might seem like a lot, molten rock rises up from 60 kilometers beneath the surface, according to the Park Service.

"It seems like it’s a ways down, but it really isn't," said Dr. Poland. 

The flow of magma presents what the geophysicist dubbed a "watershed" moment for understanding the inner machinations of Kīlauea.

"This might provide some window into how this happens. We’ve never seen magma in this part of the rift zone during a time when we’ve had a modern monitoring network," he said. "The last time it happened was 1960. And a lot was learned from that eruption. But we had relatively primitive methods of measuring deformation and seismicity" back then.

Priming the volcanic pump, so to speak, is an unusually large amount of magma.

"It’s not only taken the magma that was sitting around at Puʻu ʻŌʻō, but its drawn magma from other parts of the East Rift Zone," said Dr. Poland. "We can see a lot of subsidence in the East Rift Zone, even uprift of Puʻu ʻŌʻō. … So it seems to have sucked a lot of volume from all parts of the East Rift Zone and made this intrusion that moved down rift.”

While the East Rift Zone runs only a small handful of kilometers from the park to the ocean, it runs another 100 kilometers or so beneath the Pacific seabed, according to the geophysicist. If the current magma flow doesn't come to surface on land but follows the rift, "there’s a chance that you might have a submarine eruption, which would be amazing." he said.

"What’s interesting about this particular area of the volcano is that the East Rift Zone continues offshore for 100 kilometers or more. There’s more distance (of the rift) underwater than there is above the water," Dr. Poland said. "And we know there have been eruptions out there. There have to have been to have built this ridge that extends for more than 100 kilometers offshore. We’ve often wondered, 'How does magma get from the summit of Kīlauea over 100 kilometers down the rift zone to erupt offshore?'"

The volcano might be ready to show how.