(Volcano Watch is a weekly article written by scientists at the U.S. Geological Survey’s Hawaiian Volcano Observatory.)

Last week’s Volcano Watch focused on the East Rift Zone eruption, which continues to impact the lower Puna District on the Island of Hawaiʻi. Changes in the eruption are occurring daily, if not hourly, so by the time you read this article, the situation will likely have changed. Nevertheless, here’s the status of the eruption as of Friday, May 11 (when this was written):

Fifteen fissures have erupted in the Leilani Estates area thus far. These outbreaks have typically been short-lived, lasting minutes to hours, with pauses between the fissures ranging from minutes to days. The fissure eruptions have covered about 117 acres with lava and emitted high levels of sulfur dioxide (vog.ivhhn.org/leilani-eruption). Earthquake activity, ground deformation measured by GPS, and the opening of new ground cracks east and west of Leilani Estates suggest that this eruption is not over and that additional fissures are likely to open.

The breaking news of this past week, however, was about Kīlauea Volcano’s summit eruption.

On May 2, two days after the significant collapse in Pu‘u ‘Ō‘ō on Kīlauea’s East Rift Zone, the lava lake in “Overlook crater” within Halemaʻumaʻu at the summit of Kīlauea began to drop. The steady lowering of the lava lake over the next week increased the potential for phreatic (steam) explosions at Halemaʻumaʻu, which has resulted in the closure of Hawaiʻi Volcanoes National Park and concern among nearby residents.

Explosive events have happened at the summit lava lake since the vent first opened in 2008. But those events have been gas-driven explosions caused by rocks falling from the crater walls. Rocks hitting the lake surface cause the release of gases dissolved in the lava, which explosively propels gas and ash upward, forming a dusty ash plume that rises above the lava lake.

Since 2008, more than 60 gas-driven explosions have thrown tephra (general term for airborne volcanic rock fragments) and spatter (fragments of molten lava) onto the Halemaʻumaʻu crater rim. Fine ash particles have also been blown a few miles by wind, creating minor annoyances in downwind areas.

But, phreatic, or steam-driven, explosions are much larger in scale and impacts. These explosions can occur when the lava column within a vent drops below the water table, allowing an influx of groundwater into the still-hot conduit.

The mix of hot rock and water results in the formation of steam. If the conduit remains open, the steam can passively rise to the surface, forming an impressive, but relatively benign, steam plume.

However, rocks falling from the conduit wall can form a plug that effectively chokes the conduit. When this happens, steam below the plug can no longer passively escape, so pressure builds until an explosion blasts through the conduit.

During a phreatic explosion, large ballistics—boulders weighing up to several tons—can be blasted out in all directions, falling to the ground within about 1 km (0.6 mi) of the conduit. Smaller rocks—marble to pea-sized—can be thrown 5-6 kilometers (up to 4 miles) from Halemaʻumaʻu, but where they land is largely influenced by wind direction.

Phreatic explosions can produce towering plumes of gritty to fine ash that can be blown farther downwind, possibly tens of kilometers (20 or more miles). The fallout of ash will not be life-threatening, but it will be a nuisance for downwind communities. The thickness of ash deposits at a given location can range from a fine dusting to fractions of an inch, but the total thickness can add up with multiple explosions.

It’s important to note that it’s not 100 percent certain that a phreatic explosion will occur at the summit of Kīlauea. But, based on the disappearance of the summit lava lake (it has now drained completely from view) and geophysical models, there’s a possibility that phreatic explosions could happen in the coming days or weeks. We must also note that these explosions can occur with no—or very little—warning.

Residents living near or downwind of Kīlauea’s summit area should learn about ashfall hazards, stay informed of the volcano’s status, heed all closures, and review family and business emergency plans. For more information about Kīlauea summit ash hazards, please visit vog.ivhhn.org/summit-ash-hazar….

On May 9, U.S. Geological Survey scientists presented information about the possible phreatic explosions at Kīlauea’s summit. A video of this presentation is posted at www.bigislandvideonews.com/201….

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