It has been the repetition of the signal that has been most evident at the summit of Kīlauea: lava fountains erupt, pressure falls, and the ground settles, and then the volcano quietly refills and repeats.
This start-and-stop rhythm has sustained the Halemaʻumaʻu eruption through 40 separate fountaining events from two vents, interspersed by periods long enough for the shallow magma storage to reinflate. The rhythm will be familiar to observers monitoring tilt and GPS instruments surrounding the caldera: while fountaining, the shallow Halemaʻumaʻu magma chamber depresses rapidly; during periods of quiescence, it refills steadily until pressure buildup is in sync with the next opportunity for eruption.
What has shifted is the character of the “between” period. Following the end of episode 40 on Jan. 12, occasional groups of small earthquakes have been occurring 1.5-4 km below Halemaʻumaʻu, with the vast majority of these being smaller than magnitude 1.0. These have been occurring in short bursts, and the summit’s inflationary tilt has been punctuated by small, sharp deflation pulses during the swarms and have then returned to its general reinflation trend. The swarms have been quite randomly distributed beneath the crater and south caldera region, as opposed to the distinct seismic signature that marked a short-lived new fissure in episode 30 in August 2025.
The recurring pattern of eruption “episodes” can make Kīlauea seem predictable, but the system below does not act like a single balloon expanding and popping. There are two main magma chambers, shallow and then even deeper, which is a useful way to break up monitoring data. However, Kīlauea’s summit is also characterized as a network of interconnecting conduits and intrusions that can store and route magma in more than one direction, including into rift zones that serve like underground transportation routes to feed the flanks of the volcano.
In the ongoing eruption, the summit GPS deformation models indicate two trends that are moving in opposite directions: the level of pressurization that has been attained in the shallow Halemaʻumaʻu chamber before each eruption has been slowly increasing, while the level of pressurization in the deeper south caldera has been slowly decreasing since the eruption began in late 2024. The increased shallow pressurization has been attributed to magma supply that is consistent with a higher output rate, with the current effusion rate being characterized as double Kīlauea’s supply rate. On the other hand, the decrease in the deeper pressurization has been attributed to the slow release of the high level of pressurization that was attained from October 2023 to the onset of the summit eruption.
There is a short transition here. If the earthquake swarms were indicative of magma pushing a new shallow fault, then the data from the monitoring stations would be expected to show a more compact grouping of earthquakes and a change in deformation. However, the scattered nature of the earthquake swarms and the lack of deformation data supporting new magma intrusion have kept the attention on pressure adjustment in the summit area rather than a shift in the location of the vents.
Even during the pause, the crater can appear to be active. During the days following episode 40, there was observed persistent glow and tremor. The summit sulfur dioxide emission was recorded at 1,550 tonnes/day, which is well within the range of 1,000 to 5,000 tonnes/day during a pause, and is nowhere near fountaining levels. Episode 40 itself showed how quickly the system can change.
It lasted 9.7 hours, had fountains that reached 800 feet (250 meters), and released an estimated 5.5 million cubic meters of lava. For the residents and visitors, the lesson is that the “pause” stage is not the same as “quiet.” Swarms, subtle tilt changes, and reinflation are part of the volcano’s routine signals that help scientists bracket the likely windows for the next fountains, while also recognizing that the next turn of Kīlauea is driven by a plumbing system that never moves in a single line.
