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Kīlauea’s recent episodic lava fountains are driven by a cycle: new magma builds pressure beneath the summit, then expanding gas bubbles help propel rising lava upward. Eruption releases pressure, and the cycle can begin again. The mechanism is different from the continuous 2018 Fissure 8 fountain, which was driven mainly by a pressure gradient moving magma from summit storage toward a low-elevation flank vent.
How pressure and gas produce a fountain
- New magma builds pressure. As magma accumulates beneath Kīlauea’s summit, pressure in the system increases. USGS Hawaiian Volcano Observatory geologist Katie Mulliken describes the sequence: “As new magma accumulates, the amount of pressure builds. Eventually, lava erupts and de-pressurizes the system.” USGS Volcano Watch
- Gas expands as magma rises. Falling pressure during ascent allows dissolved magmatic gas to come out of solution and form bubbles. Those expanding bubbles help drive lava upward. The ejected lava is full of bubbles, giving fragments around the crater rim a foam-like texture. USGS Volcano Watch
- Eruption releases pressure. Once lava erupts, pressure falls. In the recent summit activity, episodes of fountaining are separated by renewed buildup rather than forming one uninterrupted fountain.
Why the 2018 Fissure 8 fountain was different
Not every Hawaiian lava fountain is driven by the same balance of forces. The 2018 Fissure 8 fountain in Kīlauea’s lower East Rift Zone was continuous and was driven primarily by a pressure gradient: magma moved from summit storage down toward a low-elevation flank vent. That transport mechanism differs from the pressure buildup and gas expansion associated with recent episodic summit fountains. USGS Volcano Watch
| Setting | Behavior | Principal driver described by USGS |
|---|---|---|
| Recent summit fountains | Episodic | Pressure buildup as magma accumulates, with expanding gas bubbles helping propel lava upward |
| 2018 Fissure 8, lower East Rift Zone | Continuous | Pressure-gradient transport from summit storage toward a low-elevation flank vent |
What gas pistoning can—and cannot—tell us
Before Kīlauea’s Episode 15, more than 100 cycles of lava rising and falling, vent overflows, and spattering were observed in the north vent. HVO calls this behavior “gas pistoning”; similar behavior has been seen in other Kīlauea eruptions. USGS notice, March 26, 2025
This is a documented precursor to that episode, not evidence that gas pistoning reliably predicts every future fountain. A single cycle, observation, or fountain height does not by itself determine when the next episode will begin or how high a fountain will rise.
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How common and how high have Kīlauea’s fountains been?
Fountaining has appeared in multiple Kīlauea eruptions. USGS reports 44 lava-fountaining episodes during the first three years of the Puʻuʻōʻō eruption and 17 during the November 14–December 20, 1959 Kīlauea Iki eruption. In episode 15 of the 1959 Kīlauea Iki eruption, the highest fountain measured on Kīlauea reached 1,900 feet (580 meters). These historical figures describe particular eruptions; they do not establish a typical height for current activity. USGS Volcano Watch
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Fountains create hazards beyond the lava flow
Fountaining can release volcanic gases and throw tephra—fragments of volcanic material—into the air. Fine fragments can travel downwind, and volcanic gas can contribute to vog. USGS identifies water vapor, carbon dioxide, and sulfur dioxide among the gases released during summit fountaining. Its Kīlauea information page reports sulfur dioxide emissions of up to 75,000 tonnes per day during episodes; that is a reported maximum, not a constant rate or a live reading. USGS Kīlauea Eruption Information
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In the conditions described on that page, summit lava flows were confined to the floor of Halemaʻumaʻu crater, while gas and fragments could affect areas downwind. Eruption conditions change, so consult the latest USGS Kīlauea update for current hazards and activity.
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