So what on Earth (or at least on Kīlauea) is a “gas piston”? —USGS Volcano Watch

If you’ve been following the ongoing eruption of Kīlauea, you may have seen the terms "gas piston" or "gas pistoning" show up in some of the USGS Hawaiian Volcano Observatory (HVO) updates, photo captions, and other places. 

If you’ve followed Kīlauea’s activity for even longer than the current eruption, you may have seen the term pop up before as well—the phenomenon has been observed in the 2008–2018 lava lake, in vents at Puʻuʻōʻō between 1983 and 2018, and even in earlier eruptions here, like at Maunaulu in the 1960s and in Halemaʻumaʻu in the early part of the 1900s (during HVO founder Thomas Jaggar’s tenure here at Kīlauea).

But what exactly is this "gas pistoning" that we’re referring to?

Sources/Usage: Public Domain. In the top panel, lava rises in the north vent in Halemaʻumaʻu prior to Episode 14 on March 19, 2025. Note that only a faint, hazy gas plume is visible near the right edge of the lava surface. In the middle panel, the lava surface has risen to the point of spilling lava out of the vent and has begun to spatter and more violently release the trapped gas, with a more obvious plume. In the bottom panel, the lava is more clearly draining down in the center of the vent, with the lava surface dropping and even more of a plume visible as more gas escapes. USGS photos.

Essentially, gas pistoning is a shallow, degassing-driven rise and fall of a lava surface. Often these pistons occur in narrow conduits—although they can happen in larger lava lakes and even in lava channels—and in bunches, as part of a series.

To start a piston, or one cycle of pistoning, lava at the surface becomes more viscous, or thick, usually by cooling. That cooler, more viscous lava is more difficult than usual for gases to escape through, so gases that would otherwise escape easily to the atmosphere instead begin to accumulate and build up a foamy, bubbly layer beneath that cooler surface lava.

Eventually, that foamy layer becomes buoyant enough to push the whole layer of viscous lava above it up to higher levels in the volcanic conduit (akin to a piston moving up within an engine).

If the lava reaches the top of the conduit, such that it can spill out, the top lava layer thins out to the point that the gas layer beneath can be released, which is often accompanied by spattering and bubble bursts. Any lava that did not spill out of the conduit can then drain back down deeper into the conduit, where it may or may not become part of another gas piston cycle.

If the lava remains within a conduit, but still at a higher level than normal because of the buoyant foam layer, the piston may destabilize on its own, or it may require an external force to destabilize it. Many gas pistons at Maunaulu and Halemaʻumaʻu have been observed to end, or drain, when rocks from a rockfall punctured the top layer of viscous lava, which then allowed the gas from the accumulated foam layer to escape.

When the lava is rising or at a static high level, with most of the gas trapped in the foam layer, volcanic tremor and sulfur dioxide emission rates drop to low levels because much volcanic tremor is caused by degassing itself. If the gas can’t escape normally—whether during a gas piston or some other circumstance—not as much tremor is generated. When a gas piston ends, with the lava draining back down accompanied by violent spattering and release of the accumulated gas, tremor spikes.

We have observed a range of gas piston types during the recent eruption sequence at Kīlauea. They began to become obvious in March of 2025, as part of precursory activity ahead of fountaining episodes 14 and 15. Since then, some episodes have had obvious precursory gas pistons, and others have not.

Some gas pistons from the current eruption involve the lava rising high enough that there are overflows that spill out of the vents, which can help initiate gas release and lava drainback. Others don’t quite reach the top of the conduit in the vents and instead drain without having lava overflows. Still more aren’t visible to our cameras or even observers in the field, but based on variations in recorded tremor and sulfur dioxide emission rate, we can surmise that the pistoning is indeed still happening, just deeper in the conduit out of view.

HVO does not yet have a full understanding of why the gas pistons are often a precursor to the high-fountaining episodes, or why they may behave differently from episode to episode. But we continue to collect geophysical and gas chemistry data, and make other geological observations, in order to better understand the gas pistoning phenomenon and the role that it plays in the current eruption.

Gas pistoning has been observed at Kīlauea for over a hundred years, and we expect that it will continue to occur during the ongoing eruption and in future eruptions as well.

Sources/Usage: Public Domain.

This video, captured on March 19, 2024, just prior to episode 14 of the Kīlauea summit eruption in Halemʻamaʻu, shows gas-pistoning cycles in the north vent. The video has been sped up 100x, and about one hour is shown in 40 seconds. During these gas-piston events, lava rises within the north vent and emits only a faint, hazy gas plume. Once the lava has risen to the point of spilling lava out of the vent, it starts to spatter and more violently release the trapped gas, with a more obvious plume. Eventually, the lava drains down in the center of the vent, with the lava surface dropping and even more of a plume is visible as more gas escapes. USGS video.