Volcano Watch: So what on Earth (or at least on Kīlauea) is a ‘gas piston’?

“Volcano Watch” is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates.

If you’ve been following the ongoing episodic summit eruption of Kīlauea volcano on the Big Island, you might have seen the terms “gas piston” or “gas pistoning” show up in some of the Hawaiian Volcano Observatory updates, photo captions and other places.

If you’ve followed Kīlauea’s activity for even longer than the current eruption, you might have seen the term pop up before as well — the phenomenon was observed in the 2008-18 lava lake as well as vents at Puʻuʻōʻō between 1983 and 2018.

It’s been seen during even earlier eruptions, such as sat Maunaulu in the 1960s and in Halemaʻumaʻu Crater in the early part of the 1900s at the summit caldera — during Hawaiian Volcano Observatory founder Thomas Jaggar’s tenure at Kīlauea.

But what exactly is “gas pistoning” to which we’re referring?

Essentially, gas pistoning is a shallow, degassing-driven rise and fall of a lava surface.

These pistons often 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. It’s more difficult than usual for gases to escape from that cooler, more viscous lava.

So gases that would otherwise escape easily into the atmosphere instead begin to accumulate and build up a foamy, bubbly layer beneath that surface of cooler lava.

Eventually, the 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 inside 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 lava spattering and bubble bursts.

Any lava that did not spill out of the conduit can then drain back deeper, where it might or might 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 could destabilize on its own, or it might 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 this ongoing episodic summit eruption at Kīlauea.

They began to become obvious in March as part of precursory activity ahead of sustained lava fountaining Episodes 14 and 15. Since then, some episodes have had obvious precursory gas pistons and others have not.

Some gas pistons during the current eruption involve lava rising high enough that overflows spilled out of both vents in Halemaʻumaʻu Crater, which can help initiate gas release and lava drainback.

Others don’t quite reach the top of the magma 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 pistoning is still happening, just deeper in the conduit and out of view.

Hawaiian Volcano Observatory does not yet have a full understanding of why the gas pistons are often a precursor to the high fountaining episodes or why they might 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 it plays in the ongoing summit eruption.

Gas pistoning has been observed at Kīlauea for more than 100 years, and we expect it to continue during the ongoing eruption and as part of future eruptions, too.

Volcano Activity Updates

Kīlauea has been erupting episodically within its summit caldera since Dec. 23, 2024. Its U.S. Geological Survey Volcano Alert Level remains at Watch.

Episode 27 of the ongoing episodic eruption in Halemaʻumaʻu Crater happened June 29, with about 11 hours of fountaining from the north and south vents.

Summit region inflation since the end of Episode 27 along with persistent tremor suggest another eruptive episode is possible and could start between July 7 and 12.

Sulfur dioxide emission rates are elevated in the summit region during active eruption episodes. No unusual activity has been noted along Kīlauea’s East Rift Zone or Southwest Rift Zone.

Mauna Loa is not erupting. Its U.S. Geological Survey Volcano Alert Level remains at Normal.

No earthquakes were reported felt in the Hawaiian Islands during the past week.

Hawaiian Volcano Observatory continues to closely monitor Kīlauea and Mauna Loa.

Visit the observatory website for past “Volcano Watch” articles, Kīlauea and Mauna Loa updates, volcano photos, maps, recent earthquake information and more. Email questions to askHVO@usgs.gov.