Volcano Watch: What is the difference between an eruption and an episode?

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

The historic episodic eruption at the summit of Kīlauea has had 49 episodes of lava fountaining so far during the past year and a half, with episode 50 forecast within the next week.

But why are these individual fountaining events considered episodes instead of separate eruptions? And what’s the difference?

There have been unique repeated cycles of behavior since the ongoing summit eruption began Dec. 23, 2024, at the Big Island volcano. These have started with precursory spatter and overflows from one or both vents — which occurred before nearly every episode — in Halemaʻumaʻu Crater, lasting hours to days.

The cycle then moves into lava fountaining from one or both vents, lasting from 4.5 hours to more than 8 days, and finally, there is a pause in lava activity.

An eruption occurs when magma rises from below ground and reaches the surface, typically as effusive lava flows, lava fountains or an explosive eruption column.

There can be short breaks, or pauses, of lava erupting at the surface in some eruptions, even as magma continues to accumulate or move underground.

Because lava fountains have repeatedly erupted from one or both of the same two main vents (north and south) since December 2024, Hawaiian Volcano Observatory distinguishes these periods as episodes within an overall eruption taking place within Halemaʻumaʻu.

When pauses in activity during eruptions at Kīlauea last more than 90 days, there will usually be a longer period of quiescence, or a new eruption will start in a different region of the volcano.

Despite having breaks in activity during this ongoing eruption at the summit, monitoring data indicates activity is still elevated and the eruption is not finished, even if we don’t always see lava at the surface.

The ongoing episodic eruption continues to display moderate to high levels of activity during pauses, including ground deformation, seismic tremor and volcanic gas emissions.

Kīlauea’s summit tiltmeters record deflation during fountaining episodes as magma is removed from the magma chamber and erupted at the surface. There is then an abrupt change from deflation to inflation at the end of each episode as the summit begins to repressurize.

This repetitive behavior of deflation during episodes and inflation during pauses is the primary way Hawaiian Volcano Observatory staff are able to forecast when the next episode could take place based on the volume erupted and rate of repressurizing.

During lava fountaining episodes, strong volcanic tremor — a signal associated with fluid movement — is recorded on seismic stations at and around Kīlauea summit. The tremor during pauses is generally at a lower intensity, but it persists at varied levels influenced by volcanic gas, depth of magma in the vents and other factors.

Gas emission rates at the summit also remain elevated during periods between fountaining episodes, with sulfur dioxide emissions typically ranging between 1,000 to 5,000 tonnes per day.

These levels are much higher than emission rates recorded prior to the onset of the ongoing eruption, which were less than 120 tonnes per day.

Episodes and pauses aren’t unique to just this eruption.

During the first 3.5 years of the Kīlauea middle East Rift Zone eruption at Puʻuʻōʻō, there were also episodic fountaining events similar to the current summit eruption.

The pauses, or repose periods, between those episodes were longer on average than we are seeing now, with the longest lasting 65 days.

As of now, these short pauses between episodes — generally lasting days to weeks — included elevated seismic tremor, volcanic gases and deformation — which often transitions to low-level eruptive activity preceding sustained fountaining, all indicating there will be another fountaining episode.

Hawaiian Volcano Observatory keeps a close eye on monitoring data for any indication of magma movement into a different region of the volcano; this might mean a change in the ongoing eruption and perhaps eventually a new eruption beginning in a different location.

We don’t know when or how this eruption will end, but each time we see expected patterns in monitoring data after a fountaining episode ends, there’s a good chance that another fountaining episode from the north and/or south vents in Halemaʻumaʻu is on the way.

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VOLCANO ACTIVITY UPDATES

Kīlauea volcano

Volcano Alert Level: Advisory

Kīlauea has erupted episodically since Dec. 23, 2024, within the summit caldera. Episode 49 of summit lava fountaining happened for 7.5 hours on June 14. Summit region. No unusual activity is noted along Kīlauea’s East Rift Zone or Southwest Rift Zone.

What’s next: Inflation since the end of Episode 49 indicates another fountaining episode is possible. Models currently indicate Episode 50 is most likely to occur between June 23 and 27.

Mauna Loa volcano

Volcano Alert Level: Normal

Mauna Loa is not erupting. Check out the most recent monthly volcanic activity report for Mauna Loa online.

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

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EARTHQUAKE ACTIVITY UPDATES

Four earthquakes were reported felt during the past week in the Hawaiian Islands:

• MAGNITUDE-2.5 earthquake at 2:11 a.m. June 18 located 9 miles south of Honoka‘a at a depth of 16 miles below sea level.
• MAGNITUDE-4.5 earthquake at 2:14 a.m. June 17 located 10 miles southeast of Pāhala at a depth of 20 miles below sea level.
• MAGNITUDE-2.8 earthquake at 11:16 a.m. June 14 located 4 miles south of Hōnaunau-Nāpōʻopoʻo at a depth of 13 miles below sea level.
• MAGNITUDE-3.1 earthquake at 9:48 p.m. June 12 located 3 miles west of Kahaluʻu-Keauhou at a depth of 3 miles below sea level.

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