Volcano Watch: What happens beneath the surface doesn’t always stay beneath the surface

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

Kīlauea has been erupting episodically for the past 2 months in Halemaʻumaʻu Crater, nested within the south end of Kaluapele, the summit caldera.

Captivating lava fountains burst from either one or both of the vents during each eruptive episode. However, as fascinating as those episodes are, scientists at the U.S. Geological Survey Hawaiian Volcano Observatory are equally as interested in what’s happening beneath the surface between episodes.

There have been 10 lava fountaining episodes from 2 vents in Halemaʻumaʻu since the eruption began Dec. 23, 2024: the north vent and the south vent. Most have been active for less than a day (16 hours on average), though a couple were longer (up to 8 days).

Hawaiian Volcano Observatory staff rely on several key monitoring datasets to track the status of Kīlauea using a network of instruments across the volcano.

A variety of seismoacoustic instruments record earthquakes and other ground vibrations, as well as low-frequency sound.

Gas sensors sniff the volcanic gas emissions in areas downwind.

Webcams, including the livestream camera, provide near real-time visual and thermal views.

Ground deformation is documented using GPS units and tiltmeters.

Together, these datasets can be used by scientists to analyze the processes occurring within Kīlauea.

In between each lava fountaining episode, datasets such as tilt, seismicity and gas emissions, have shown similar behaviors. This repeating nature of activity has allowed observatory scientists to identify patterns that can be used to estimate windows of probability for future eruptive episodes in Halemaʻumaʻu.

When lava fountains are erupting in Halemaʻumaʻu, ground deformation instruments — tiltmeters, in particular — show deflation as magma that was stored in the volcano is erupted on the surface. In between lava fountaining episodes, those sensors show inflation as magma again accumulates beneath the surface and repressurizes the magma chambers.

The amount of repressurization required for a new episode to start has ranged from 2 to 10 microradians, taking anywhere from a day to nearly 2 weeks. This range of repressurization is what informs probability estimates for when a new episode most likely will begin.

The rate of inflation can vary though, which causes the probability window for the start of the next episode to be shifted.

While deformation shows us how magma is behaving underground between episodes, we also have data telling us surface activity continues even while fountaining is paused.

During episodes of lava fountaining, the rumblings of magma making its way out of the conduit produce an intense tremor signal seen on seismic stations across Kīlauea. The tremor rumbles at lower frequencies, caused by rapid expansion and contraction of magma within the vents.

These continuous vibrations result from the formation and release of gas bubbles that drive the fountains high into the air.

Yet, even between eruptive episodes, tremor has remained present.

The intensity drops significantly between episodes but continues to exist until the next episode. This indicates that gas is continuing to be released by the vents and magma continues to stir near surface even when lava is not visibly erupting.

As the weak tremor churns in the background, other seismic signals are also observable.

Low frequency signals that range in size are seen on the seismic network around the summit area and suggest complex patterns of magma migration, bubble formation and transport within the volcano.

The observation of continuing tremor between episodes is corroborated by volcanic gas measurements and persisting glow at the vents. When an episode of lava fountaining is occurring, sulfur dioxide emission rates are highly elevated, in the range of 10,000 tonnes per day.

Even when lava is not actively fountaining, sulfur dioxide emission rates have remained moderately elevated, in the range of 1,000 t/d. If Kīlauea were truly quiet, we would expect only about 100 t/d of sulfur dioxide.

Likewise, in between eruptive episodes, glow remains visible at night at the north vent, and sometimes at the south vent. This incandescence is another clue that magma is just beneath the surface, continuing to gurgle, glow and degas.

New episodes of lava fountaining have been brief and impressive, but the build up to them is an exciting glimpse into how Kīlauea volcano works.

If you happen to visit Kīlauea in between Halemaʻumaʻu lava fountaining episodes, know that the degassing and glow are evidence of the continuous activity happening, hidden just beneath the surface.

Volcano Activity Updates

Kīlauea has been erupting intermittently within Halemaʻumaʻu Crater in the summit caldera since Dec. 23, 2024. Its U.S. Geological Survey Volcano Alert level is at Watch.

The eruption continued during the past week, with one eruptive episode. Episode 10 was active from the night of Feb. 19 until the morning of Feb. 20. Kīlauea summit has been inflating since episode 10 ended, suggesting another eruptive episode is possible.

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 is at Normal.

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

• Magnitude-3.5 earthquake 9 miles east-southeast of Nāʻālehu at a depth of 22 miles at 6:10 p.m. Feb. 18.
• Magnitude-3.8 earthquake 11 miles east-southeast of Nāʻālehu at a depth of 21 miles at 5:04 a.m. Feb. 14.
• Magnitude-3.1 earthquake 4 miles south of Kapaʻau at a depth of 14 miles at 6:31 a.m. Feb. 13.

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

Visit the observatory’s 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.