Volcano Watch: What fans the flames observed at volcanic vents?

Volcano Watch is a weekly article and activity update written by U.S. Geological Survey Hawaiian Volcano Observatory scientists and affiliates. This week’s article is by Mike Cappos, a gas field engineer with HVO.

Recently, something has been observed “dancing” above the active eruptive vents inside Halemaʻumaʻu…Hydrogen flames! Let’s take a look into what causes this and other flame phenomenon on volcanoes.

During pauses between the high-fountaining episodes of the ongoing Kīlauea volcano summit eruption, which began on Dec. 23, 2024, USGS Hawaiian Volcano Observatory (HVO) scientists—and astute watchers of the HVO livestream cameras—have periodically observed yellow to orange-colored flames emanating from the two vents inside of Halemaʻumaʻu. These flames, which look most impressive at night, are the result of burning hydrogen gas.

Flames of this nature have been observed in and near eruptive vents during past eruptions of Kīlauea and Mauna Loa volcanoes, and at other volcanoes around the world—often basaltic ones like ours in Hawaii. Flames were observed at Kīlauea in the early 1900s by Thomas Jaggar, the original founder of HVO, while he was observing eruptions inside Halemaʻumaʻu.

Other occurrences of flames have been observed during the 1969–1974 Maunaulu eruption, the 1986–2018 Puʻuʻōʻō eruptive era on Kīlauea’s East Rift Zone, the 2018 lower East Rift Zone eruption, the 2021–2022 Kīlauea summit eruption, and others.

Hydrogen gas is a minor constituent of volcanic gases. Water—which is made of hydrogen and oxygen—is a major component of volcanic gases, but in oxygen-poor magmas, some hydrogen that might otherwise be part of water remains as hydrogen itself in very small amounts.

How do we get flames from that hydrogen though? The temperature at which hydrogen ignites in air depends on its concentration. Generally, the higher the concentration of hydrogen present, the lower the temperature required for ignition.

One example of this was the Hindenburg airship disaster of 1937. The gas envelope of the Hindenburg was filled with pure hydrogen gas, which caught fire and exploded from only a stray spark when the airship was beginning to land, all at normal ambient temperatures in the atmosphere near the ground surface.

The low concentrations (less than 2%) of hydrogen gas present in and near volcanic vents require significantly higher temperatures for ignition to be achieved. Hydrogen gas such low concentrations needs temperatures above about 1380°F (around 750°C). Eruptive vents at Kīlauea have temperatures in excess of 1,830°F (1,000°C), which is more than sufficient to ignite the hydrogen.

However, we believe another key player for hydrogen to be efficiently ignited at the vent is the vent geometry. In most cases, flames are observed emanating from a hornito or small spatter cone within the vent.

In this sort of geometry, water-rich magmatic gas can accumulate inside the orifice and stay in an oxygen-poor state because it has not yet mixed with the oxygen-rich ambient atmosphere. The hornito or small spatter cone effectively acts as a super-hot nozzle to help induce the combustion of hydrogen, which creates the flames we have seen recently and during earlier eruptions.

But what about blue flames? The yellow/orange hydrogen flames in volcanic vents are somewhat elusive compared to the blue flames that are commonly observed around active lava flows.

Lava flows themselves typically don’t emit flames. Their broad surfaces disperse any remaining volcanic gases quickly and temperatures are too low to ignite any further diluted levels of hydrogen gas at the surface of the flow.

Blue flames that are sometimes seen around lava flows are more likely to be caused by the burning of methane gas, which is produced by the “cooking” of vegetation beneath the lava flow. This was a common observation during Kīlauea’s 2018 lower East Rift Zone eruption in the Leilani Estates subdivision. Although those dancing blue methane flames may seem beautiful… they are dangerous themselves and can even create a secondary danger. As methane gets trapped beneath the lava flow, the methane may not just burn, it may explode in and around lava flows in vegetated areas. This is a constant danger to those in close proximity to a lava flow and extreme caution must be maintained.

Volcanic flames are an interesting offshoot of volcanic degassing; their intriguing presence is an informative clue about vent or lava flow conditions. Still, they are dangerous. So, we’re happy to have our current flames mesmerizing us from a safe distance away but easy to see in HVO livestreams.

Volcano Activity Updates

Kīlauea has been erupting episodically within the summit caldera since December 23, 2024. Its USGS Volcano Alert level is WATCH.

Episode 24 of the Kīlauea summit eruption in Halemaʻumaʻu crater occurred from June 4-5, with approximately 7.5 hours of fountaining from the north and south vents. Lava fountains likely reached above 1,000 feet and the eruption plume reached 16,500 feet above ground level. Summit region inflation since the end of episode 24, along with persistent tremor, suggests that another 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 USGS Volcano Alert Level is at NORMAL.

One earthquake was reported felt in the Hawaiian Islands during the past week: a M2.8 earthquake 12 miles south of Pa‘auilo at 15 miles depth on June 1 at 8:22 a.m.

HVO continues to closely monitor Kīlauea and Mauna Loa.

Visit HVO’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.