What Fans the Flames Observed at Volcanic Vents? — USGS Volcano Watch

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 December 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.

Sources/Usage: Public Domain.

Image of burning hydrogen above the north vent inside Halemaʻumaʻu taken during the evening of May 21, 2025. USGS photo.

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.

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Image of burning methane (blue flames in foreground) from ground cracks taken during the 2018 lower East Rift Zone eruption of Kīlauea in Leilani Estates subdivision. USGS photo.

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 requires 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 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.