Lava from Alaska’s Great Sitkin volcano has spread into the valleys around it

Lava has filled most of the summit crater at Alaska’s Great Sitkin volcano and pushed out into the valleys below it, the U.S. Geological Survey’s Alaska Volcano Observatory said in a hazard notice issued September 24. The observatory kept the volcano at a Watch alert level with an Orange aviation color code, the same status it has held through a slow eruption that has now run for more than five years. No explosions have occurred at the volcano since May 2021, the notice says, even as the lava dome inside the crater keeps growing and spilling outward.

Lava filling the crater and pushing into the valleys below

The September 24 notice describes the activity in the same terms the observatory has used for weeks: “Slow eruption of lava within the summit crater continues,” with lava that “has filled most of the summit crater and advanced into surrounding valleys.” A companion notice from September 22 recorded earthquake activity consistent with continued dome growth, though cloud cover blocked satellite and webcam views of the crater itself during that reporting period.

Great Sitkin sits on a remote island in the central Aleutians, about 23 miles northeast of Adak, according to VolcanoDiscovery’s profile of the volcano, with Atka as the other nearby community exposed to its ash and gas. The island itself measures roughly 14 by 16 kilometers, built almost entirely by the same stratovolcano now sending lava into its own lower slopes. No towns sit close enough to the summit to be threatened by the slow-moving lava directly, but the observatory tracks the flow anyway because a change in eruption style, from effusive to explosive, could send ash into the flight corridors that cross the Aleutian chain, a hazard the volcano has produced before.

The Aleutian air corridor carries a steady stream of cargo and passenger flights between North America and Asia, which is why the observatory maintains the same aviation color-code system, running from Green through Yellow and Orange to Red, across every volcano it monitors in the chain. Orange sits one tier below the level reserved for eruptions that are actively producing hazardous ash above the vent, the kind of event that has forced airlines to reroute around other Aleutian volcanoes in the past.

Two magma reservoirs beneath a five-year eruption

The eruption began in July 2021 after roughly four years of unrest, and it has outlasted most of the volcano’s earlier episodes, which produced lava domes in 1945 and 1974 and a series of steam-driven explosions in 2018 and 2019. A 2024 imaging study using ambient seismic noise from ocean waves and storms mapped the volcano’s internal plumbing and found two separate magma reservoirs feeding the current activity, work reported by Alaska public radio station KUCB. Researchers built the model from years of accumulated background noise rather than a single active survey, effectively listening to the mountain rather than sending signals into it.

Matthew Haney, the Alaska Volcano Observatory’s chief scientist, said the resulting three-dimensional model of the system matters for the people watching it day to day: “Having this picture looking inside the volcano is important for volcanologists involved in” monitoring the mountain’s behavior. The study’s authors, drawn from Purdue University, the Carnegie Institution, and the observatory itself, compared the technique to a medical CAT scan, built from years of ambient noise rather than a single imaging pass.

That subsurface picture helps explain why the eruption has stayed slow rather than turning explosive again. With two reservoirs supplying magma along mapped pathways rather than a single pressurized chamber, the volcano has behaved more like a mountain building itself up from within than one racing toward a blast, the kind of steady effusion that has let lava creep into the surrounding valleys over months rather than erupting outward in a single event. Volcanologists watching the mountain treat that plumbing map as a baseline: any future notice describing a sudden pressure spike or a new pathway opening would mark a departure from the pattern the 2024 study established.

An Orange code that has held for years, not days

The Orange aviation color code, one step below the highest Red warning reserved for imminent or ongoing explosive eruption, has applied to Great Sitkin continuously since the July 2021 eruption began, according to the observatory’s volcano profile page. Its September 24 hazard notice and a companion notice from September 22 both describe Alaska Volcano Observatory scientists monitoring the mountain through a combination of local seismic and infrasound sensors, satellite imagery, webcams, and regional infrasound and lightning networks, tools that let them track the dome’s growth even when weather blocks a direct view, as cloud cover did for parts of the past week.

The observatory’s public notices have changed little week to week through September, repeating the same core facts: no explosions since May 2021, continued lava effusion, and an alert status that has not moved. That consistency is itself the story the agency is tracking, since a sudden shift in seismicity, gas output, or dome shape would be the signal that the long, quiet phase of the eruption is ending. For now, the observatory says, the mountain is doing what it has done for five years: filling its own crater and letting the overflow find its way into the valleys around it.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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