Great Sitkin has been quietly erupting lava for more than five years

On a remote island in the central Aleutians, lava has been oozing out of Great Sitkin’s summit crater for more than five years without a single explosion to interrupt it. The volcano’s current eruption began in July 2021 and has continued largely unnoticed by anyone outside Alaska’s small monitoring community, slowly filling the crater and pushing flows into the valleys below. It is one of the longest-running effusive eruptions in the Aleutian chain in decades, and scientists still watch it daily for any sign that the pattern could change.

What AVO observed at the summit dome this week

In its update on the morning of September 9, the Alaska Volcano Observatory described the same pattern that has defined the eruption for years: lava continuing to erupt slowly within the summit crater, minor earthquakes detected in the previous 24 hours, and clear webcam and satellite views showing steaming and elevated surface temperatures at the summit dome. The volcano alert level remains at WATCH, with the aviation colour code at ORANGE, the same status AVO has assigned since the eruption’s explosive opening in 2021.

A slow-motion eruption with no explosions since 2021

Since lava effusion began, flows have filled most of Great Sitkin’s roughly mile-wide summit crater and advanced into the valleys beyond it, building up the volcano’s dome in a gradual, non-explosive way that is common among Alaska’s Aleutian volcanoes. According to AVO’s September 9 update, no explosions have been recorded at Great Sitkin since a single event in May 2021, meaning the entire eruption since then has consisted of lava slowly effusing rather than being violently ejected. The observatory tracks the volcano using local seismic and infrasound sensors, satellite radar and thermal imagery, webcams, and regional infrasound and lightning networks, a monitoring mix suited to a volcano whose activity changes gradually rather than in sudden bursts.

Four years of unrest that built toward 2021

The current eruption did not begin without warning. Volcanic unrest at Great Sitkin was first detected in July 2016, marked by elevated seismicity, unusual steam emissions from the summit crater and a handful of small explosive events; AVO logged thousands of small earthquakes beneath the volcano over the following years. That build-up culminated in a Vulcanian explosion on May 25, 2021, an event AVO successfully forecast hours in advance based on the seismic signals it was tracking. High-resolution satellite radar then detected a small new lava dome inside the explosion crater on July 23, 2021, marking the start of the effusive phase that has continued ever since. It was Great Sitkin’s first eruption since 1974, when an earlier explosive event sent an ash cloud above roughly 25,000 feet.

A 3D model reveals why the lava keeps coming

Researchers have since worked out why the eruption has proven so durable. A study by scientists from Purdue University, the Carnegie Institution for Science and AVO built a three-dimensional model of the ground beneath Great Sitkin using seismic data paired with ambient noise generated by ocean waves and storms passing over the island, a technique AVO scientist-in-charge Matthew Haney compared to running a CAT scan of the Earth. The resulting image showed two magma reservoirs beneath the volcano and traced the pathway connecting them to the surface, giving researchers a clearer picture of what has been feeding the dome’s slow growth. Haney said the model helps scientists judge the hazards the eruption could pose to air travel over the Aleutians and to the nearby communities of Adak and Atka, even though the current activity remains non-explosive.

Why the alert level has stayed at Watch and Orange

Great Sitkin’s alert status has not moved from WATCH and ORANGE since the 2021 explosion, a level AVO reserves for volcanoes showing heightened or escalating unrest, including ongoing eruptions with low-level explosivity or effusion. Rather than a warning of imminent danger, the sustained status reflects the simple fact that lava is still actively erupting at the summit: AVO’s own hazard framework ties the alert level to observed activity, not to a forecast of change, so the classification will likely stay in place for as long as the effusion continues. Unlike its neighbour Shishaldin, which cycles between advisory-level unrest and short explosive episodes, Great Sitkin’s status has been unusually stable for more than five years.

Monitoring a volcano far from any city

Great Sitkin Island sits among the Andreanof Islands in the central Aleutians, about 26 miles east of Adak, itself one of the most remote inhabited communities in North America. That isolation shapes how AVO watches the volcano: rather than routine ground visits, the observatory leans on satellite radar and thermal data, a network of webcams, and seismic and infrasound stations to track the dome’s growth and the flows spreading into the surrounding valleys. The same remoteness is also why the eruption has drawn so little public attention despite running longer than almost any other in the region’s recent history, its hazard confined mostly to the aviation traffic that crosses the Aleutian chain overhead.

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

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