Alaskan volcano Mount Spurr showing activity that will ‘most likely end in an explosive eruption,’ scientist says

an aerial view of a snowy volcano and mountain range

A huge volcano in Alaska appears to be “moving closer to an eruption,” scientists monitoring it have said.

Mount Spurr, which sits 81 miles (130 kilometers) west of Anchorage, is now releasing unusual levels of volcanic gases near its summit and from a flank vent that last erupted in 1992.

The 11,000-foot (3,370-meter) volcano has been undergoing an uptick in earthquakes and snow and ice melt on its slopes in the past year, indicating magma movement under the surface. Now, according to scientists at the Alaska Volcano Observatory (AVO), it’s most likely that this unrest will end in an eruption.

That’s an increase in risk from the observatory’s last assessment in February, which gauged that Mount Spurr was equally likely to simmer down as it was to erupt. Now, observations of increased carbon dioxide and sulfur dioxide emissions from the volcano have tipped the balance toward an eruption, said Matt Haney, the scientist-in-charge of the AVO at the U.S. Geological Survey.

“This time period of unrest will eventually most likely end in an explosive eruption like the ones that happened in 1953 and 1992,” Haney told Live Science.

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Those eruptions both took place at Crater Peak, a flank vent about 2 miles (3.2 km) from the stratovolcano’s summit. The last time the mountain’s peak erupted was likely more than 5,000 years ago, Haney said, so scientists don’t expect an eruption there — most likely, the rock between the eruptible magma and the summit crater is well-solidified and would be hard for any magma to burst through.

Any eruption will probably occur at Crater Peak, which has been more recently active and which probably has easier pathways to the surface for magma to move.

Crater Peak exploded three times over several months in 1992 and once in 1953. In both cases, ash erupted at least 50,000 feet (15,240 meters) in the atmosphere, Haney said. One of the explosions in 1992 sent the cloud drifting over Anchorage, blanketing the city in an eighth an inch (3.1 millimeters) of dust. In 1953, Anchorage experienced a quarter-inch (6.4 mm) ashfall.

If the magma movement beneath the volcano doesn’t settle down, the next sign of an eruption will likely be volcanic tremor, Haney said.

Unlike the brief, small earthquakes that have been shuddering the volcano over the last year, volcanic tremor is a long, ongoing shaking that can last for minutes, hours, or days. It indicates that magma is rising and that an eruption is likely imminent.

In 1992, volcanic tremor started about three weeks before Mount Spurr erupted. Another nearby volcano that erupted in 2009, Mount Readout, showed volcanic tremor for two months before it blew its top.

“If we see [tremor],” Haney said, “that will be the next sign that Spurr is further progressing toward an eruption.”