ANCHORAGE — A volcano in Alaska’s Aleutian Islands spewed ash into flight paths, prompting a warning to pilots by the National Weather Service.

Shishaldin Volcano erupted at 5 a.m. Tuesday, the Alaska Volcano Observatory announced, and sent up an initial ash cloud to 19,000 feet. Clouds initially obscured the mountain, but satellite imagery confirmed the ash cloud, U.S. Geological Survey geophysicist Hans Schwaiger said.

Seismicity diminished for a few hours, but it then increased again. During the increase, the volcano spewed an ash cloud to 25,000 feet, the observatory announced. The later eruption increased the volume of ash.

Wind continued to push the ash cloud northeast into the eastern Bering Sea and away from jet airliners flying between North America and Asia.

However, the National Weather Service upgraded its alert level to a warning. Trace amounts of ash were expected to fall on communities as far away as Sand Point about 140 miles east of the volcano.

Seismic activity greatly diminished after noon.

The eruptions were confirmed by lighting and satellite data, the observatory said. More explosions could occur or activity could significantly decrease with little warning, according to the observatory.

The observatory first detected increased seismic activity at the volcano. Elevated surface temperatures indicated active lava at the vent in the volcano summit, the observatory said. Volcanic ash is angular and sharp and has been used as an industrial abrasive. The powdered rock can cause a jet engine to shut down.

The volcano is 679 miles southwest of Anchorage near the center of Unimak Island, the largest island in the Aleutians. False Pass, a village of 40 people, is on the island’s east side. Unless winds change, the cloud would move north of False Pass and would not pose a threat, Schwaiger said.

Shishaldin also erupted Dec. 12, producing an ash cloud that reached up to 25,000 feet, and on Friday, sending an ash cloud to about 24,000 feet.

The volcano is a symmetrical cone that is 10 miles in diameter at its base. It rises to 9,373 feet and is the highest peak in the Aleutians.

Shishaldin is one of the most active volcanoes in the Aleutian volcanic arc. It has had at least 54 episodes of unrest, including more than 24 confirmed eruptions since 1775, according to the observatory. Most eruptions are small. An event in 1999 spewed an ash column that reached 45,000 feet.

The volcano is monitored with seismic and infrasound sensors, satellite data, a web camera and distant infrasound and lightning networks.

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• By DAN JOLING, Associated Press

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