The Augustine volcano has sent plumes of smoke 15km (10 miles) into the sky.
The mountain, on an uninhabited island, has been erupting since Wednesday. Scientists said the "eruptive period" might last for months.
Alaska Airlines cancelled 28 flights using Anchorage and Fairbanks airports, while people living many miles away were warned about possible ash falls.
Augustine rises 1,260m (4,134 feet) out of Alaska's Cook Inlet, forming its own island, and is one of the most active volcanoes in the area, the Alaska Volcano Observatory says.
Months of rumbling earthquakes warned experts of the imminent explosions - the first eruptions in 20 years.
"This is typical Augustine behaviour," said the observatory's Tina Neal.
"This is one big eruption period, and it's going to have several sub-events that we might call eruptive pulses," she added.
Ash plume
A pilot had reported ash as high as 15,850m (52,000 feet).
Ash can be a danger to aeroplanes if it gets into their engines.
Alaska Airlines said flights, including those servicing Los Angeles and Seattle, would be suspended until late on Saturday, at least, while Era Aviation also cancelled flights.
A "very light dusting" of ash was reported near Homer, about 120 km (75 miles) north-east of Augustine.
Meanwhile, to the south, about 16,000 residents of Kenai Peninsula and Kodiak Island were warned about the ash cloud, and schools were closed.
Ash in the atmosphere can pose a health risk - especially for people with respiratory problems.
"Just don't go out if you don't have to," said Charlie Franz, chief executive officer of South Peninsula Hospital in Homer.
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