The explosions on Friday, lasting three-and-a-half to 11 minutes each, followed two similar events on Wednesday and were part of an eruptive period that could last for months, said Tina Neal, a geologist with the federal-state Alaska Volcano Observatory.

"This is one big eruption period, and it's going to have several sub-events that we might call eruptive pulses," she said of the island peak 275km southwest of Anchorage.

Alaska Airlines announced the cancellation of 28 flights between Friday afternoon and Saturday morning as a precaution.

Flight restrictions around the volcano have been in place for the past few days. 

Ash clouds

A pilot reported ash from the second explosion as high as 52,000 feet (15,850 metres) above sea level, Neal said. 

Ash clouds from the other two explosions reached about 11,000 metres, according to the observatory. 

The 1260m tall Augustine forms
its own uninhabited island

A "very light dusting" of ash was reported near Homer, a community about 120km northeast of Augustine, Neal said.

The ash fall was reported by a National Weather Service observer, said Dave Schneider, a US Geological Survey official at the volcano observatory.

"You can almost taste it in your mouth, but you can't perceive it any other way," said Schneider. 

Previous eruptions

Augustine's previous eruptions were in 1986 and 1976. Both times the volcano had several ash- and steam-producing explosions that ran over a prolonged period, according to the Alaska Volcano Observatory. 

The current activity, which was preceded by months of small but intensifying earthquakes below the volcano, is similar to that in the past, Neal said. 

"This is typical Augustine behaviour," she said. 

Augustine, a conical peak, rises 1260m out of Cook Inlet, forming its own uninhabited island. It is the most active of the Cook Inlet volcanoes, according to the Alaska Volcano Observatory.  

The Alaska Volcano Observatory (http://www.avo.alaska.edu/) has an Augustine activity page and offers a pre-defined search for Augustine images.