San Francisco  - Alaska's Mount Redoubt volcano erupted twice Thursday in what the Alaska Volcano Observatory termed a "major explosive event," sending razor-sharp ash over the city of Anchorage.

The second eruption reached a height of around 20 kilometres, following an eruption earlier Thursday that send a plume of ash about 9 kilometres high.

The volcano, roughly 160 kilometers southwest of Anchorage, last erupted during a four-month period in 1989-90.

The ash will pass over the city of Anchorage but is not expected to fall to the ground there, the National Weather Service said.