The director of the island’s volcano observatory, Roderick Stewart, said no one was injured when the mountain shot debris more than a mile into the air and down its western flank.
The observatory's Web site said the pyroclastic flow appears to have reached the sea, and vacant buildings set on fire in Plymouth could be seen burning for several hours after the blast.
The former capital city was abandoned when the volcano erupted in 1997, killing 19 people. "I'm quite surprised there's still stuff that can burn there," Stewart said.
Electrical charges generated by the soaring ash cloud triggered several lightning strikes visible from the town of Salem, the observatory said.
More than half the British Caribbean territory's 12,000 inhabitants have moved away since Soufriere Hills roared back to life in 1995.