New Zealand's Mount Ruapehu volcano erupted suddenly, spewing mud down a ski slope and forcing police to close nearby roads.
The minor eruption Tuesday evening on central North Island sent two mud flows, or lahars, down the mountain's slopes, including the Whakapapa ski field, local district council spokesman Paul Wheatcroft said.
A cloud of ash and steam was reported rising about 4 kilometers (15,000 feet) above the mountain peak by airplane pilots shortly after the mid-evening eruption, vulcanologist Craig Miller said.
One lahar temporarily trapped a snow groomer, and two other ski field workers suffered minor injuries, Wheatfield said.
Two other people were reported injured by rocks tossed out by the explosion. One, a 22-year-old teacher, suffered serious leg injuries when rocks crashed through Dome Hut mountain shelter and was flown by rescue helicopter to nearby Waikato Hospital, rescuers told local media.
Two ski lodges with about 60 skiers were evacuated from the mountain before midnight.
Wheatcroft told National Radio that "at this stage it is not a civil defense emergency."
Miller said the eruption was "level 2, minor eruptive activity" and the lahars were a "lot smaller" than one earlier this year which released millions of liters (gallons) of water from the mountain's crater lake.
He said roads and rail tracks near the mountain had been closed by police, but were later reopened.
Miller said the eruption had occurred without any warning.
"Looking at our seismic instruments it was ... from nothing to full (eruption) in the space of a minute," he said, adding that such occurrences are "reasonably rare."
Vulcanologists and conservation staff are to fly over Mt. Ruapehu early Wednesday to assess the state of the volcano and crater in the wake of the eruption.