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Mount Tongariro's eruption on Monday was the mountain's first in decades.
Mount Tongariro, one of three volcanoes on New Zealand's North Island, erupted late Sunday night, spewing ash into the atmosphere and putting the area on alert.
The eruption happened at 11:50 p.m., and sent rocks and ash flying from the Te Mari craters on the northern side of the mountain, Stuff.co.nz reported.
Witnesses of the explosion, which was the mountain's first in decades, described seeing "flame-like explosions," red hot rocks being thrown into the air, and lightning, TV New Zealand reported. Loud explosions were also reported, and another witness said they saw a cloud of ash coming out of "a new hole in the side of the mountain."
GNS Science, a government-owned environmental research firm which monitors natural disasters, issued the highest level warning—a red aviation code—for planes in the region, the Associated Press reported.
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"This eruption caught us by surprise," GNS Duty volcanologist Michael Rosenberg told TV New Zealand. "We've been monitoring the area after earthquakes, but we didn't expect this. This is an unknown situation and we're therefore unsure of how it will progress. Ash is the main hazard for the area, and it's drifting east."
The country's Civil Defense also raised the emergency alert from one to two, and said volcanic activity could pose a threat to several areas, including Waikato, Hawke's Bay, Gisborne, Manawatu-Whanganui, Bay of Plenty and Taranaki.
They advised citizens to stay indoors with their windows and doors closed and to protect their eyes, mouth and nose from volcanic ash, according to Radio New Zealand.
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Several roads have also been closed due to ash and debris, according to Stuff.
There have not been any reports of injuries or damages so far, Radio New Zealand reported.
Tongariro, which is one of three volcanoes in Tongariro National Park, last erupted in 1897, TV New Zealand reported. However, Ngauruhoe, which is considered a vent of Tongaririo by geologists, erupted in 1974-75.