There were no reports of injuries or damage from the eruption of Mount Asama, which is about 90 miles (145 kilometers) northwest of Tokyo.
The volcano erupted at 1:51 a.m. (0451 GMT, 11:51 p.m. EST) Monday, belching out a plume that rose about a mile (1.6 kilometers) high, according to Japan's Meteorological Agency. The plume was still roiling over the volcano's crater late Monday.
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Chunks of rock from the explosion were found about 3,300 feet (1,000 meters) away from the volcano. Ash was detected over a wide area, including central Tokyo. In the town of Karuizawa, southeast of the volcano, the ash was thick enough to obscure road markings in some areas, town official Noboru Yanagishi said.
"Some people said they heard a strange noise in the morning when the eruption occurred," he said.
The eruption was not big enough to disrupt daily life near the volcano, though many people awoke to find their cars covered in a fine layer of powder. National broadcaster NHK showed people in Tokyo lining up to get carwashes, or wiping the ash from their windows.
In Tachikawa, a district in the northwest area of Tokyo, some farming areas were coated with ash.
"Because it's February and not harvesting season, there was no real damage to any crops," said Shoichi Matsumoto, a local official.
At Tsumagoi, a small town on the volcano with ski resorts and hot spring baths, residents went about their business as usual. Travelers planning vacations to the area had called to inquire, but no one had canceled, said Masaru Yoshida, a spokesman for the local tourist association.
"The wind has blown the ash to the other side of the mountain," he said. "It's a clear day so you can see some smoke, but not as much as we've had in the past."
Mount Asama has been active for thousands of years.
Its last major eruption took place in September 2004, spewing enough ash to damage local crops. It frequently spews smaller amounts of ash from its crater.
With 108 active volcanos, Japan is among the most seismically busy countries in the world. The country lies in the "Ring of Fire" — a series of volcanoes and fault lines that outline the Pacific Ocean.
An alert level of three was in place Monday for a 2.5-mile (4-kilometer) radius, which bars entry into the area and urges nearby residents to take caution. Alert level four advises residents to prepare for evacuation, while level five, the highest, orders evacuation, according to the Meteorological Agency.
"We don't think there will be any stronger eruptions, but we don't see the activity stopping anytime soon, either," agency official Kazuya Kokubo said.