Dramatic footage of the massive eruption was streamed live, with volcanic ash visibly filling a large area.
There were no immediate reports of damage or injuries, but local authorities have pushed up the danger level to three out of five.
The 1,592-meter (5,222-feet) Mount Aso is located on Japan’s southernmost main island, Kyushu, and is considered a popular hiking destination for tourists. It is also the second largest mountain in Japan and is one of the biggest calderas in the world, encompassing an area 25 kilometers (15.5 miles) north to south and 18 kilometers (11.2 miles) east to west.
Officials said that around 30 people were parked near the peak when the volcano erupted, but that they had been evacuated. Overall, about 100 tourists were transported to safety.
There had been no indication that an eruption was coming, according to meteorologists.
A second eruption has not been ruled out, according to Sadayuki Kitagawa, senior coordinator for volcanic affairs at Japan’s Meteorological Agency. Kitagawa told people to be careful of falling rocks due to a “possibility of volcanic rocks landing in an area over a 1-kilometer radius.”
Video footage shows black smoke billowing 2,000 meters (6,561 feet) into the sky and ash raining down, NHK national television reported.
No nuclear plants have been affected by the eruption, Kyushu Electric Power Co stated, including the Sendai nuclear plant, which was restarted in August and is located about 160 kilometers (100 miles) south of Mount Aso.
Arrivals and departures have become “irregular” at the Kumamoto Airport because of the eruption, Japan Air Lines said, adding that several flights have already been canceled and more cancellations are expected.
Japan is located in a region known as the “Ring of Fire,” which consists of a horseshoe-shaped band of fault lines and volcanoes running around the edges of the Pacific Ocean. There are over 100 active volcanoes in the area.
Volcanic activity in Japan has intensified in the last year. Japan’s biggest volcanic disaster in nearly 90 years killed 63 last year when Mount Ontake erupted in central Japan. Authorities also had to evacuate a remote island south of Kyushu in May due to another eruption.