Image copyright Reuters Image caption Several climbers were rescued from Mount Aso
Japan’s Mount Aso volcano has re-opened to hikers after erupting for the first time in 32 years, spewing plumes of ash.
Japan’s weather agency issued a “yellow alert” for air traffic in the area, with some flights forced to re-plan.
Some 27 hikers were taken to safety after a fast-moving pyroclastic flow engulfed them near the peak.
The three-week eruption could wreak extensive havoc, with thousands of residents directly affected.
The chances of people fleeing Mount Aso are unlikely, said a spokesman for the Meteorological Agency, who added that people should not think that others would take risks.
“The likelihood of people fleeing the volcano are very slim. But if you get close to the area, you don’t need to feel faint,” he told Reuters news agency.
Following the volcanic eruption on Saturday, residents of dozens of homes in the area were given temporary shelters.
The volcano, which was dormant for 30 years until it erupted in 1994, previously erupted 40 times between 1600 and 1983.
Image copyright Getty Images Image caption An eruption at Aso in 1958 led to more than 600 deaths
However, history is littered with examples of pyroclastic flows, rocks that can inflate on impact, and when they do, they move at hundreds of kilometres an hour.
After Aso’s eruption in 1954, the peak led to more than 600 deaths. And on 20 September this year, lava burst out of Mount Sinabung on the Indonesian island of Sumatra, sparking a big eruption.
Pictures from Aso, about 160km (100 miles) south of Tokyo, showed huge plumes of ash on Monday morning, with the threat of more eruptions continuing.
The mountain’s main peak is relatively high and there was no immediate threat to people below it, but a smaller peak above it was experiencing increased activity.
The red alert – the highest on Japan’s four-tier warning system – remained in place for the largest peak.
Image copyright Yomiuri Shimbun Image caption It is unclear what will happen now that the volcano has re-opened
The most affected region is Nagano prefecture, where at least 47 villages, nearly 4,000 people and the Kagoshima city of Kiondo are covered with ash.
Local authorities warned residents to avoid the area, and hundreds of vehicles were parked outside Aso’s town hall, aiming to prevent it being blocked from the outside.
Image copyright Yomiuri Shimbun Image caption More than 3,000 people living in the area are at risk
More than 12,000 people who live and work in and around the mountain were evacuated, most of them clearing away and gathering food and medicine for their families and pets.
The volcano’s first eruption in 29 years happened last Saturday. Pictures taken from nearby Fuchu Mountain showed huge plumes of grey ash that could be seen rising hundreds of metres high, the BBC’s Alan Sykes in Tokyo said.
Officials told people not to enter the ash-coated area, although they were allowed to go back home.
Hikers have been bussed to other areas to continue their treks, but officials have warned others are not being taken to the old volcano.
“I’ve been told to go to Kagoshima, which is a 40-minute drive [from my village],” said Jiro Uchida, an Aso resident who walked to work.
“I’m not sure what I’m supposed to do now. It was an active volcano so I thought I could safely climb up it. But now I don’t know whether I can climb back down.”
What is pyroclastic flow?
The very large and very fast-moving stream of magma, gas and rock flows toward the surface.
“Fireballs,” or superheated ash-like dust, can at times reach hundreds of metres high.
The form of lava – also known as magma – can be pushed upwards.
The whole passage of magma moves quickly and can create two types of flow – a fire, or pyroclastic, flow and a steam or boiling steam flow.
Pyroclastic flows burn at temperatures of around 2,000°C (3,200°F), according to scientists.
“This is a major volcanic event of the year, and the government is mobilising all necessary equipment and people in a massive operation,” said Prime Minister Shinzo Abe.