The world’s largest active volcano, Hawaii’s Mauna Lo, has erupted for the first time in nearly 40 years.
The lava flow is mostly contained within the summit, but residents are on alert and have been warned in advance of the risk of ash fall.
The US Geological Service (USGS) said the situation could change rapidly.
The volcano’s warning level has also been upgraded from “advisory” to “warning” – the highest rating.
Emergency officials say no evacuation orders have been issued and populated areas are unlikely to be affected at this stage.
Mauna Loa is located within Hawaii Volcanoes National Park, which covers half of the US state’s Big Island. The volcano is 13,679 feet (4,169 m) above sea level and covers an area of more than 2,000 square miles (5,179 square km).
It erupted at Moku’āweoweo, the volcano’s summit caldera, at 23:30 local time on Sunday (09:30 GMT Monday). Calderas are hollows that form below the summit at the end of an eruption.
It said after a series of warnings that an eruption was possible following recent earthquakes in the region, including more than a dozen reported aftershocks on Sunday.
An advisory for ash – which can contaminate water supplies, kill plants and irritate the lungs – was in effect overnight for the surrounding area but has since been lifted.
“Based on past events, the early stages of a Mauna Loa eruption can be very dynamic and the location and advance of lava flows can change rapidly,” the USGS said.
He added that if the eruption moves beyond the walls of the peak caldera, the lava flow “could move rapidly downwards”.
According to the USGS, Mauna Loa has erupted 33 times since 1843. A previous eruption in 1984 sent lava flows within 5 miles of Hilo, the island’s most populous town.
But the Big Island’s population has more than doubled since 1980 to nearly 200,000 residents, and the Hawaii Civil Defense Agency has warned that residents could face “lava devastation.”
Dr Jessica Johnson, a British volcano geophysicist at the Hawaiian Volcano Observatory, said: “These lava flows rarely pose a threat to life, but they can be extremely destructive to infrastructure. “
He warned that the lava flows threatened Hilo and Kona, another major population center, adding that volcanic gases could cause breathing problems for locals.
Mauna Loa is the largest active volcano in the world. Others are large volcanoes but are classified as inactive, meaning they have not erupted for a long time, or extinct, meaning they are almost certain not to erupt in the future.
Mauna Loa shares the Big Island with Mauna Kea, the world’s tallest mountain when measured from its underwater base, about 20,000 feet below sea level.
