Hawaii’s Mauna Loa, the world’s largest active volcano, has erupted for the first time in nearly four decades, causing volcanic ash and debris to fall nearby, authorities say.

Flows of lava remained contained within the summit caldera of Mauna Loa in the US island state, but the eruption could pose a threat to nearby residents should conditions change, the United States Geological Survey reported at 11:45 pm Sunday (09:45 GMT Monday), about 15 minutes after the eruption in Hawaii Volcanoes National Park .

“At this time, lava flows are contained within the summit area and are not threatening downslope communities,” the Geological Survey said on its website, noting that locals on the Big Island of Hawaii should review preparedness procedures.

While the eruption on the main island of the Pacific state remains confined within the basin at the top of the volcano, “if the eruptive vents migrate outside its walls, lava flows may move rapidly downslope,” according to the Geological Survey.

Hours later on Monday morning, the survey’s volcano monitoring office tweeted: “Lava does seem to have flowed outside the caldera, but for now the eruptive vents remain confined to the caldera.”

Mauna Loa began to erupt at 11:30 PM HST on Sunday. The eruption is currently confined to the summit, and there is no indication that magma is moving into either rift zone. HVO is closely monitoring. Follow @USGSVolcanoes for updates. Find webcams here: https://t.co/PCmuqZqpcB pic.twitter.com/dv6vJBsASo

— USGS (@USGS) November 28, 2022

“However, lava flows in the summit region are visible from Kona,” the Hawaiian Volcano Observatory said in a statement. “There is currently no indication of any migration of the eruption into a rift zone.”

A rift zone is where the mountain splits apart. The rock is cracked and relatively weak, and it’s easier for magma to emerge.

How long the volcano erupts and whether it could cause lava to flow to populated areas is impossible to predict, said Miel Corbett, a Geological Survey spokeswoman.

“But I can tell you, we’re in constant communication right now with Hawaii Civil Defense, and they’re providing updates to community members,” she said.

The Geological Survey said the Hawaiian Volcano Observatory was in consultation with emergency management personnel and its staff would conduct an aerial reconnaissance over the 4,168-metre (13,674-foot) volcano as soon as possible.

Hawaii authorities said no evacuation orders have been given although the summit area and several roads in the region have been closed.

A Geological Survey webcam on the Mauna Loa summit’s north rim showed long, bright eruptive fissures within the crater.

Portions of the Big Island were under an ashfall advisory issued by the National Weather Service in Honolulu, which said up to 0.6 centimeters (a quarter-inch) of ash could accumulate in some areas.

The Hawaiian islands are home to six active volcanoes. Mauna Loa, the largest on Earth, has erupted 33 times since 1843, according to the Geological Survey.

The most recent eruption, in 1984, lasted 22 days and produced lava flows that came within seven kilometres (four miles) of Hilo, a city with a population of 44,000 people today.

Last month, scientists said Mauna Loa was in “a state of heightened unrest” after a series of earthquakes were felt in the area.

Mauna Loa is the much larger neighbour of the Kilauea volcano, which erupted in 2018 and destroyed 700 homes. Some of Mauna Loa’s slopes are much steeper than Kilauea’s, so when it erupts, its lava can flow much faster.

During a 1950 eruption, the mountain’s lava travelled 24 kilometres (15 miles) to the ocean in less than three hours.