Hawaii's Mauna Loa volcano erupts for 1st time in nearly 40 years

Hawaii's Mauna Loa, the world's largest active volcano, has started to erupt, prompting volcanic ash and debris to fall nearby, authorities said Monday.

The eruption began late Sunday night in the summit caldera of the volcano on the Big Island, the U.S. Geological Survey (USGS) said. Early Monday, it said lava flows were contained within the summit area and weren't threatening nearby communities.

The agency warned residents at risk from Mauna Loa lava flows to review their eruption preparations, though there are no evacuation orders. Scientists had been on alert because of a recent spike in earthquakes at the summit of the volcano, which last erupted in 1984.

Scientists will have to wait to see if this remains a summit-only eruption or a rift zone eruption.

There is currently no indication of any migration of the eruption into a rift zone, officials said. A rift zone is where the mountain is splitting apart and the rock is cracked and relatively weak — making it easier for magma to emerge.

"At this time, it's not a time to be alarmed," Big Island Mayor Mitch Roth said.

The average Mauna Loa eruption is not typically prolonged, lasting a couple of weeks, said Ken Hon, the scientist-in-charge at the Hawaiian Volcanos Observatory.

"Typically, Mauna Loa eruptions start off with the heaviest volume first," Hon said. "After a few days, it starts to calm down a little bit."

WATCH: Night sky illuminated by Hawaii volcano eruption:

In some previous eruptions, lava has overflowed the caldera but never made it close to populated areas.

"Right now we are looking at indications, trying to figure out if this is going to be an eruption that remains within the summit of Mauna Loa or moves down one of the rift zones either to the southwest and to the northeast," Hon said.

"We don't want to try and second guess the volcano," he went on. "We have to let it actually show us what it's going to do and then we inform people of what is happening ASAP."

Even though it noted there is no indication of lava exiting the summit, the civil defence agency said it has opened shelters in Kailua-Kona and Pahala because it has reports of people self-evacuating along the South Kona coast.

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