The world’s largest active volcano was erupting Monday and wasn’t immediately threatening communities on Hawaii’s Big Island, but officials warned residents to be ready for worse.

Many current residents weren’t living there when Mauna Loa last erupted 38 years ago. The U.S. Geological Survey warned the roughly 200,000 people on the Big Island that an eruption “can be very dynamic, and the location and advance of lava flows can change rapidly.”

The eruption began late Sunday night following a series of fairly large earthquakes, said Ken Hon, the scientist-in-charge at the Hawaiian Volcano Observatory.

There’s been a surge of development on the Big Island in recent decades — its population has more than doubled, from 92,000 in 1980.

Most of the people on the island live in the city of Kailua-Kona to the west of the volcano, which has about 23,000 people, and Hilo to the east, with about 45,000. Officials were most worried about several subdivisions about 30 miles to the south of the volcano, which are home to about 5,000 people.

A time-lapse video of the eruption from overnight showed lava lighting up one area, moving across it like waves on the ocean.

The U.S. Geological Survey said that the eruption had migrated to a rift zone — a place where the mountain rock is cracked and relatively weak — making it easier for magma to emerge.

An eruption from the zone could send lava toward the county seat of Hilo or other towns in East Hawaii but it could take the lava weeks or months to reach populated areas.

“We don’t want to try and second-guess the volcano,” Hon said. “We have to let it actually show us what it’s going to do and then we inform people of what is happening ASAP.”

Hawaii County Civil Defense announced that it had opened shelters because it had reports of people evacuating from along the coast on their own initiative.

The average Mauna Loa eruption is not typically prolonged, lasting a couple of weeks, Hon said.

“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.”

The USGS warned residents at risk from Mauna Loa lava flows to review their eruption preparations. Scientists had been on alert because of a recent spike in earthquakes at the summit of the volcano, which last erupted in 1984.

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 centimeters) of ash could accumulate in some areas.

Mauna Loa is one of five volcanoes that together make up the Big Island of Hawaii, which is the southernmost island in the Hawaiian archipelago.

Mauna Loa, rising 13,679 feet (4,169 meters) above sea level, is the much larger neighbor of Kilauea, which erupted in a residential neighborhood and destroyed 700 homes in 2018. Some of its slopes are much steeper than Kilauea’s, so lava can flow much faster when it erupts.

During a 1950 eruption, the mountain’s lava traveled 15 miles (24 kilometers) to the ocean in fewer than three hours.

Tourism is Hawaii’s economic engine but Roth predicted few problems for those on vacation during the eruption.

“It will be spectacular where it is, but the chances of it really interrupting the visitor industry — very, very slim,” he said.

For some, the eruption might cut down on some travel time, even if there is more volcanic smog caused by higher sulfur-dioxide emissions.

Read more: Hawaii's Kilauea volcano erupts, lava fountains form in park

“But the good thing is you don’t have to drive from Kona over to Hawaii Volcanoes National Park to see an eruption anymore,” Roth said. “You can just look out your window at night and you’ll be able to see Mauna Loa erupting.”

Julia Neal, owner of Pahala Plantation Cottages, said the eruption brings some relief after many preparedness meetings, and much wondering about what the volcano will do.

“It’s exciting,” she said. “It’s kind of a relief that it’s happening and we’re not waiting for it to happen.”

A few future guests from the U.S. mainland called Neal “asking me to make a prediction, which I can’t,” she said. “So I said, just stand by.”

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