A bad-case scenario for a Mauna Loa eruption threatening populated areas within only a few hours did not happen Monday on Hawaii island.

Instead of flowing quickly to South Kona, which has happened before, lava from the world’s largest volcano erupted from its Mokuaweoweo summit caldera at about 11:30 p.m. Sunday, then migrated Monday to a northeast flank through three fissures, each between 1 and 2 miles long, where red lava spurted as high as 100 to 200 feet into the air.

Hawaii island Mayor Mitch Roth described the lava flow as heading in “the best possible” direction.

That direction is toward Hilo, but the path’s topography and history of Mauna Loa eruptions put the city on the island’s east side relatively far from danger.

”We feel pretty comfortable with the situation,” Roth said Monday afternoon at a news conference. “Right now it seems like the lava is going to a positive place as far as keeping away from the public and everything like that — away from property.”

Ken Hon, scientist-in- charge of the U.S. Geological Survey’s Hawaiian Volcano Observatory, said the eruption as of Monday afternoon appeared similar to one from Mauna Loa in 1984 and that if the current flow is maintained, then it could be about a week until it approaches any outskirts of Hilo.

Hon, along with state and county emergency management officials, cautioned that Mauna Loa eruptions can be dynamic in early phases, so the public should pay attention to alerts.

By 1:30 p.m. Monday only one of the three fissures remained active, and most fountaining was only around 10 feet or so high, USGS reported.

The 1984 eruption began in Mauna Loa’s caldera and moved down the mountain’s northeast flank through fissures. After four days, lava moving 300 to 700 feet per hour was about 4 miles from the outskirts of Hilo, but got no farther after lava channel levees upslope of the flow’s far edge broke, creating somewhat parallel new channels that slowed the advance. The eruption ended after 21 days.

The current eruption is Mauna Loa’s first since 1984, and follows weeks of increased seismic activity with scores of small tremors each day that kicked emergency planning and preparations into gear.

Hawaii County Civil Defense Administrator Talmadge Magno said preparation work had been underway since September, and two emergency shelters — one at the Robert Herkes Gymnasium in Kau and one in the Old Airport Gymnasium in Kailua-Kona — were opened Monday as a precaution and remained open overnight.

The Hawaii Emergency Management Agency activated its emergency operations center at about 1 a.m. Monday while the Hawaii National Guard was put on standby alert, according to Maj. Gen. Ken Hara, adjutant general for the Hawaii National Guard.

Lava flows from Mauna Kea over the past 200 years have reached populated areas in South Kona in as little as a few hours, including one in 1950 where lava traveled 15 miles to the ocean in less than three hours.

That compares with days to weeks for populated areas of North Kona and southeastern parts of the island including Naalehu and Pahala. For Hilo the travel time over less steep terrain has been weeks to months for lava coming out of the mountain’s northeast rift zone, according to USGS.

Hon said that in the 33 previous Mauna Kea eruptions over two centuries, lava has come out of only one rift zone during each eruption, leading to one side of the island, so he does not expect that the direction of the current flow will change.

“So we’re hoping that this lava flow, while it will be a big, spectacular event, occupies a fairly small portion of the island and hopefully will have a relatively minor effect on the residents and visitors to the island,” he said.

Schools on Hawaii island remained open Monday, no evacuations were ordered and no major roadways were threatened as the leading edge of lava from the lasting fissure was more than 10 miles from Saddle Road.

Gov. David Ige on Monday encouraged tourists who plan to visit Hawaii not to change any plans they have for travel.

“Lava flows are not threatening any downslope communities,” he said.

The eruption is producing volcanic fog, or vog, and could also produce fine ash as well as volcanic glass fibers commonly known as “Pele’s hair” that can be carried downwind.

The National Weather Service issued and later canceled an ashfall advisory for Hawaii island Monday.

USGS said Monday afternoon that a visible gas plume from the erupting fissure fountains and lava flows was primarily being blown to the northwest.

Dr. Libby Char, director of the state Department of Health, said the agency is monitoring air quality at 10 existing stations on Hawaii island and is working to set up more monitors, including ones that can measure potentially deadly sulfur dioxide gas.

Char warned the public not to get close to lava because gas plumes can shift with the wind.

Poor air quality due to fine ash can be very localized, and people with respiratory illness should remain indoors or use a face covering if going outdoors, state and county officials advised.

Mauna Loa is one of five volcanoes that together make up the Big Island.

Rising 13,679 feet above sea level, Mauna Loa is the much larger neighbor to Kilauea, which erupted in the residential Leilani Estates community in a rift zone in 2018 and destroyed about 700 homes in Puna. Parts of Mauna Loa are much steeper than Kilauea, so when it erupts, its lava can flow much faster.

Hon said the volume of lava being produced and the speed of the flow will take a couple of days to assess. He roughly guessed that lava was flowing Monday morning at between 1 and 3 miles per hour and that the order of magnitude for lava production was comparable to Kilauea’s last eruption.

A bigger unknown is how long this eruption will last.

Previous Mauna Loa eruptions over the past two centuries lasted on average one to two weeks but have been as short as a few days and as long as a year.

“We won’t really know until the eruption is over how long it is going to be,” Hon said.

VOLCANO INFORMATION

>> To stay informed about Mauna Loa’s eruption, visit hawaii-county-volcano-hazards-hawaiicountygis.hub.arcgis.com.

———

Star-Advertiser staff contributed to this report.