GRINDAVIK, Iceland >> A volcano in southwestern Iceland erupted on Thursday for the third time since December, sending jets of lava into the sky and triggering the evacuation of the Blue Lagoon spa, one of the island nation’s biggest tourist attractions.

The eruption began at about 0600 GMT (1 a.m. EST) along a nearly two-mile fissure northeast of Mount Sýlingarfell, the Icelandic Meteorological Office said. The site is about 2½ miles northeast of Grindavik, a coastal town of 3,800 people that was evacuated before a previous eruption on Dec. 18.

The Meteorological Office said that lava was flowing to the west and there was no immediate threat to Grindavik, and civil defense officials said that no one was believed to be in the town at the time of the eruption.

“They weren’t meant to be, and we don’t know about any,” Víðir Reynisson, the head of Iceland’s Civil Defense, told national broadcaster RUV.

The Civil Defense agency said that lava was heading for a pipe that supplies communities on the peninsula with hot water from the Svartsengi geothermal plant. Authorities asked people to use hot water sparingly, as workers rushed to lay an underground water pipe as a backup.

The nearby Blue Lagoon thermal spa, created using excess water from the power plant, was closed when the eruption began and all the guests were safely evacuated, RUV said. A stream of steaming lava later spread across the exit road from the spa.

The Icelandic Met Office earlier this week warned of a possible eruption after monitoring a buildup of magma, or semi-molten rock, below the ground for the past three weeks. Hundreds of small earthquakes had been measured in the area since Friday, capped by a burst of intense seismic activity about 30 minutes before the latest eruption began.

Dramatic video from Iceland’s coast guard showed fountains of lava soaring more than 50 meters (165 feet) into the darkened skies. A plume of vapor rose about 1½ miles above the volcano.

Iceland, which sits above a volcanic hot spot in the North Atlantic, averages an eruption every four to five years. The most disruptive in recent times was the 2010 eruption of the Eyjafjallajokull volcano, which spewed huge clouds of ash into the atmosphere and led to widespread airspace closures over Europe.

This is the third eruption since December of a volcanic system on the Reykjanes Peninsula, which is home to Keflavik, Iceland’s main airport and several large towns There was no disruption reported to the airport on Thursday.

Dave McGarvie, a volcanologist who has worked extensively in Iceland, said it’s highly unlikely the “gentle, effusive” eruption will disrupt aviation because such volcanoes produce only a tiny amount of ash.

Grindavik, about 30 miles southwest of Iceland’s capital, Reykjavik, was evacuated in November when the Svartsengi volcanic system awakened after almost 800 years with a series of earthquakes that opened large cracks in the earth north of the town.

The volcano eventually erupted on Dec. 18, sending lava flowing away from Grindavik. A second eruption that began on Jan. 14 sent lava towards the town. Defensive walls that had been bolstered since the first eruption stopped some of the flow, but several buildings were consumed by the lava, and land in the town has sunk by as much as 1½meters (4½ feet) because of the magma movement.

No confirmed deaths have been reported, but a workman is missing after falling into a fissure opened by the volcano.

Both the previous eruptions lasted only a matter of days, but they signal what Icelandic President Gudni Th. Johannesson called “a daunting period of upheaval” on the Reykjanes Peninsula, one of the most densely populated parts of Iceland.

It’s unclear whether the residents of Grindavik will ever be able to return permanently, McGarvie said.

“I think at the moment there is the resignation, the stoical resignation, that, for the foreseeable future, the town is basically uninhabitable,” he said.

He said that after centuries of quiet, “people thought this area was fairly safe.”

“It’s been a bit of a shock that it has come back to life,” he added, “Evidence that we gathered only quite recently is that eruptions could go on for decades, if not centuries, sporadically in this particular peninsula.”

Danica Kirka and Jill Lawless in London contributed to this story.