The world’s largest active volcano, Hawaii’s Mauna Loa, has erupted for the first time in almost four decades, authorities said Monday.

Mauna Loa erupted at 11:30 p.m. local time Sunday, in what was its first eruption since 1984, according to the Hawaii Volcano Observatory.

As the eruption began in Moku‘āweoweo, the summit caldera of Mauna Loa located inside Hawaii Volcanoes National Park, the US Geological Survey raised the alert level for the volcano from “advisory” to a “warning.”

“At this time, lava flows are contained within the summit area and are not threatening downslope communities,” the USGS said.

“Winds may carry volcanic gas and possibly fine ash and Pele’s hair downwind.”

The USGS warned that the early stages of the eruption could be “very dynamic and the location and advance of lava flows can change rapidly.”

“If the eruption remains in Moku‘āweoweo, lava flows will most likely be confined within the caldera walls,” the statement said. However, if the eruption spreads outside the walls of Mauna Loa’s summit caldera, “lava flows may move rapidly downslope.”

Residents at risk from Mauna Loa were warned to review preparedness guides and refer to Hawai’i County Civil Defense information for any further guidance.