Hawaii’s Mauna Loa volcano is erupting for the first time since 1984

The world’s largest active volcano, Mauna Loa in Hawaii, is erupting for the first time in almost 40 years, with US officials warning nearby residents to prepare for possible evacuation.

The eruption, the volcano’s first since 1984, began at 11.30pm local time on 27 November at Moku‘āweoweo, the summit’s caldera located inside Hawai‘i Volcanoes National Park on the island of Hawaii.

So far, the eruption is confined to the summit area but officials warned the situation could change rapidly.

Along the sides of the volcano are two so-called rift zones, extending north-east and south-west from the caldera, where the surface of the volcano can crack and split. “Very fluid and fast-moving lava” can quickly run from a new fissure, threatening populated neighbourhoods downstream, said Ken Hon at the Hawaiian Volcano Observatory in a briefing on 28 November.

“Right now, we just don’t know what’s going to happen – if this is going to stay as a summit-only eruption or move into one of the rift zones,” he said.

On 28 November, officials from the US Geological Survey said lava had started to spill out of the summit’s caldera, but said there is no evidence of lava erupting from the rift zones.

A warning for ashfall has been issued for Hawaii’s Big Island, with medically vulnerable residents advised to stay inside or wear filter masks. Shelters have been opened to provide safety for islanders, although there is no immediate threat to populated areas, according to officials.

Mauna Loa is a giant shield volcano, standing at around 4 kilometres tall and covering a land area of about 5000 square kilometres.

It has erupted 33 times since 1843, most seriously in 1950 when it inundated the coastal town of Hoʻōpūloa within 3 hours, destroying houses, a church and the local highway.

During its last eruption in 1984, lava flows came within 5 miles of the city of Hilo.

Volcanologists have been reporting an uptick in activity at Mauna Loa for weeks, with dozens of small, shallow earthquakes recorded around the summit in the last month. “Then it really kicked up right before the eruption started,” says Paul Segall at Stanford University in California.

Deeper earthquakes around the volcano over the past several years have indicated Mauna Loa might be ready to erupt. “This has been on our radar for a while,” he says.

In a statement, the Hawaiian Volcano Observatory said it will conduct aerial reconnaissance “as soon as possible” to better understand the risks posed by the current eruption.

Segall says the eruption may help volcanologists understand more about the connection between Mauna Loa and another Hawaiian volcano called Kīlauea. “We don’t really understand the plumbing system where the magma separates” to flow to the two volcanos, he says. One theory behind the long delay since Mauna Loa’s last eruption is that magma was being diverted to Kīlauea, which saw a major eruption in 2018 that caused a caldera collapse.

Segall says he sees no indication at this point that the Mauna Loa eruption could become as forceful as the 2018 eruption of Kīlauea. “Statistically speaking, this thing is going to be relatively modest,” he says.