When Did Mauna Loa Last Erupt and What Happens When it Next Blows?

Hawaii's Mauna Loa volcano has been showing signs of heightened activity in recent weeks. The volcano, which is one of the five making up the island of Hawaii, is the highest volcano on the planet, rising 13,000 feet above sea level.

It is also considered one the most active volcanoes on Earth.

The volcano has been rumbling to life in recent weeks. Heightened activity began in mid-September, when the United States Geological Survey's (USGS) Hawaiian Volcano Observatory recorded increased earthquakes in the area. The earthquake activity increased from 10 to 20 a day, to 40 to 50 a day, according to an update from the USGS.

Officials have gathered in recent days to discuss the ongoing activity, and staff from the USGS Hawaiian Volcano Observatory and Hawai'i County Civil Defense Agency will gather on October 27 to discuss the activity with the public.

Currently, the volcano is under a yellow advisory warning. The color is a step above green, indicating normal behavior. The next level above yellow is orange, indicating that the activity is being watched, and then it moves to red, which constitutes an active warning. Experts have stressed that the volcano is not currently posing a danger.

There is no way of telling whether this activity will lead to an eruption, and experts are not worried yet, but are keeping a close eye on the activity.

Matt Watson, professor of volcanoes and climate at the University of Bristol told Newsweek that there is a line to be crossed between "being prepared" and not being an alarmist" when it comes to these increases in activity.

"There has been an uptick in activity. Around a month ago, there was some doubling in earthquakes. So those quakes are often a portend for an eruption but they're often not as well, but they do tend to signify that something is happening," Watson said. "It's a very, very, very well-monitored volcano and is amongst the best monitored on Earth. It's worth noting that some indications that the USGS are not yet worried is that they haven't changed the alert level yet."

Watson said that if the USGS did change the alert level, that would be indicative of their belief that something would happen "quite quickly." But at the moment, they have just put everyone on a watch level, he said.

When Did Mauna Loa last Erupt?

Mauna Loa last erupted on March 25 to April 15, 1984. This followed nearly two years of increased seismic activity.

The eruption began quite suddenly, according to the USGS. This followed a three year period where there was a slow increase in earthquakes.

While nobody was hurt in the 1984 eruption, there was damage to power lines and a few roads were blocked due to the lava flow.

What Happens If it Erupts Again?

While Mauna Loa is the largest volcano on earth, the way it tends to erupt is not as dangerous when compared to other volcanoes.

"Keeping people prepared so they don't get blindsided is important. I think it's unlikely the eruption would start very, very rapidly. But, these sorts of systems could go from [zero] to 60 relatively quickly, and people do need to be prepared," Watson said. "It's not a system I would suggest that is highly likely to kill a lot of people. [...] The style of eruption you typically have there is relatively benign. [But] I guess from its base, it's the highest volcano on Earth. There's a lot of material."

University of Calgary earth science professor Rajeev Nair, told Newsweek that in the worst case, an eruption of Mauna Loa could be a threat compared to Kilauea—another, huge Hawaii volcano.

"The flanks of the volcano are steeper and the volume of eruptions historically from Mauna Loa eruptions are larger. This can pose risks to neighboring communities if the eruptions occur along the rift zones," Nair said. " Even if not directly affecting communities, a significant Mauna Loa eruption has the potential to cut off communities. But if the eruption occurs in the summit crater, it could likely be contained within the caldera without significantly impacting communities."

Update 10/25/22 9:04 a.m ET: This article was updated to add more quotes from Matt Watson.