Indonesia's Mount Agung finally erupted Saturday, after more than 2 months of seismic rumbling that had led authorities to evacuate more than 100,000 residents from an exclusion zone ringing the mountain on Bali Island. Over the past 3 days, the volcano has sporadically sent ash plumes up to 9 kilometers (km) into the sky, leading airlines to cancel all flights into and out of Bali's main Denpasar International Airport, stranding a reported 50,000 travelers. Today, the Indonesian National Disaster Management Authority raised the alert to four, the highest level, and ordered residents to stay 10 km away from the mountain because the "probability of a bigger eruption is increasing." Scientists are now watching to see whether the volcano repeats the violent eruptions of 1963, when superheated gases and volcanic material raced down the mountain in a series of pyroclastic flows, killing more than 1000 people.
"The volcanic eruption has now moved on to the next, more severe, magmatic eruption phase, where highly viscous lava can trap gases under pressure, potentially leading to an explosion," says Mark Tingay, a geologist at the University of Adelaide in Australia. But Agung might still subside, as it did in 1989 after spewing gas, and in 2007, when the crater swelled with an apparent buildup of magma that gradually decreased.
The 3000-meter volcano, the highest peak on Bali, has erupted repeatedly throughout geologic history. Eruptions in 1963 and in 1843 had a volcanic explosivity index of five, on a scale of zero to eight, says Heather Handley, a volcanologist at Macquarie University in Sydney, Australia. The eruption of Mount Pinatubo in the Philippines in 1991 had an explosivity index of six and resulted in more than 800 deaths, devastated the surrounding landscape, and pumped enough aerosols and dust into the atmosphere to cause significant global cooling. (A zero-level eruption is a slow lava flow that people can walk away from.)
At the moment, "it is very hard to tell at this time whether there will be a bigger eruption," Handley says. Prior to an eruption, gases—water vapor, carbon dioxide, and sulfur dioxide—bubble out of the magma as it rises, adding more pressure to the volcanic system, she explains. That pressure likely led to the eruptions over the weekend.
Now, red-hot lava can be seen in the crater. One possibility is that the eruption column of hot volcanic ash, gas and rock fragments now extending into the sky above the summit could collapse into what's called a pyroclastic flow and race down the flanks of the mountain. Or if there is a lull in activity, lava in the crater could start to solidify, plugging the vents. This could lead to a build up of pressure within the volcano and a more explosive eruption. But "it's really not clear at this stage quite how the eruption will progress," Handley says.
That's despite being one of the most intensely monitored and perhaps best-communicated eruptions ever, she says. Indonesian authorities have a livestream of seismic activity at the volcano and have been posting regular updates on observations. "You can see what's going on minute by minute," Handley says.
Correction, 28 November, 8:19 a.m.: The description of the volcanic mechanisms at work was clarified to reflect current conditions at Agung.