Bali’s Mount Agung has been spewing columns of ash into the sky since last week. Authorities are concerned that this could be a precursor to a major eruptive event, and as a result, 100,000 people have been ordered to evacuate, and regional airports have closed.
Officials are also observing lava pool within the crater, which suggests that the magma chamber just beneath the surface is quickly depressurising. The constant tremors being felt around the volcano also suggest the rapid and continual movement of magma toward the vent.
The alert level is currently at its highest. The exclusion zone – an area in which no-one may enter – was expanded from 6.4 to 12.1 kilometers (4 to 7.5 miles) after scientists expressed concern about the danger of a major eruption, which will feature extensive pyroclastic flows.
The volcano is quite clearly erupting right now, but the apparent switch to a magmatic phase is what's worrying researchers on the ground.
“The rays of fire are increasingly observed at night. This indicates the potential for a larger eruption is imminent,” the National Board for Disaster Management said in a statement, per The Jakarta Post.
Agung has been dormant for 50 years, and the ash columns represent the beginning of its return to activity. This eruption was driven by the explosive release of superheated groundwater as steam, which is a surefire sign that magma is ascending. The appearance of the "rays of fire" within the crater now suggests it's moved to a phreatomagmatic stage, where said groundwater mixes explosively with fresh lava.
Although exactly when and how Agung will erupt more dangerously is difficult to say at this point, the authorities aren’t taking any chances. The last major eruption back in 1963 killed 1,600 people, but the area is far more populated these days.
The government is particularly worried about what some are calling “cold lava flows”, which are technically named “lahars”.
Lahars, whose name derives from the Javanese word for “flowing lava”, are a little like mudslides. They’re very viscous and have the properties of particularly wet concrete. Unlike conventional mudflows, however, they’re formed from the ashy remnants of volcanic eruptions – and they’re far more dangerous.
Ash, unlike plenty of soil, is often very loose and unconsolidated shortly after it’s deposited. When rain falls on it, it mobilizes as a slurry incredibly quickly, especially on slopes. If you get caught in one that’s moving at 36 kilometers (22 miles) per hour, you can quickly be smothered.