Volcanic eruptions can be triggered by weather events

On Saturday 4 December, Semeru – at 3,676 metres the tallest mountain on the densely populated island of Java, in Indonesia – burst catastrophically into life, throwing up columns of ash so vast that they obscured the sun on many parts of the island, plunging residents into darkness.

A suffocatingly thick blanket of ash and mud now cloaks several villages in east Java, hampering efforts to extricate those who are trapped and signalling a long and arduous recovery effort ahead. Fourteen lives have been lost, and more than 900 people evacuated.

But this isn’t Semeru’s first rodeo – it also erupted in January this year, in an entirely uneventful fashion. What is it that made Saturday’s eruption so disastrous, but renders other eruptions harmless? How do researchers know what’s coming when a volcano starts to stir? And most importantly for those that live under a volcano’s shadow: how much warning can researchers give that a catastrophic eruption is about to begin?

Each volcano has a distinct geological personality, with its own unique signs that eruption is imminent – things like a sudden, unusual increase in the strength of earthquakes below it, or a swelling and bulging of the volcano itself, as magma builds below the surface with enough pressure to physically deform the mountainous vessel.

But for some volcanoes, such as Semeru, even the most watchful researchers could not have foreseen Saturday’s disaster. Despite being generally active, with a number of small eruptions occurring over the past year, Semeru exhibits a volcanic feature known as a “summit lava dome” that makes it particularly difficult to predict major eruptions. The dome is a bulging bubble-like formation at the volcano’s pinnacle, where the mounting pressure of magma and gas below the surface gathers. These domes are highly variable between volcanoes, with some forming over centuries, and others emerging within mere months.

The sides of these domes are composed primarily of unstable rocky debris, and collapse can theoretically happen at any point.

“We can monitor dome deformation via remote sensing to get a sense of how much pressure might be building inside, and by visually monitoring for new cracks – but these are mostly important for dormant volcanoes,” says Dr Szabolcs Kosik, researcher at the Horizons Regional Council at Palmerston North, New Zealand. “The current active case is different, because collapse could happen at any time.”

Small collapses can act as something of a pressure-relief valve, releasing a fraction of the pressure that has built below the surface. These small pressure releases result in minor eruptions, like Semeru’s January hiccough that produced an ash plume no more than 2km high. With ash gliding generally no further than around 4.5 km, minor eruptions rarely pose much risk to nearby settlements.

What made Saturday’s eruption so much more dramatic was the scale of the dome collapse. Days of unrelenting rainfall and thunderstorms destabilised the sides of Semeru’s magma dome, leading to large-scale collapse and exposing the pressurised magma below.

The wet weather didn’t just trigger the dome collapse, it fundamentally shaped the nature of the unfolding disaster. Anyone who has absent-mindedly tried to rinse a hot, oily frypan under a cold tap will know the explosive potential of dousing scorching materials in water. In the case of Semeru, torrential rainfall met roiling magma with devastating consequences.

“The interaction between rainwater and magma probably strongly contributed to the explosive energy of eruption and thus the height of the eruption plume,” says Kosik.

Semeru’s dome collapse resulted in an eruption column that reached approximately 15km into the sky. Under normal conditions, the pyroclastic flows generated by the dome collapse would have been unlikely to reach human settlements.

However, in the sodden weather, Semeru’s ash plume took on water and quickly became unsustainably heavy, collapsing to form the violent flows of volcanic slurry known as “lahars” that devastated nearby villages. Large lahars such as this one are powerful enough to crush, bury or lift away just about anything that lies in their path.

At their origin point on the summit, lahar flows can reach temperatures up to 600°C. Despite the cooling effect of the heavy rain that triggered Semeru’s eruption, and the river flow with which it intermingled, Semeru’s lahar flows would have retained horrific heat as they hit nearby villages, notes Emeritus Professor Ray Cas of Monash University: “Clearly they would have been at least at water-boiling temperature at the time they hit the valleys and the villages.”

Indonesia is home to over 130 currently active volcanoes, and almost all of them are situated in densely populated areas. Living alongside a volcano clearly carries a significant burden of risk – but is this rick increasing? The days of ceaseless rainfall in the build-up to this natural disaster were clearly key to way it unfolded. As weather patterns around the world look set to become more extreme under climate change, should we expect to see more eruptions of this nature?

While researchers are beginning to understand some of the complex feedback loops between climate change and volcanic eruptions, the effect of altered rainfall regimes remains unclear.