Sinabung & dangerous lava domes

Sinabung in Sumatra has been erupting for the last few months, prompting regular evacuations of people living near to it.

(Incandescent lava dome, flow deposit and burning vegetation - Reuters/YT Haryono)

Currently, the style of activity is producing a lava dome: where viscous (less fluid) magma builds up on top of the vent. The lava, although seemingly solid, can have a lot of gas trapped inside at high pressures, and is also still very hot. Sometimes chunks of it fall off the dome, these blocks or slabs break apart, releasing the gas, mixing with and heating the surrounding air, forming pyroclastic flows and surges.

Depending on where the lava is being emplaced on the volcano, material may fall off in different directions, thus the areas most at risk can change quite quickly. Dome building eruptions often produce a LOT of flows - the size of which can often follow a power law relationship: many small events, with a diminishing number of larger events*. Lava domes make both managing and communicating the risk very difficult. Often people are evacuated in the anticipation of larger flows, which may not happen for a while (if at all). After a time of small flows, many people naturally want to return to their homes. Then they may be evacuated again, and subsequently return. This process can occur many times, and ultimately people can become very reluctant to leave.

Unfortunately, this kind of relationship is very different to grasp.  In Montserrat, 1997, this process (among other factors) occured - the danger perhaps obvious to the scientists, but people became used to where flows were going and how big they were. Many thought that they understood the speed of them, and unfortunately thought they could escape in time. On June 25th 1997, people on the slopes of the Soufrière Hills, in areas that they were advised not to visit, were caught off guard (despite numerous warnings from scientists) by a sudden increase in the magnitude and energy of the flows during a partial dome collapse, which lead to a tragic loss of life*. 

This has unfortunately been the case in many similar eruptions, from Soufrière Hills Volcano to Merapi. An added danger is that apart from larger flows related to small collapses...lava domes can also produce large vulcanian explosions - which create even more energetic flows, that can sweep down all sides of the volcano at once.

The key thing is to not expect a volcano to always behave in the same way, but rather to think "what could it do to surprise me?".

Despite the fact that the Indonesian scientists are very capable volcanologists and communicators, we just have to hope that the eruption calms down again.




*Loughlin, S. & Baxter, P., 2002. Eyewitness accounts of the 25 June 1997 pyroclastic flows and surges at Soufrière Hills Volcano, Montserrat, and implications for disaster mitigation. Geological Society of London. 
*Loughlin, S., Calder, E. & Clarke, A., 2002. Pyroclastic flows and surges generated by the 25 June 1997 dome collapse, Sonfière Hills Volcano, Montserrat. Geological Society of London.