Dr Carmen Solana, Volcanologist, University of Portsmouth http://www.port.ac.uk/school-of-earth-and-environmental-sciences/staff/carmen-solana.html
On the Bali volcano activity back in November, 2017, Dr Solana said:
“A lot remains uncertain about what will happen. We are still far from being able to forecast how eruptions are going to develop. It could rapidly increase in activity and produce a vast eruption or it could die down. Volcanic tremors are being registered, indicating that fluids (gas and magma) are moving under the ground. So if it follows the most frequent trend, it is likely to continue increasing in explosivity – but at what rate and how large, nobody knows.
“The people affected were feeling very frustrated after the long lived crisis, but the volcano is proving the Indonesian volcanologists and emergency managers right in their decision to evacuate. With volcanic eruptions, being on the cautious side is better than being overconfident.
“The volcanologists in the Indonesian Volcanic Survey have raised the alert to IV. I will expect that, if the explosive activity increases further, they will expand the exclusion zone accordingly.
“Indonesian volcanoes erupt frequently and generally in an explosive or very explosive fashion. In the past, eruptions have also produced Calderas, which are hollowed depressions several kilometres in size, left after vast explosive eruptions empty large reservoirs of magma and blast the rocks above.”