2. Magmatic processes: A stoping model
To understand the current ongoing volcanism, we proposed a scenario based on a stoping model as follows (Fig. 2):
1) Continuous loss of magma from the chamber as a lateral intrusion has occurred without any fissure eruptions because a magma in the chamber is denser than the surrounding country rocks possibly be Tertiary volcanics. As the magma escaped from the chamber, pressure in the chamber decreases, then, vesiculation and/or expansion of bubbles in the magma might have occurred.
2) Decrease in density of the chamber induced a gravitational instability of the roof rocks of the chamber, and solid rock mass started to fall into the chamber, which resulted in the piston cylinder-type collapse crater formation at the summit. Vesiculated magma which is lighter than the chamber roof ascended simultaneously.
3) Ascended magma caused phreato-magmatic explosions by the interaction with groundwater in shallow aquifers.
4) After the major eruption on 18 August, a magma conduit might have established and the significant amount of SO2 started to be emitted from the summit without scrubbing of the gas by groundwater. (events)