Hydromagmatic volcanism is a common eruptive style in the Azores. It is characterised by a high fragmentation and the associated deposits are fine ashes and alternating ash and lapilli layers, reflecting the oscillation between hydromagmatic and magmatic activity.
The fine granulometry accounts for an important control of the transport and emplacement mechanism of hydromagmatic deposits by secondary factors such as atmospheric conditions. In the case study of Furnas-C eruption, rain played a determinant role in the important lateral facies variation of the deposit and on its dispersal. It promoted the production of a dispersal axis different from the wind direction and the thickening of the deposit in some areas due to localised premature ash fall.
These observations introduce one more variable on hazard and risk assessment