Chapter 2 Cyclic phenomena at the Soufrière Hills Volcano, Montserrat
Cycles of eruptive activity are generally interpreted as evidence of one or more mechanisms operating in equilibrium. Modulation of cycle characteristics thus reflects changes in the conditions affecting those mechanisms. This kind of semi-deterministic behaviour at the Soufrière Hills Volcano has occurred on multiple timescales and with a range of eruptive intensity. By documenting cyclic phenomena, it is possible to investigate the mechanisms that modulate the state of the eruption and examine conceptual models. Pattern recognition and model development allows some degree of short-term forecasting ability for volcanic activity. We report the cyclic eruptive phenomena that have occurred on Montserrat on scales of hours to centuries. We identify four dominant types of cyclicity: sub-daily variations in lava flux; sub-annual cycles in eruption intensity; multi-annual ‘on–off’ switching of lava extrusion; and multi-decadal recurrence of seismic crises. We exploit a wealth of multi-parameter datasets (including seismic, geodetic, thermal, archive and visual observations), and present the evidence and observations for each type of cyclicity, some of which are documented here for the first time. Wavelet time-series analysis is used to constrain cycle characteristics, where appropriate. We discuss the implications of these observations in understanding the eruptive mechanisms of the Soufrière Hills Volcano.