Gravity and deformation changes at two persistently active volcanoes: Insights into magmatic processes

Insights on some of the mechanisms responsible for persistent volcanism can be best achieved through the synergy of temporal geophysical and geochemical data sets. Gravity changes combined with ground deformation have been shown to provide important information on magma reservoir mass changes while measurements of gas flux have been influential in determining the rate of magma emplacement. The integration of long-term micro-gravity and ground deformation data with SO₂ flux and total sulphur budgets collected at Poás and Masaya volcanoes (since 1983 and 1993, respectively) now allows for the identification of significant cycles of activity. Recent eruptive activity at Poás volcano (Costa Rica) has been characterised by the disappearance and subsequent reappearance of the summit crater lake following intrusive episodes in 1980 and 1986-1989. Magma approached the surface on both occasions and was detected by the observation of concurrent increases in micro-gravity. These increases can be best modelled in terms of brittle fracturing of a shallow magma carapace allowing magma ascent through the conduit system to beneath the crater. This process allows for the vertical transfer of heat and gas and is driven by convection of buoyant, volatile-rich magma displacing colder, degassed magma. As magma pressure drops, the connection between the deeper magma reservoir and shallow conduit system is severed allowing the hydrothermal system to resume its role as a cooling mechanism. In contrast, recent activity at Masaya volcano (Nicaragua) has been characterised by repeated periods of significant passive degassing (>2000 t/d SO₂) with the eruption of only negligible amounts of juvenile material. The resulting cycle gravity and gas flux variations is clearly not driven by intrusion of additional magma into the shallow system. Rather, it may be due in part to blocking and gas accumulation caused by restrictions in the shallow volcano substructure. However, as with Poás, this cycle of activity is also controlled by the convective overturn of shallow degassed, cooled and dense magma that is replaced periodically by lower density, hot, gas-rich magma from depth. Buoyant gas-rich magma rises from depth and is emplaced near the surface, resulting in the formation and fluctuation of a low-density gas-rich layer centred beneath the pit crater complex. The presence of a well developed hydrothermal system such as at Poás, can act as a physical and chemical buffer to the volcanic system and therefore moderate any volcanic processes. Similarly, the chaotic but essentially "open" nature of Masaya's substructure can obscure short-term variations. However, in both situations, long-term multi-component monitoring has proven valuable in explaining the observed cyclic changes, specifically, that gas-driven convection is a fundamental process at these persistently active volcanoes.