Integrated geophysical studies at Masaya volcano, Nicaragua.
Research into the mechanisms responsible for the lasting, cyclic activity at Masaya
volcano can lead to a better understanding of persistently degassing volcanoes. This study is
greatly enhanced by the integration of dynamic micro-gravity, deformation and gas flux
measurements. The acquisition of extended temporal and spatial geophysical data will also
allow for the development of robust models for the dynamics of magmatic systems. Masaya
volcano, Nicaragua, is one of the most active systems in Central America, making it an
excellent natural laboratory for this study. It is noted for repeated episodes of lava lake
formation, strong degassing and subsequent quiescence.
Ground-based geophysical measurements show two episodes of similar magnitude
gravity decreases in 1993-1994 and 1997-1999, separated by a period of minor gravity
increase. A major increase in S02 gas flux from 1997-1999 correlates well with the most
recent episode of gravity decrease. The gravity changes are not accompanied by deformation
in the summit areas and are interpreted in terms of sub-surface density changes. The persistent
degassing at Masaya suggests that up to -15 krrr' of magma may have degassed over the last
150 years, only a minute fraction of which has been erupted. Furthermore, thermal flux
calculations suggest that 0.5 krrr' of magma (the estimated volume of the shallow reservoir)
would cool from liquidus to just above solidus temperatures in only 5 years. The high rates of
degassing and cooling at open-system volcanoes such as Masaya raise questions as to the
ultimate fate of this degassed and cooled magma. A number of models have been proposed to
explain this, but the most likely mechanism to explain persistent activity at Masaya and other
similar volcanoes is convective removal of cooled and degassed magma and subsequent
recharge by volatile-rich magma from depth.
Another fundamental question in modem volcanology concerns the manner in which a
volcanic eruption is triggered; the intrusion of fresh magma into a reservoir is thought to be a
key component. The amount by which previously ponded reservoir magma interacts with a
newly intruded magma will determine the nature and rate of eruption as well as the chemistry
of erupted lavas and shallow dykes. The physics of this interaction can be investigated through
a conventional monitoring procedure that incorporates the Mogi model relating ground
deformation (~) to changes in volume of a magma reservoir. Gravity changes (.1.g)combined
with ground deformation provides information on magma reservoir mass changes. Models
developed here predict how, during inflation, the observed .1.gI~ gradient will evolve as a
volcano develops from a state of dormancy through unrest into a state of explosive activity.