Gravity fields and the interpretation of volcanic structures: Geological discrimination and temporal evolution

Abstract

Abstract A review of gravity data reflecting shallow concealed structures on volcanoes demonstrates that valuable information may be derived on the geological development of such structures and on the dynamic evolution of active volcanoes. To a first approximation, all recorded examples of anomalous gravity fields associated with volcanoes are concentric with and have sources that lie within the volcanic edifice. Positive anomalies with wavelengths less than 20 km and amplitudes up to ca. 30 mGal, characterise mainly basaltic volcanoes from various tectonic settings, and are caused by a relatively dense intrusive complex/magma body which contrasts with its surroundings either because the body is more mafic than average or, more likely, because near surface, previously erupted materials are uncompacted. Negative anomalies with wavelengths up to 100 km and amplitudes up to ca. 60 mGal, occur over much larger volcanic calderas, many of which have erupted highly silicic pyroclastic ash and pumice; uncompacted silicic caldera infill, with a possible contribution from low-density magma bodies, is responsible for the observed anomalies. There is some evidence for a continuum of gravity anomaly types that corresponds to the geological evolution of volcanic systems generally from primitive rift or subduction-related basaltic andesite to mature, high-level silicic calderas. Repeated microgravity observations over active volcanoes enable magma movements, variations in magma input and changes in magma density to be monitored closely. Dynamic modelling of volcanic systems is providing new evidence on the behaviour of concealed magma bodies and could have considerable potential for predicting eruptive events.