Caldera Deflation: Constraining Magma Chamber Physics and Predicting Hazards From Gravity and Elevation Changes

Caldera forming eruptions are environmentally and economically the most devastating volcanic events. Inflation is usually considered to be an important pre-cursor to activity. Here, we show that during caldera deflation a volcanic system may also enter a hazardous stage. We have evaluated published gravity-height (Δ g/Δ h) data on Krafla, Askja, Kilauea and Campi Flegrei in order to discriminate between subsurface processes during caldera subsidence. With respect to end-member gravity-height correlations such as the free air gradient (FAG) and the Bouguer corrected free air (BCFAG), Δ g/Δ h gradients may be interpreted in terms of subsurface mass redistribution or density changes. Δ g/Δ h gradients during subsidence will either plot along the FAG or the BCFAG or fall above, below or between these predicted lines. Within Δ g vs Δ h diagrams, we have discriminated between three regions, each of which is evaluated in terms of subsurface processes occurring during volcano subsidence. These processes include either mass and density decrease, mass and density increase or mass decrease and density increase within the magma chamber. We have interpreted Δ g/Δ h gradients as possible indicators of precursors of volcanic activity and propose that gravity - height surveys may help to detect precursors of caldera collapse due to magma drainage. The case of the Askja 1875 eruption in Iceland has been evaluated in terms of the beginning of the eruptive episode being induced by roof collapse of an evacuating magma chamber. Based on other examples of recent volcanic roof collapse, we evaluate the contribution of gravity-height surveys in assessing volcanic risks during caldera subsidence including the trigger mechanism for caldera-forming explosive eruptions.