Linking magmatic processes to geophysical signals at Rabaul (Papua New Guinea)

Caldera-forming eruptions are the most voluminous (up to 5000 km3) and violent eruptions on earth. Caldera unrest in many occasions consists of long periods (years to decades) of deformation and earthquake activity, however not all periods of unrest culminate in a volcanic eruption and not all eruptions are related to a caldera collapse. This complex behavior has created dilemmas that volcano scientists are confronted to during the reactivation of caldera volcanoes (e.g. Campi Flegrei - Italy, Yellowstone - USA, or Long Valley – USA). The unrest at the Rabaul caldera is a perfect example of the complexity and problems associated with not understanding the processes behind unrest. Above background seismic and deformation signals started in 1971 and intensified between 1983 and 1985, but eruption only occurred in 1994. This provides a unique opportunity to unravel the processes at the origin of such a strong and protracted deformation and seismic activity, and their link with the eruption that occurred ten years later.

This project uses a number of petrologic (SEM, EPMA, SIMS), geochemical modeling (MELTS), and numerical modeling (lattice-Boltzmann, finite differences) techniques. Some of the results are the following: Three types of magmas were involved in the recent period of activity (1994-present): basalt, high-K dacite and low-K dacite. Basalts were injected in the high-K dacitic reservoir a couple of decades to a couple of days prior to eruption, suggesting that they were the main drivers of unrest. Basalts were injected from the E and migrated toward the W prior to the 1994 eruption. The direct relationship between magma injection and unrest is still being investigated (overpressure due to mass addition, volatile exsolution or both?). Magmas are stored in at least two reservoirs: a main dacitic reservoir at ~7km depth beneath the bay and a basaltic reservoir at similar depths to the E-NE. The high-K dacite (the major constituent of the last caldera-forming eruption 1400 y BP), was likely produced by fractional crystallization of the replenishing basalts at shallow pressures and under relatively dry conditions.