Integrating experimental petrology with InSAR deformation constraints for the magmatic system of Kelud volcano, Indonesia

Abstract:

The parameters that govern the eruptive style at volcanoes are critical to understand, since the volcanic hazards posed to the nearby populations are directly related on whether an eruption is explosive or effusive. Eruptions from Kelud volcano located in East Java, Indonesia are difficult to forecast in that sense, because the eruptive style varies considerably, from effusive eruptions e.g. 1920 & 2007 to explosive eruptions in 1990 and 2014, despite near identical bulk rock compositions. Experiments were undertaken to constrain the magma storage conditions such as pressure, temperature and H2O-CO2 ratios prior to both explosive and effusive eruptions at Kelud and compared to monitoring data before the eruptions. Experimental matrix glass and mineral rim compositions, as well as crystal contents were measured and compared to the natural mineral and groundmass characteristics erupted in explosive and effusive eruptions. The experimental constraints suggest the pre-eruptive Kelud magmas are stored under low pressures (<75 MPa) equating to <4 km depth. However, experiments showed that the magma stored before effusive eruptions had slightly hotter temperatures (1050°C compared to 1000°C for 2014 eruption) and also lower fractions of water in the fluid phase (XH2O=0.55) compared to the magmas that generated the explosive eruptions, which were near water saturation. In 2007, precursors (deep seismic, CO2 degassing) and petrology (crystal zoning) point to a magma injection, that lowered viscosity and supressed water contents, leading to effusive eruption (lots of warning). However, before 2014 explosive eruption, there’s no evidence of magma injection trigger. InSAR data from 30 CosmoSkyMed images spanning 2^nd December 2013 – 19^th May 2014 from ASI were used to assess the degree of deformation during the 2014 explosive eruption. Surprisingly, no pre-eruptive deformation was detected, suggesting that either deformation occurred outside the period of observation, or the magma already existed and was highly compressible. This a cautionary tale when interpreting deformation signature alone, as this study shows that explosive eruptions can occur without recent magma injections or inflation akin to the eruption of Calbuco volcano in 2015.