The dynamics of volcanic conduits: views from the inside and outside
Kathy Cashman (Univ of Bristol) and Diana Roman (Carnegie Institution for Science).
A fundamental goal of volcanology is to use geophysical signals to infer conditions of magma ascent, eruption initiation, transitions in eruptive style and to identify when an eruption has ended. The dynamics of magma ascent and eruption are, however, determined in large part by the physical properties of the magma, which can be measured only once an eruption begins, and then only the state of the magma at the Earth’s surface. Case studies of high-frequency volcanic microearthquakes (volcano-tectonic earthquakes, or ‘VTs’) the precede, accompany and/or follow volcanic eruptions indicate an apparent link between magma bulk viscosity and an approximately 90° horizontal rotation of accompanying VT earthquake fault-plane solutions (FPS), which may reflect stresses produced in the walls of an inflating or pressurizing dike (Roman and Cashman, 2006). Recent investigations of eruptions with and without 90° FPS rotations provide a tight constraint on the minimum bulk viscosity associated with stress field reorientation. Additionally, detailed reanalysis of seismicity and magma properties from four representative eruptions indicates (1) that the time-depth history of FPS rotations provides insight into the changing dynamics of magma ascent and eruption and (2) the type of faulting (normal, reverse, strike-slip) may help to identify conditions of magma ascent (rapid, slow or stalling). In summary, we suggest that combining analysis of faulting outside magma conduits with insight from analysis of changing magma properties during ascent may provide a means to qualitatively assess, in near real-time, conditions of magma ascent and, by extension, the likelihood of different types of eruptive behaviour.