Gas mobility in rheologically-layered volcanic conduits: The role of decompression rate and crystal content on the ascent dynamics of magmas

Unravelling the rheological behaviour of magmas is fundamental for hazard assessment. At shallow depth the combined effects of degassing, vesiculation and crystallization are likely to produce dramatic changes in the rheology, hence modulating flow dynamics and eruptive style. The rheological evolution from a low viscosity crystal-poor, bubble-free, water-rich melt to a highly viscous crystal-rich, vesicular magma containing a water-poor melt often occurs in the conduit. To clarify the viscous flow dynamics of rheologically-layered volcanic conduits, we performed decompression experiments using a magma analogue system characterized by a low-viscous Layer L (10 Pa s) at the bottom and a high-viscous particle-bearing Layer H (≥1000 Pa s) at the top. Silicone oils and spherical glass beads are employed as magma and crystal analogues, respectively. Three sets of experiments address the effects of: 1) decompression rate (ca. 10−2 and 104 MPa/s); 2) crystal content in the high viscosity magma (0, 10, 30 and 70 vol.%); and 3) volume ratio of the two rheological layers (0.6 or 0.3). Our results indicate that decompression rate exerts the most dramatic role, yielding changes in time-scale of outgassing up to two orders of magnitude, and affecting the style of decompression response (permeable outgassing or fragmentation). The solid fraction 1) strongly modulates gas mobility, 2) influences the pervasiveness of fragmentation and 3) affects the extent of mingling in the experimental conduit. These results demonstrate that the properties of a shallow, partially-crystallized portion of the magmatic column and its response to varying ascent rate are primary controls on eruptive style.