We used 12 high-temperature deformation experiments on fabricated cores of rhyolite ash to explore the effect of porosity on the rheology of pyroclastic deposits. During deformation, the cores of ash accommodate strain mainly by shortening and reduction of porosity. Porosity loss creates a strain-dependent rheology expressed by a marked and continuous increase in effective viscosity. The effective viscosity () of pyroclastic materials is predicted as a function of porosity () and melt viscosity (o) by: .
The optimal value for the parameter α, based on the experimental data, is 0.78 ± 0.15. These experiments provide constraints on the time scales of compaction and flow processes in volcanic systems under variable conditions (i.e., temperature, load stress). Our results indicate that welding and compaction processes in pyroclastic deposits (e.g., ignimbrites, conduits) can occur on time scales of tens of minutes to hours. At these time scales, welding is fully decoupled from cooling of the deposits and may be coupled to depositional processes. Within volcanic conduits, welding processes operate on fragmented magma rapidly enough to eliminate porosity in a matter of hours, thereby sealing off permeability and supporting cyclical Vulcanian-style explosive eruptions.