Rheology of porous volcanic materials: High-temperature experimentation under controlled water pressure

Abstract

We present a suite of 21 high-temperature, uniaxial deformation experiments performed on 25 by 50 mm unjacketed cores of porous ( Φ ∼ 0.8) sintered rhyolitic ash. The experiments were performed at, both, atmospheric (dry) and elevated water pressure conditions (wet). Experiments used a constant displacement rate of 2.5 × 10 − 6 m s − 1 corresponding to a strain rate ( ɛ o) of ∼ 5 × 10 − 5 s − 1 ; a single experiment was run at 2.5 × 10 − 5 m s − 1 (ɛ o ∼ 5 × 10 − 4 s − 1 ). Dry experiments were conducted mainly at 900 °C, but also included a suite of lower temperature experiments at 850, 800 and 750 °C. Wet experiments were performed at ∼ 650 °C under water pressures of 1, 2.5, and 5 MPa, and at a fixed P H 2O of ∼ 2.5 MPa for temperatures of ∼ 385, 450, and 550 °C. During deformation, strain is manifest by shortening of the cores, reduction of porosity, flattening of ash particles, and radial increase of the cores. The continuous reduction of porosity leads to a dynamic transient strain-dependent rheology and requires strain to be partitioned between a volume (porosity loss) and a shear (radial increase) component. These data demonstrate the effect of porosity on the rheology of dry and hydrous melts. The effect of increasing porosity is to expand the window for ductile deformation for dry melts by delaying the onset of brittle deformation by ∼ 50 °C (875 °C to 825 °C). The effect is more pronounced in hydrous melts (∼ 0.67–0.78 wt.% H 2O) where the ductile to brittle transition is depressed by ∼ 140 to 150 °C. Increasing water pressure also delays the onset of strain hardening due to compaction-driven porosity reduction. These rheological data are pertinent to any volcanic processes involving high-temperature porous magmas (e.g., magma flow in conduits, welding of pyroclastic materials).