EXPERIMENTAL WELDING OF PYROCLASTIC DEPOSITS: RHEOLOGY AND STRAIN

We describe results from 16 high-temperature, constant strain rate and constant load deformation experiments on natural pyroclastic materials that simulate welding. Fifteen experiments were run unconfined at temperatures between 800 and 900C and one was run under ~200 bars H₂O confining pressure. Starting materials comprised 4.3 cm diameter, ~6 cm length cores of sintered Rattlesnake Tuff rhyolite ash with starting porosities of ~70%. Experimental displacement was controlled to achieve total strain values between 10 and 90%.

In thin section, the deformed experimental end products exhibit striking similarities to all facies of natural welded pyroclastic rocks including variably flattened pumice fiamme and systematically deformed bubble wall shards. To quantify the amount of strain accumulation, we placed three manually rounded ~1 cm diameter pumice lapilli at different heights in each experimental product. Axial ratios (x-axis dimension/y-axis dimension) of the deformed lapilli (fiamme) show a systematic increase with increased deformation. To further quantify strain, we measured flattening ratios of originally spherical bubble wall shards. These analyses are compared to similar measurements on natural samples to evaluate current methods of quantifying deformation in welded pyroclastic facies.

Stress-strain and strain-time experimental results indicate that the glassy, porous aggregates have a strain-dependent rheology; the effective viscosity of the mixture increases non-linearly with decreasing porosity. Temperature, rather than stress is the dominant factor controlling the rheology of these materials. However, preliminary results indicate that the presence of moderate H₂O pressure allows for viscous deformation (e.g., welding) to occur at significantly lower temperatures than in anhydrous conditions. Quantifying the effects of H₂O on the rheology of glassy materials has significant implications for our understanding of the welding process in pyroclastic deposits. Our analysis explores the effects of H₂O on the temperature-timescale relationships during welding of pyroclastic deposits.