Rheology and timescales of welding

We describe results from 15 high-temperature, constant strain rate and constant load deformation experiments on natural pyroclastic materials that simulate welding. Experiments were run on unconfined samples at temperatures between 835° and 900° C. Samples comprised 4.3 cm diameter, ∼6 cm length cores of sintered Rattlesnake Tuff rhyolite ash. Porosity of starting materials is ∼78%. The experiments used uniaxial load stresses of 0.2 to 5 MPa which corresponds to overburden depths of < 200 m in ignimbrite deposits. The experimental results track strain (porosity loss) and strain rate as a function of time at fixed conditions (load and temperature). Our results show that deformation of pyroclastic material has a strain dependent rheology. The effective viscosity (η _e) of the samples increases during the experiment as strain acccumulates and porosity (φ ) is reduced. We describe this behaviour using the relationship: (1) log η _e = log η _o - α [φ /(1-φ )]. where effective viscosity is related to the viscosity of the framework material (melt), the sample porosity, and a fit-parameter for the material (α ). Our experimental work suggests a value of 0.63 for compaction of natural pyroclastic materials. Equation 1 is the basis for an empirical equation that describes the total strain during viscous compaction as a function of original porosity (φ _o), the viscosity of framework melt (η _o),load (σ ) and time: (2) \epsilon = \phi_{o} + (1-\phi_{o})/\alpha \times ln [(\alpha \sigma \Deltat)/(\eta_{o} (1-\phi_{o} ) + exp[-(\alpha \phi_{o})/(1 - \phi_{o} ) ] ]. In this relationship, the values of φ _o and η _o are physical properties of the specific deposit and load relates to the thickness of the deposit and the position (depth) of the sample. Eq. 2 can be used to predict ɛ vs. time paths to compare against the original experimental data and to model natural deposits. By rearranging the above equation to isolate time (Δ t) we predict the times required for strain accumulation (reduced φ ) during welding of natural pyroclastic deposits. We show that the timescales of welding for even moderate emplacement temperatures, relative to glass transition temperatures, can be very short (i.e., days) and within an order of magnitude of the timescales of deposition or assembly of large ignimbrite sheets.