THERMO-RHEOLOGICAL FEEDBACKS IN SILICIC LAVAS AND IGNIMBRITES

The rheology of lava is highly dependent on temperature, and feeds back to temperature, because rapidly sheared melts can undergo viscous heating (heat production = viscosity [strain rate]²), and rapid disequilibrium crystallization can cause heating due to latent heat release (H_xt). The heat budget of partially crystalline lava offsets these gains with conductive losses. We present two case studies. We measured the apparent viscosity of crystalline dacitic lavas from Santiaguito, Guatemala. At conditions appropriate to lava flows (shear stress 0.1 to 0.4 MPa, strain rate 10^-8 to 10^-5 s^-1), apparent viscosity is best modeled as a power-law. Viscosity of the flow core, at 850 °C, is estimated ~5x10¹⁰ Pa.s. There is no evidence for significant crystallization during flow emplacement at Santiaguito, but viscous heating may be significant ongoing heat source within these flows 100Wm^-3 if most shearing is restricted to a 1m wide zone), enabling highly viscous lava to travel long distances (4 km in 2 yrs for Santiaguito).

The lava-like and rheomorphic Grey’s Landing ignimbrite, Idaho, provides abundant field evidence supporting the upward migration of a transient, 1-2 m thick, sub-horizontal ductile shear zone at the interface between the pyroclastic density current and deposit, through which all of the deposit passed. Using rheological experiments and thermo-mechanical modeling, we demonstrate that syn-depositional welding and ductile flow is achievable within a very restricted field of likely time-temperature-strain space where rapid high-strain deformation (1000%) is favored by higher emplacement temperatures (850 °C). The field of ductile deformation is broadened significantly by accounting for strain-heating, which permits a sustained temperature increase up to 250 °C within the shear zone, and helps to explain the enormous extents of lava-like lithofacies and intense rheomorphism recorded in extremely high-grade ignimbrites. We conclude that strain heating, an inevitable result of magma transport that feeds back to rheology and transport, should be taken into account in thermal modeling of volcanic processes at both high and low strain rates, and both pre- and post-eruption.