SOME PHYSICAL CONSTRAINTS ON THE ERUPTION AND EMPLACEMENT OF LAVA-LIKE TUFFS

Rheomorphic lava-like tuffs are characteristic of the very largest volume silicic eruptions on Earth, for example, in the Etendeka-Parana and Snake River Plain silicic large igneous provinces (SLIPs). In contrast they are uncommon in other magmatic environments and the eruption and emplacement of one has never been observed. Therefore our understanding of lava-like tuffs comes from studies of deposits and geochemical and rheological analysis of their samples. A recent upsurge in studies of SLIPs (e.g., Etendeka-Parana, SRP, Gawler Range, Sierra Madre Occidental, etc.) and their constituent parts, principally rheomorphic lava-like tuffs like the Grey's Landing ignimbrite, coincides with the first rheological and geospeedometric studies of such deposits, providing important new insights and quantitative constraints on the timescales, rates, and thermal states of the erupting rhyolitic magmas, pyroclastic density currents, and ignimbrites deposited. Most importantly welding, (most) rheomorphism, local quenching, and exsolution of volatiles within the Grey's Landing ignimbrite is demonstrated to be syn-depositional; and all these processes occur simultaneously and at timescales similar to eruption and pyroclastic density current transport. Therefore changes in eruption or transport properties maybe reflected in subtle differences of welding, deformation, and vesicle distribution styles. Experiments to understand the evolving rheology of pyroclasts from PDC transport to deposition, welding, and rheomorphism are on-going; initial results indicate that sufficiently low effective viscosities probably require a combination of dissolved water content (~1 wt. %) and shear heating within the syn-depositional rheomorphic shear zone to produce the strain fabrics and structures commonly observed.