DOMES VS. FLOWS: DACITE LIQUID VISCOSITY AND MAGMA RHEOLOGY

Dacitic magmas commonly form steep-sided lava domes, often in the craters of stratovolcanoes, such as St. Helens (USA), Santa Maria (Guatemala) and Unzen (Japan). The current St. Helens lava dome has grown rapidly in the last year to ~425 m high. The lava dome of Santiaguito, in the crater left by the 1902 explosion of Santa Maria, has formed a complex of four overlapping domes since 1922, with one currently active vent (Caliente) that rises ~500 m above the crater floor. Santiaguito is notable for the extrusion of several long blocky lava flows, which extend up to ~4 km from the source vent. Studies of the Santiaguito domes and flows may provide useful information regarding the likely future evolution of the dome at St. Helens, especially because the chemistry and crystal fraction of the erupted dacites is very similar.

Here we present preliminary data on the rheology of dacitic liquids and magmas. Viscosities of dacitic liquids, and crystal- or bubble-bearing dacitic magmas, can be measured at the low temperatures pertinent to dome growth by parallel-plate viscometry, in the range 10⁸ to 10¹² Pa.s. Measurements at these low temperatures have the advantages that (i) samples containing dissolved water and other volatile species can be analyzed at ambient pressure, and (ii) stresses and strain rates are low, and similar to those in block lava flows (~10⁵ Nm^-2 and 10^-7 to 10^-3 s^-1). High-temperature measurements using concentric cylinder viscometry in the range 1 to 10⁴ Pa.s, provide important additional constraints on the viscosity of non-Newtonian magmas at higher strain rates, closer to those in volcanic conduits.

Domes at both Santiagito and St. Helens, and flows at Santiaguito, are of dacitic bulk composition, comprising a glassy rhyolitic matrix with plagioclase and amphibole phenocrysts. Viscosities of anhydrous rhyolitic and dacitic liquids are similar, but crystallization and cooling of a hydrous dacitic liquid will result in significant viscosity increase due to increased crystal fraction, decreased temperature, and volatile loss. Santiaguito magmas may have retained small but important quantities of volatiles during ascent, allowing slow magma flow under normal conditions, punctuated by flow-front collapse events that can produce hazardous block and ash flows during core depressurization.