Viscosity and Flow Dynamics of the Rhyolitic Lava of Mount Arteni, Armenia: Insights from Numerical Modeling of Emplacement and Effusion Rates

The Arteni Volcanic Complex lies within the Ararat Plain, within the Aragats Volcanic Province (AVP) of western Armenia, and is noted for its remarkable diversity of obsidian types. K-Ar dates show Early Pleistocene ages of 1.26 to 1.60 Ma, while fission track dating yields slightly younger ages of 1.17 to 1.38 Ma. The Arteni Volcanic Complex was formed through successive eruptions from two adjacent centers: Mets Arteni and Pokr Arteni, both dome-shaped rhyolitic volcanoes. The complex consists of rhyolitic domes, dome-coulees, necks, fissures, extrusions, and dikes—typical of high-viscosity silicic volcanism. The Arteni rhyolitic lava flow extends a remarkable 8 km from the summit area of Mets Arteni. Covering an area of ~15 km², its volume is estimated at 0.6 km³. The unit consists of a basal layer of pyroclastics and crystalline rhyolitic-dacitic rocks, followed by crystal-clear obsidian, and capped by vesicular water-rich perlite. The obsidian typically contains between 0.06 and 0.2 wt% H₂O, while the overlying perlite contains up to 4 wt% H₂O. Although rhyolitic melts are usually highly viscous, the Arteni flow’s excessive length suggests lower viscosity due to temperature, composition and water content during emplacement. Consequently, we modeled viscosity using 850-970 °C obtained from similar rheological calculations for the Dry Fountain obsidian complex located 22 km north of Yerevan, Armenia. We note that the upper perlite layers of the Aragats flow contain 3.5–4 wt% H₂O. If the underlying obsidian formed by rapid degassing under lithostatic pressure applied by the overlying perlite, the original melt likely had a water content of ~3.8 wt%. For this water content and temperature, the viscosity of Arteni rhyolite was 3.86–4.79 log η (Pa·s), much lower than anhydrous obsidian (7.45–9.18 log η Pa·s). We suggest that this low viscosity led to the exceptional length of the rhyolite flow due to depolymerization of melt structure as well as lack of phenocrysts.