High-Viscosity Silicic Lavas (Rhyolite, Dacite, and Andesite)

The eruption processes, flow behaviour, and characteristics of high-viscosity felsic and silicic lavas are now considered. Magma differentiation and country rock assimilation, usually in crustal magma reservoirs, generate felsic magma bodies with progressively lower temperature, increasing silica content and higher viscosity. These in turn affect the mobility, flow distance of any erupted lava flow which generally decreases as the SiO2 content and viscosity increase. Such lavas are characterised by their high aspect ratio, forming very thick lava domes, mesa lavas, or short lava flows (coulées), depending on their viscosity and the topography onto which they flow, or intrude in the case of cryptodomes. Subaerial domes and lavas are commonly texturally zoned from stony or crystallised interiors to obsidian, coherent pumice, a carapace of autobrecciated obsidian and pumice clasts outwards, and an apron of block and ash flow deposits and resedimented talus laterally. The most common settings for eruption of evolved, high-viscosity lavas are stratovolcanoes and calderas at convergent volcanic arcs, rift and hot spot or plume settings. Here they are often erupted subaerially from point-source vents or along caldera ring fractures. Such domes and lavas may generate explosive eruptions, producing a range of deposit types from rings of phreatic lithic breccias to pumice-tuff cones surrounding the lava and lateral aprons of block and ash flow and pumice and ash flow deposits. Felsic domes are also commonly erupted in subaqueous settings in lakes, including caldera lakes, subglacially, and in the oceans. Modern seafloor felsic domes and lavas are found at the summits of submarine arc volcanoes, including those with summit calderas, as well as on the seafloor of back-arc basins. They can be distinguished from subaerial counterparts by abundant hyaloclastite breccia and may have significant coherent pumice carapaces and pumice hyaloclastite breccias. We highlight high-resolution bathymetry studies of the rhyolite domes and short lavas erupted during the 2012 eruption of the wholly submarine Havre Volcano, in the Kermadec Arc. In deep subaqueous and subglacial settings, explosive exsolution of volatiles is suppressed by hydrostatic and glaciostatic pressures, leading to eruption of coherent domes and lavas, and their autoclastic (autobreccia, quench fragmented hyaloclastite) carapaces. For both subaerial and subaqueous domes, lavas, and cryptodomes, we summarise the facies characteristics and provide facies models.