Lava flow morphology: The roles of external confinement and lava-ice interaction

Landforms associated with lava and ice interaction are common at glaciated stratovolcanoes. These landforms reflect the steep slopes and thin valley glaciers typical at stratovolcanoes, and differ from well-documented landforms associated with shallow slopes and/or thick ice. At stratovolcanoes, meltwater may travel freely downslope, eroding ice and rapidly enlarging pre-existing glacial drainages. As a result, lava flow eruptions have produced few catastrophic floods that occur when large volumes of water are released suddenly. Lava flowing into the open ice channels and voids becomes confined and grows upward, filling the available space and producing steep-sided ridges arid bulbous, smooth-sided domes. Many of the ridges produced in this way had previously been interpreted as erosional remnants of valley-fill lavas. Smaller scale lava features (fractures, glass, and hyaloclastites) formed during this process are similar to those produced during subaqueous eruptions (including those involving entrapped meltwater). Identification of these features in the field can be used to reconstruct local environments and climatic conditions during past eruptions.; Laboratory simulations of confined lava flows show a systematic relationship between flow thickening and a ratio of flow cooling to flow advance rate. Low viscosity flows thicken through flow inflation. More viscous flows thicken primarily through a process of breakouts on the flow surface at points upslope from the flow front. The volume and duration of these breakouts (ephemeral vents) is directly related to the surface morphology that develops on the flow. The rate of flow thickening increases as the confining channel width is decreased (while other experimental parameters are held constant). Although flow thickening is more pronounced in confined flows, it occurs at similar scale and by similar processes in unconfined flows. Scaling of experimentally determined relationships allows thicknesses to be estimated for natural lava flows of known length and surface morphology. In addition, effusion rates and durations can be estimated for flows with known thicknesses. These observations are applicable to the emplacement of confined lava flows (within river canyons or meltwater channels through ice) and large volume unconfined lava flows (such as found in Columbia River Basalt and on Mars and Venus). In addition, experiments demonstrate that low confining walls can control flow emplacement. Confinement by subtle topography may play a major role in the production of long and narrow lava flows found on Mars, Venus and the Earth.