Mount St. Helens and Santiaguito lava domes: The effect of short- term eruption rate on surface texture and degassing processes

In order to assess the effect of eruption rate on the surface morphology and degassing mechanisms of silicic lava flows, we studied surface characteristics and obtained water content and hydrogen isotopic values of samples from flows at the Mount St. Helens and Santiaguito lava domes. We compared the surface textures and inferred degassing processes to short-term extrusion rates and found that when domes are small and eruption rates are high, lava will not completely degas in transit to the surface, allowing additional volatile loss through surface vesiculation which results in the formation of a scoriaceous carapace. When domes exceed a critical size and/or their cooled crusts reach a critical strength, emergence of new magma is impeded, shortterm eruption rates decline, and more thorough degassing can take place leading to smooth-textured flows lacking scoria development. At Mount St. Helens, this transition occurred during the dome’s third year, when it grew from 31.8 to 53.2 X lo6 m3. Santiaguito attained a comparable state after 2-3 years of growth, and for most of its 70-year history has produced nonvesicular lava. Degassing patterns that combine closed, open, and kinetic processes can be distinguished using isotope data obtained from samples whose positions on a flow and emplacement histories are well-constrained. Evidence for these patterns is most clearly preserved in lavas erupted during early, rapid stages of dome growth. Petrologists and volcanologists seeking to infer magma chamber conditions from the volatile contents of extruded lavas thus need to sample flows early in their emplacement while paying attention to surface texture, position relative to the vent and flow front, and time of emergence.