MOUNT-ST-HELENS AND SANTIAGUITO LAVA DOMES - THE EFFECT OF SHORT-TERMERUPTION RATE ON SURFACE TEXTURE AND DEGASSING PROCESSES

In order to assess the effect of eruption rate on the surface morpholo gy and degassing mechanisms of silicic lava flows, we studied surface characteristics and obtained water content and hydrogen isotopic value s of samples from hows at the Mount St. Helens and Santiaguito lava do mes. 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 vesicu lation which results in the formation of a scoriaceous carapace. When domes exceed a critical size and/or their cooled crusts reach a critic al strength, emergence of new magma is impeded, short-term eruption ra tes decline, and more thorough degassing can take place leading to smo oth-textured flows lacking scoria development. At Mount St. Helens, th is transition occurred during the dome's third year, when it grew from 31.8 to 53.2 x 10(6) m(3). Santiaguito attained a comparable state af ter 2-3 years of growth, and for most of its 70-year history has produ ced nonvesicular lava. Degassing patterns that combine closed, open, a nd kinetic processes can be distinguished using isotope data obtained from samples whose positions on a flow and emplacement histories are w ell-constrained. Evidence for these patterns is most clearly preserved in lavas erupted during early, rapid stages of dome growth. Petrologi sts and volcanologists seeking to infer magma chamber conditions from the volatile contents of extruded lavas thus need to sample flows earl y in their emplacement while paying attention to surface texture, posi tion relative to the vent and how front, and time of emergence.