Sw. Anderson et al., MOUNT-ST-HELENS AND SANTIAGUITO LAVA DOMES - THE EFFECT OF SHORT-TERMERUPTION RATE ON SURFACE TEXTURE AND DEGASSING PROCESSES, Journal of volcanology and geothermal research, 69(1-2), 1995, pp. 105-116
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.