Physical properties of the 1980 Mount St. Helens cryptodome magma

Physical properties of cryptodome and remelted samples of the Mount St. Helens grey dacite have been measured in the laboratory. The viscosity of cryptodome dacite measured by parallel-plate viscometry ranges from 10.82 to 9.94 log₁₀η (Pa s) (T= 900-982°C), and shrinkage effects were dilatometrically observed at r>900°C. The viscosity of remelted dacite samples measured by the micropenetration method is 10.60-9.25 log₁₀η (Pa s) (7=736-802 °C) and viscosities measured by rotational viscometry are 3.22-1.66 log₁₀η (Pas) (7=1298-1594°C). Comparison of the measured viscosity of cryptodome dacitic samples with the calculated viscosity of corresponding water-bearing melt demonstrates significant deviations between measured and calculated values. This difference reflects a combination of the effect of crystals and vesicles on the viscosity of dacite as well as the insufficient experimental basis for the calculation of crystal-bearing vesicular melt viscosities at low temperature. Assuming that the cryptodome magma of the 18 May 1980 Mount St. Helens eruption was residing at 900 °C with a phenocryst content of 30 vol.%, a vesicularity of 36 vol.% and a bulk water content of 0.6 wt.%, we estimate the magma viscosity to be 10^10.8 Pa s.