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 viscosi ty of cryptodome dacite measured by parallel-plate viscometry ranges f rom 10.82 to 9.94 log(10 eta) (Pa s) (T = 900-982 degrees C), and shri nkage effects were dilatometrically observed at T>900 degrees C. The v iscosity of remelted dacite samples measured by the micropenetration m ethod is 10.60-9.25 log(10 eta) (Pa s) (T = 736-802 degrees C) and vis cosities measured by rotational viscometry are 3.22-1.66 log(10 eta) ( Pa s) (T = 1298-1594 degrees C). Comparison of the measured viscosity of cryptodome dacitic samples with the calculated viscosity of corresp onding 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 cry stal-bearing vesicular melt viscosities at low temperature. Assuming t hat the cryptodome magma of the 18 May 1980 Mount St, Helens eruption was residing at 900 degrees C with a phenocryst content of 30 vol.%, a vesicularity of 36 vol.% and a bulk water content of 0.6 wt,%, we est imate the magma viscosity to be 10(10.8) Pa s.