Although the fragmentation process of the cryptodome magma during the catastrophic lateral blast of the Mount St. Helens on May 18, 1980 is likely to have been strongly influenced by the physical and rheological properties of the cryptodome magma, such properties have been poorly investigated. Such properties are also important for the interpretation of experimental results on fragmentation of samples of cryptodome magma (Annual Report 95).
Light-grey, vesicular, hypersthene-hornblende dacite was the most abundant
lithology (72%) erupted during the lateral blast of May 18, 1980. In our
fragmentation experiments, dacite samples corresponding to typical grey
dacite of 1980 Mount St. Helens cryptodome were employed. The grey dacite
contains 30 vol% phenocrysts (on a vesicle-free
basis) of 210 µm average size, comprising plagioclase, hypersthene
and hornblende and Fe-Ti oxides, which are easily visible in SEM images
of samples. The proportion of microlites and microphenocrysts in the microcrystalline
groundmass is 45 vol% calculated on
a vesicle-free basis from a published (back scattered electron) image of
grey blast dacite. Microlites and microphenocrysts occupy some of the inter-bubble
partitions, are dominantly plagioclase, and have widths ranging from 1-10
µm. Most vesicles range in size from several microns width (average
5 µm) to several tens of microns. The smaller vesicles are usually
elongated with aspect ratios (ratio of longest to shortest apparent axis)
less than 8. Many vesicles are interconnected and the characteristic thickness
of interbubble partitions is 1 - 4µm.
Physical properties of raw and remelted samples of the Mount St. Helens
grey dacite, corresponding to the 1980 cryptodome magma, have been measured
in the laboratory. The densities of the vesicular cryptodome dacite and
the glass obtained from remelted cryptodome dacite are 1.59 g/cm3
and 2.48 g/cm3 correspondingly. The room temperature compressional
and shear wave velocities were determined by the phase comparison method
at 20 MHz using the 1 and 2 transducer technique. P-wave velocity was determined as 6021±
9 m s-1 and s-wave velocity
as 3612± 4 m s-1. A pulse
transmission technique was employed for velocity measurements of vesicular
cryptodome dacite with 1 MHz p-wave transducers. At low confining
pressures (1 MPa) p-wave velocities depend on the sample orientation
and vary over the range 2900-3100 m/s. This variation is likely a function
of sample fracture anisotropy. In contrast, at high confining pressures
(350 MPa) the measured p-wave velocity is 4000 m/s. At these confining
pressures, there is no directional dependence (with respect to sample orientation)
of p-wave velocity and the sample appears isotropic.
The linear expansion coefficient of a raw porous grey dacite sample
was measured using a push-rod dilatometer (Model TMA 402®
, Netzsch Gerätebau, Selb, Germany). Over the temperature range 20-800°C,
the linear expansion coefficient was estimated to be 4.85x10-6
C-1. The glass transition has been determined as the temperature
of the peak in the linear expansion curve as a function of temperature.
At a heating rate of 10°C/min, the dilatometric peak temperature of
the porous grey dacite was determined to be 800 - 810°C. Bulk chemical
composition of natural samples of Mount St. Helens grey dacite used in
experiments was determined by solution ICP-AES methods. The chemical
composition of the matrix glass of raw samples and composition of samples
of remelted glass was determined by microprobe (SX-50, CAMECA) at 15 kV
and 15 nA. The bulk water content of samples used in experiments was determined
to be 0.61-0.62 wt% by the Karl-Fischer titration method.
The viscosity of vesicular dacite samples measured by parallel plate
viscometry ranges from log10 = 10.82
to 9.94 (Pa s) (T=900-982°C), and shrinkage effects were
dilatometrically observed at T>900°C. The viscosity of remelted
samples measured by the micropenetration method is log10
= 10.60-9.25 (Pa s) (T= 736-802°C) and viscosities
measured by rotational viscometer are log10
= 3.22-1.66 (Pa s) (T= 1298-1594°C). Comparison
of the measured viscosities of raw and remelted dacitic samples with calculated
viscosities of corresponding dry and water-bearing melts demonstrates significant
deviations between measured and calculated values for T=900°C.
This difference reflects a combination of the effects of crystals and vesicles
on the viscosity of dacite as well as the insufficient experimental basis
for the calculation of melt viscosities at low temperature. At the
temperature of 900°C, assumed to be that of the cryptodome magma of
the May 18, 1980, Mount St. Helens eruption, a phenocryst content of 30
vol% and a vesicularity of 36 vol% we estimate the viscosity to be 1010.8
Pa s. The rheology and the elasticity of the 1980 cryptodome constrain
the location of the brittle-ductile transition of such material during
the rapid decompression expected to have occurred on May 18, 1980.