Nature and efficiency of pyroclast generation from porous magma: Insights from field investigations and laboratory experiments

Enhanced knowledge of pre- and syn-eruptive processes is vital to
deal with the increasing threat imposed to population and
infrastructure by volcanoes that have been active historically and
may potentially erupt in future. For many years, most of this
knowledge was received from experiments on analogue materials
and/or numerical models. In order to increase their significance
and applicability for the “real” case, the natural complexity may
not be oversimplified and the input parameters must be reliable and
realistic. In the light of this, a close connection of field and
laboratory work is essential. Volcanic eruptions may be phreatic,
phreatomagmatic or magmatic, the latter scenario of which was
addressed in this study. Rising magma is subject to decreasing
lithostatic pressure. As a direct consequence, volatiles become
increasingly oversaturated and bubbles will nucleate and grow
depending on initial volatile content and melt viscosity. Both
factors directly influence the diffusivity that limits the rate of
bubble growth. Increasing amounts of bubbles increase the buoyancy
difference to the surrounding rocks and lead to an acceleration of
the rising melt-bubble mixture. Beside these limiting factors, the
overpressure in the gas bubbles greatly depends on the magma’s
ascent speed as it controls the residence time to conditions
favourable to degassing (a combination of lithostatic pressure and
magma temperature) and the time of overpressure reduction due to
degassing. Volcanic eruptions occur when the bubbly melt can no
longer withstand the exerted stress that derives from the overlying
weight (lithostatic pressure), the expanding gas bubbles (internal
gas overpressure) and different ascent velocities in the conduit
(velocity profile). The melt will be fragmented and the
gas-pyroclast mixture will be erupted. This study has combined
close investigation of the deposits of the 1990-1995 eruption of
Unzen volcano, Japan and detailed laboratory investigations on
samples of this eruption and other volcanoes. The field work
intended to reveal the density distribution of samples from within
the eruptive products. Although all samples already underwent one
eruption, their physical state (e.g. crystallinity, porosity)
mostly remained close to sub-surface pre-eruption conditions due to
their high viscosity and accordingly allowed their usage for the
analysis of the fragmentation behaviour. In that purpose, rapid
decompression experiments that simulate volcanic eruptions
triggered by internal gas overpressure have been performed at 850
°C to evaluate fragmentation threshold and fragmentation
efficiency. Laboratory investigations of that kind are one approach
to bridge the gap between observational field volcanology and risk
assessment as they reveal information on what can not be
investigated closely but what is essential to know for realistic
eruption models and the adjacent hazard mitigation. Changing the
experimental conditions and close investigation of the artificial
products reveals the influence of physical properties on the
fragmentation behaviour. The density distribution inside a dome and
the upper part of the conduit is crucial to the eruptive style of
an explosive volcano. This information cannot be collected during
an ongoing eruption but is important for future hazard assessment
via modelling conduit flow and dome collapse/explosion behaviour.
Therefore, the percentage of the mass fractions of all rock types
in the primary and secondary volcanic deposits must be evaluated.
For this purpose and at the lowest logistic effort, field-based
density measurements have been performed on Unzen volcano, Japan.
The resultant density distribution was found to be generally
bimodal at constant peak values but changing peak ratios. The most
abundant rock types at Unzen exhibited an open porosity of 8 and 20
vol.%, respectively. The porosity was found to be arranged in
layers of cm- to dm-scale that were oriented subparallel to flow
direction, i.e. subvertical within the conduit and flank-parallel
within the dome lobes. The achieved results allowed for an internal
picture of the dome during the last eruption of Unzen volcano. The
evaluated picture of the density distribution within the uppermost
parts of the conduit and the dome itself allowed for insights into
and a better understanding of magma ascent and degassing conditions
at Unzen volcano during its last eruption. Knowledge of the density
distribution is additionally required to draw conclusions from the
results of laboratory investigations on the fragmentation behaviour
to the monitored behaviour of Unzen volcano during its last
eruption. In the laboratory, the fragmentation behaviour upon rapid
decompression has been investigated in a modified fragmentation
bomb (Spieler et al., 2004). At 850 °C, initial overpressure
conditions as high as 50 MPa have been applied to sample cylinders
(25 mm diameter, 60 mm length) drilled from natural samples. In a
first step, the minimum overpressure required to cause complete
sample fragmentation (= fragmentation threshold, ΔPfr) has been
evaluated. Results from samples of several volcanoes (Unzen,
Montserrat, Stromboli, and Mt. St. Helens) showed that ΔPfr mainly
depended on open porosity and permeability of the specific sample
as these parameters were controlling pressure build-up and loss.
The experiments further revealed that sample fragmentation was not
the result of a single process but the result of a combination of
simultaneously occurring processes as indicated by Alidibirov et
al. (2000). The degree of influence of a single process to the
fragmentation behaviour was found to be porosity-dependent. Further
experiments at initial pressure conditions above ΔPfr and close
investigation of the artificially generated pyroclasts allowed
evaluating the fragmentation efficiency upon changing physical
properties of the used samples. The efficiency of sample size
reduction was investigated by grain-size analysis (dry sieving for
particles bigger than 0.25 mm and wet laser refraction for
particles smaller than 0.25 mm) and surface area measurements (by
Argon adsorption). Results of experiments with samples of different
porosities at different initial pressure values showed that the
efficiency of fragmentation increased with increasing energy. The
energy available for fragmentation was estimated from the open
porosity and the applied pressure. A series of abrasion experiments
was performed to shed light on the grain size reduction due to
particle-particle interaction during mass movements. The degree of
abrasion was found to be primarily depending on porosity and
experimental duration. The results showed that abrasion may change
the density distribution of block-and-ash flows (BAF) by
preferentially abrading porous clasts. However, during the short
drying interval prior to the analysis of the experimental
pyroclasts, abrasion-induced grain-size reduction only played a
minor role and was assumed to be negligible.