Permeability and porosity as constraints on the explosive eruption of magma: Laboratory experiments and field investigations

Porosity and permeability are both parameters which may have a
considerable impact on the characteristics of a volcanic eruption.
Various processes, from magmatic flow during ascent to the point of
magmatic fragmentation during an explosive eruption are influenced,
and sometimes even controlled by the amount of volatiles trapped in
a magma’s pore space and by the efficiency of their escape.
Detailed investigations of the porosity of pyroclastic rocks and
its relation to the gas permeability are therefore crucial for the
understanding of such processes and may provide an important
database for physical models. The combination of experimental work
and field investigation represents in this context an effective
approach to obtain a statistically relevant amount of data on the
one hand, and, on the other hand, experimentally quantify the
correlation between different parameters. For this study, density
data of pyroclastic deposits from eight circum-pacific volcanoes
were recalculated to porosity values using the determined matrix
density of the corresponding rocks. The pyroclasts density was
determined directly in the field with a method based on the
Archimedean principle; the matrix density was determined in the
laboratory using a He-Pycnometer. The comparison of the resulting
porosity distribution histograms allows (a) the investigation of
local features related to depositional mechanisms, if the
distribution of single measurement points is evaluated, and (b)
statements about large scale coherencies regarding the eruptive
style and the explosivity of a volcano, if the compiled datasets of
the volcanoes are compared. The shape and the variance of the
distribution curves, as well as the positions of the porosity peak
or mean porosity values are parameters that can be used for further
interpretation. The differences in the porosity distribution
patterns allowed the classification of the investigated volcanoes
into three groups, corresponding to their eruptive characteristics:
(1) dome-building volcanoes with predominantly block-and-ash-flow
activity and occasional Vulcanian explosions (Merapi, Unzen,
Colima), (2) cryptodome-forming volcanoes with a subsequent
lateral-blast eruption (Bezymianny, Mount St. Helens), and (3)
Subplinian to Plinian explosive eruptions (Krakatau, Kelut,
Augustine). Furthermore, possible coherencies between the mean
porosity values of selected eruptions and their explosivity,
expressed in two different explosivity indexes, were evaluated. The
‘Volcanic Explosivity Index’ (VEI), introduced by Newhall &
Self (1982), is mainly based on the volume of the erupted tephra,
and shows a rough positive correlation to the mean porosity of
eruptive products. A qualitative enhancement of this correlation,
especially considering low-porosity, low-explosive deposits, was
achieved by using the measured porosity values to determine the
index of the ‘Eruption Magnitude’, introduced by Pyle 1995.
Volcanoes with not only pure explosive (Vulcanian and/or Plinian)
activity were found to deviate systematically from this
correlation. Besides their relevance for the understanding and
modeling of eruption physics, the interpretation of porosity data
may help to discriminate eruption characteristics and explosivities
also at historic and pre-historic eruption deposits. The main focus
of this work was the experimental investigation of the gas
permeability of volcanic rocks. In order to simulate degassing
processes under strongly transient conditions, the experiments were
performed on a shock-tube like apparatus. The permeability of a
natural porous material depends on a complex mixture of physical
and textural parameters. Evidently, the volume fraction of the
materials pore space, i.e. its porosity, is one of the prominent
factors controlling permeable gas flow. But, as a high scatter of
measured permeability values for a given porosity indicates, it
seems that parameters like vesicle sizes, vesicle size
distribution, vesicle shape, the degree of interconnectivity et
cetera may likewise influence filtration properties. Therefore it
is almost impossible to predict the permeability development of
natural material with theoretical cause-and-effect relations, and
experimental work in this field is essential. By performing more
than 360 gas filtration experiments on 112 different samples from
13 volcanoes, a comprehensive permeability and porosity database
was created with this study, giving rise to profound empirical as
well as quantitative investigations. The dependency of porosity and
permeability of volcanic rocks was found to follow two different,
but overlapping trends, according to the geometries of the gas-flow
providing pore-space: at low porosities (i.e. long-term degassed
dome rocks), gas escape occurs predominantly through microcracks or
elongated micropores and therefore could be described by simplified
forms of capillary (Kozeny-Carman relations) and fracture flow
models. At higher porosities, the influence of vesicles becomes
progressively stronger as they form an increasingly connected
network. Therefore, a model based on the percolation theory of
fully penetrable spheres was used, as a first approximation, to
describe the permeability-porosity trend. To investigate possible
influences of high temperatures on the degassing properties of
volcanic rocks, a measuring method that allowed permeability
experiments at temperatures up to 750 °C was developed and tested.
A sealing coat of compacted NaCl, which was, if required, further
compressed during the high-T experiment, was found to be the most
promising approach to avoid gas leaking due to different thermal
expansivities of the materials involved. The results of three dome
rock samples showed distinct lower gas filtration rates at high
temperatures. As this may, for the largest part, be attributed to
changed gas properties at high temperature, the obtained
permeability values must be corrected for the enhanced gas
viscosity. The corrected permeability values of the samples were
higher than those obtained at room temperature, possibly caused by
thermal expansion of the pores. Since, however, compressional
forces of the salt coating upon the sample cylinder may lower the
permeability particularly of highly fractured rocks to a not
quantifyable degree, these results must be interpreted accordingly
and seen under certain restrictions. Comparison of the permeability
values before and after the heating process revealed that no
permanent structural changes in the pore network occurred. This was
confirmed by a 5h-experiment on a trachytic sample, with
permeability tests in an interval of 60 minutes. The influence of
permeability on magmatic fragmentation is of special interest for
the modelling of eruptive processes. In particular the
‘fragmentation threshold’, i.e. the physical conditions, at which
magma is no longer able to reduce gas overpressure by filtration
and fragments, represents an important boundary condition for
explosive eruption models. Former studies defined this threshold to
depend on either the porosity of the magma, or a combination of
porosity and overpressure. The experimental results of this work,
however, reveal that, in addition to porosity and applied
overpressure, the permeability strongly influences the
fragmentation threshold. By quantifying this influence in a simple,
analytical equation, these results will provide a valuable tool for
physical models of eruption mechanics.