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