A qualitative and quantitative characterization of porosity in volcanic ash Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/z029p726f

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  • Soils developed on volcanic ash exhibit several unique properties not commonly observed in soils formed on other geologic material. Low thermal conductivities, low volumetric heat capacities, slow transport phenomena, excess macro-drainage and development of unique suites of clay minerals are some properties thought to be directly related to the vesicular nature of the parent ash. In order to establish cause and effect relationships in these soils one must first define the porous nature of the system. The purpose of this study was to obtain this basic information, particularly on Mazama ash. Since internal vesicular porosity of volcanic ash is thought to be a function of the properties of its parent melt, ejecta from other sources was included in the study. Pore size distributions, determined by mercury intrusion methods, and microscopic analysis of pore shape and arrangement indicate that differences between sources do exist. The percent pores between 30.0 and 0.2 microns is different for Mazama and Newberry ash. Newberry ash is more dense and exhibits a twisted knotty texture compared to Mazama's sub-parallel tubular pore arrangement. Analysis of samples from a transect indicates that characteristic porosity of a given particle size of Mazama ash does not change with distance. As particle size increases, percent of pore space between 30.0 and 0.2 microns increases. For all particle sizes percentage of total volume between these pore diameters ranges from 80 to 95 percent. Larger particles are also more vesicular. The range of vesicularity for all particle sizes is 0.8 to 2.0 ml /gm. All median pore diameters are between 4 and 10 microns, regardless of particle size. Analysis with depth in an ash deposit have implications to the effects of weathering as well as to differences in eruptive nature of the source. A major effect of weathering is to reduce the diameter of particles. In addition, weathering processes tend to decrease vesicularity of the particles by filling surface pores with weathering products. The fact that vesicularity in Newberry ash increases with depth indicates that the deposit represents one eruptive sequence. Data from Mazama deposit samples are consistent with the view that the ash fall was deposited under two levels of violence.
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