Field-based density measurements as tool to identify preeruption dome structure: set-up and first results from Unzen volcano, Japan

• Published in: Journal of volcanology and geothermal research Vol. 141; no. 1; pp. 65 - 75
• Main Authors: Kueppers, Ulrich, Scheu, Bettina, Spieler, Oliver, Dingwell, Donald B.
• Format: Journal Article
• Language: English
• Published: Lausanne Elsevier B.V 01.03.2005; Amsterdam Elsevier; New York, NY
• Subjects: block-and-ash flow; Crystalline rocks; dome; Earth sciences; Earth, ocean, space; Exact sciences and technology; explosive eruption; field-based density measurements; fragmentation behaviour; Igneous and metamorphic rocks petrology, volcanic processes, magmas; Unzen volcano; volcanology; Kyushu; Nagasaki Japan; Unzen; Far East; Japan; fragmentation behaviour; volcanology; block-and-ash flow; field-based density measurements; dome; explosive eruption; Unzen volcano; experimental studies; physical properties; domes; fragmentation; lava; ash flows; instruments; stability; models; density; accumulation; field studies; porosity; volcanic rocks; boulders; explosive eruptions; igneous rocks; pyroclastics; Asia
• ISSN: 0377-0273; 1872-6097
• DOI: 10.1016/j.jvolgeores.2004.09.005

For an improvement in the quality of conduit flow and dome-related explosive eruption models, knowledge of the preeruption or precollapse density of the rocks involved is necessary. As close investigation is impossible during eruption, the best substitute comes from quantitative investigation of the eruption deposits. The porosity of volcanic rocks is of primary importance for the eruptive behaviour and, accordingly, a key-parameter for realistic models of dome stability and conduit flow. Fortunately, this physical property may be accurately determined via density measurements. We developed a robust, battery-powered device for rapid and reliable density measurements of dry rock samples in the field. The density of the samples (sealed in plastic bags at 250 mbar) is determined using the Archimedean principle. We have tested the device on the deposits of the 1990–1995 eruption of Unzen volcano, Japan. Short setup and operation times allow up to 60 measurements per day under fieldwork conditions. The rapid accumulation of correspondingly large data sets has allowed us to acquire the first statistically significant data set of clast density distribution in block-and-ash flow deposits. More than 1100 samples with a total weight of 2.2 tons were measured. The data set demonstrates that the deposits of the last eruptive episode at Unzen display a bimodal density distribution, with peaks at 2.0±0.1 and 2.3±0.1 g/cm 3, corresponding to open porosity values of 20 and 8 vol.%, respectively. We use this data set to link the results of laboratory-based fragmentation experiments to field studies at recently active lava domes.