Another Look at the Mechanisms of Formation of Ash Aggregates in Pyroclastic Deposits
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Ash aggregation has been a subject of great interest in volcanology, due to its importance in removing the finer-grained fraction of the fragmented material generated during explosive eruptions. In such events, the amount of ash (<2 mm) represents a large fraction of the total erupted mass, and is dispersed into the surrounding atmosphere by vertical plumes and/or pyroclastic density currents (PDCs). Aggregation enhances sedimentation, reducing the residence time of solid particles in the atmosphere; therefore, understanding the processes that govern particle accretion is of critical importance for hazard assessment. Observations and experimental studies to date indicate that water, either in liquid or solid states, is able, in certain proportions, to provide the strongest bonds between particles, which are necessary to form spherical to oblate aggregates able to survive impact with the ground and to be preserved in pyroclastic deposits. In contrast, electrostatic attraction between particles forms only dry, loosely bound aggregates, several hundreds of microns in size, which rapidly disintegrate. In general, aggregates are sub-mm to a few mm in size, even if maximum sizes of several centimetres are sometimes reported. Nevertheless, the individual accreted particles rarely exceed 1 mm. Several types of aggregates were described in the PDCs produced during the 1982 eruption of El Chichón volcano (Mexico), characterized by the injection of 8 million tons of SO2 into the atmosphere, and responsible for a 5–6 °C warming in the tropical lower stratosphere. In such aggregates, individual components are strongly cemented by an S-rich film, in which particles between 1 and a few mm in diameter are common. Even if not visible at the outcrop scale, they represent a consistent proportion of the deposits and are extremely resistant to disaggregation, as shown by their capacity to survive not only the impact with the ground after falling, but also collisions with other clasts. Their similarities with aggregates found in sulphur cones at Poás volcano suggest that liquid sulphur is the cementing material. The explosive ejection of sulphur may occur in volcanoes with active hydrothermal systems. The ability of liquid sulphur to cement particles larger than 1 mm in diameter indicates that size fractions of lapilli can be efficiently removed from eruptive clouds at distances of a few km from the vent, which has important implications for hazard assessment.
KeywordsAggregation Liquid sulphur Eruptive clouds Hazard assessment
- Bonadonna, C., Mayberry, G. C., Calder, E. S., Sparks, R. S. J., Choux, C., Jackson Lejeune, A. M., et al. (2002). Tephra fallout in the eruption of Soufriére Hills Volcano, Montserrat. In T. H. Druitt & P. Kokelaar (Eds.), The Eruption of Soufriére Hills Volcano, Montserrat from 1995 to 1999 (pp. 483–516). London: Geological Society of London Memoirs.Google Scholar
- Gilbert, J. S., Lane, S. J., Sparks, R. S. J., & Koyaguchi, T. (1991). Charge measurement on particle fallout from a volcanic plume. Nature,349, 589–600.Google Scholar
- Sheridan, M. F., & Wohletz, K. H. (1983). Origin of accretionary lapilli from the Pompeii and Avellino deposits of Vesuvius. In R. Gooley (ed), Microbeam Analysis. Conference Proceedings August 6–12 (336 p.). Phoenix, Arizona: San Francisco Press.Google Scholar