Surface crusting of volcanic ash deposits under simulated rainfall
- 46 Downloads
Explosive volcanic eruptions can have severe impacts on watershed hydrology. Among them, surface crusting of volcanic ash fallout following rainfall has been shown to favour runoff, erosion and lahar initiation. It may also hamper seed emergence and depress plant growth. However, ash crust formation is poorly understood. Reconstructed ash deposits were subjected to simulated rainfall to investigate the microscale morphological modifications of the ash deposit surface in response to raindrop impact. Ash samples from three volcanic eruptions (Mt. Merapi, MER, Indonesia; Eyjafjallajökull, EYJA, Iceland; and San Cristobal, SC, Nicaragua) with different particle size distributions and soluble salt contents were used in the experiment. Microcraters and micropeaks formed on the surface of all ash deposits after rainfall initiation. This was accompanied by fine material (also referred to as micromass) accumulation in the form of one or several layers, a few tens to hundreds of micrometres thick. Such morphological changes point to structural crust formation. The crusts consisted of a thin layer of tightly packed clay and silt-size ash particles (SC), overlain by loose coarser materials in micro-craters (MER) or by an almost continuous coarse-grained layer (EYJA). In all cases, the surface crust had a reduced porosity compared with the bulk material. Depending on ash particle size, crusting was governed by splash, compaction and vertical sorting (MER), vertical particle sorting (EYJA) or compaction (SC). No samples showed evidence of particle cementation through secondary salt precipitation. Our results shed new light on the mechanisms responsible for post-depositional crusting of a natural ash deposit.
KeywordsVolcanic ash Ash deposit Structural crust Lahar initiation Rainfall simulation
We thank the Departamento de Metalurgia Extractiva, Escuela Politécnica Nacional, Quito, for access to their scanning electron microscope facility. The authors are grateful to an anonymous reviewer and to V. Manville, whose detailed and constructive comments greatly improved the quality of the manuscript. The authors would also like to thank the Associate Editor for his careful and swift handling of this contribution.
This study was supported by the Marie Curie Initial Training Network “VERTIGO” funded through the European Seventh Framework Program (FP7 2007-13) under Grant Agreement number 607905. I.T. benefited from a UCLouvain complementary PhD studentship (2017-18) funded through the Fonds Spéciaux de Recherche (FSR).
- Antos JA, Zobel DB (2005) Plant responses in forests of the tephra-fall zone. In: Dale VH, Swanson FJ, Crisafulli CM (eds) Ecological responses to the 1980 eruption of Mount St. Helens. Springer, New York, pp 47–58Google Scholar
- Assouline S (2004) Rainfall-induced soil surface sealing. Vadose Zone J 3:570–591Google Scholar
- Baumhardt RL, Schwartz RC (2004) Crusts. In: Hillel D (ed) Encyclopedia of soils in the environment. Elsevier Press, Oxford, pp 347–356Google Scholar
- Bresson LM, Valentin C (1993) Soil surface crust formation: contribution of micromorphology. Dev Soil Sci 22:737–762Google Scholar
- Dale VH, Swanson FJ, Crisafulli CM (2005) Disturbance, survival, and succession: understanding ecological responses to the 1980 eruption of Mount St. Helens. In: Dale VH, Swanson FJ, Crisafulli CM (eds) Ecological responses to the 1980 eruption of Mount St Helens. Springer, New York, pp 3–11CrossRefGoogle Scholar
- Damby DE, Horwell CJ, Baxter PJ, Delmelle P, Donaldson K, Dunster C, Fubini B, Murphy FA, Nattrass C, Sweeney S, Tetley TD, Tomatis M (2013) The respiratory health hazard of tephra from the 2010 centennial eruption of Merapi with implications for occupational mining of deposits. J Volcanol Geotherm Res 261:376–387CrossRefGoogle Scholar
- Epstein E, Grant WJ (1973) Soil crust formation as affected by raindrop impact. In: Hadas A, Swartzendruber D, Rijtema PE, Fuchs M, Yaron B (eds) Physical aspects of soil water and salts in ecosystems. Springer, Berlin, Heidelberg, Ecological Studies 4, pp 195–201Google Scholar
- Global Volcanism Program (2000) Report on San Cristobal (Nicaragua). In: Wunderman R (ed.), Bulletin of the Global Volcanism Network. Smithsonian Institution, 25:2. http://www.volcano.si.edu/. Accessed 11 May 2018
- Poss R, Pleuvret C, Saragoni H (1990) Influence des réorganisations superficielles sur l’infiltration dans les terres de Barre (Togo méridional). Cahiers-ORSTOM Pédologie 25:405–415Google Scholar
- Segerstrom K (1950) Erosion studies at Paricutin, state of Michoacan, Mexico. Report, USGS Numbered Series, Bulletin 965-A, pp 1–164Google Scholar
- Waldron HH (1967) Debris flow and erosion control problems caused by the ash eruptions of Irazu volcano, Costa Rica. US Geol Surv Bull 1241:1–37Google Scholar
- Yamakoshi T, Suwa H (2000) Post-eruption characteristics of surface runoff and sediment discharge on the slopes of pyroclastic flow deposits, Mount Unzen, Japan. Trans Jpn Geomorphol Union 21:469–497Google Scholar