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Centre for Earth and Environmental Science Research

A hazard evaluation of very large-scale lahars at Mount Shasta, Northern California

Executive Summary
ResearchersMr Ray Keeling, CEESR
Dr Scott Bryan, CEESR
Dr Tom Pierson, USGS
Dr Michael Smith, CEESR
Funding Body/SourceKingston University & ELCAS (MoD)
Duration2007 - 2008
Project SummaryThis project is evaluating the hazard potential of very large-scale lahars from Mount Shasta volcano in northern California. Mount Shasta is a 4317m high, glaciated, compound stratovolcano, located at the southern end of the Cascades volcanic range (Fig. 1). Comprehensive hazard assessments of the risks and effects of large-scale lahars have been carried out at all the major Cascades volcanoes except Mount Shasta. Therefore, this research project has the following objectives:
1) To map and characterise past lahar deposit(s) related to Big Creek Canyon of Mount Shasta to constrain their areal extents, run-out lengths and deposit volumes; 2) To undertake stratigraphical and sedimentological analysis of the Big Creek Canyon lahar deposits, to understand the main transport and depositional processes in their emplacement and to determine what the triggering mechanism for these large lahar events have been (i.e. whether they were syn or post eruptive); 3) To run specific Geographical Information Systems (GIS) software called LAHARZ (Schilling, 1998) for predictive modelling of large-scale lahar events related to the formation of the Big Creek Canyon scar of Mount Shasta (Fig. 2) to identify the area of impact from past events as a guide to future impacts from the volcano; and 4) from the field observations and predictive modelling, to produce a lahar hazard zonation map for Mount Shasta and surrounding areas. The emergency plan for future large scale lahar flow can then be re-assessed.

Fig. 1 Location map showing the volcanoes of the Cascades Range (From the USGS).

Background

Lahars are a rapidly flowing mixture of rock debris and water (other than normal stream flow) from a volcano (Smith & Fritz, 1989). Lahars are a highly destructive volcanic hazard capable of causing major loss of life, extensive damage to infrastructure (roads, bridges, buildings, power grids, water supplies etc) over large areas and at a high social and economic cost (Rodolfo, 2000). Lahar initiation usually requires three main components; a trigger mechanism, a source of sediment that can readily be mobilised by the gravity flows, and a drainage network to focus flows and promote greater run-out lengths (Vallance, 2000). Lahars are generally classed in two ways; primary (syn-eruptive) and secondary (post-eruptive). Primary lahars are formed when pyroclastic flows come into contact with glacial ice, releasing melt water, allowing it to erode unconsolidated pyroclastic debris and soils as reported by Pierson et al. (1989) at Nevado del Ruiz. Secondary lahars are created from mechanisms that don't require a co-incident eruption and include; remobilisation of pyroclastic deposits by crater-lake breakout, intense and/or prolonged rainfall and rapid melting of snow and ice cap cover (Vallance, 2000). The lahar then "bulks up" due to the entrainment of debris as it moves downstream eventually becoming a debris flow in the lower reaches of the channel (Vallance, 2000). This study will investigate the relevance of all these mechanisms with respect to Mount Shasta.

Fig. 2 Mount Shasta with Shastina cone at left and the Big Creek Canyon collapse scar on the southwestern side of the main stratocone (centre right of photo). Photo by S Bryan.

REFERENCES

Pierson, T. C., Janda, R. J., Thouret, J. C., Borrero, C. A., (1989) Peturbation and melting of snow and ice by the 13 November 1985 eruption of Nevado del Ruiz, Columbia and consequent mobilisation, flow and deposition of lahars, J. Volcanol. Geotherm. Res., 41, pp. 17-66.

Rodolfo, K. S., (2000) The hazard from lahars and Jokulhlaups, In: Sigurdsson, H. et al., (Ed), Encyclopedia of Volcanology, Academic Press, San Diego, pp. 973-995.

Schilling, S, P., (1998) LAHARZ; GIS programs for automated mapping of lahar-inundation hazard zones, USGS, Open-File Report, 98-638.

Smith, G. A. and Fritz, W. J., (1989) Volcanic influences on terrestrial sedimentation, Geol. 17, pp 375-376.

Vallance, J. W., (2000) Lahars, In: Sigurdsson , H. et al., (Ed), Encyclopedia of Volcanology, Academic Press, San Diego, pp. 601-616.

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