Mt St Helens Images - The debris avalanche and Coldwater Lake

Structure of the deposits

Schematic representation of the debris avalanche deposit. The material comprises two main types: relatively intact blocks from the old mountain from slide block I and sandy matrix derived from the blast penetration of slide block III. Based on subsequent studies of Mt Rainier (John et al., 2008) it seems likely that much of the fine matrix also derives from previous hydrothermal alteration of the volcano edifice. The proportion of matrix increases with tranport distance as shown below. The length of the deposit is 25 km, about 16 of which are shown here.

Source US Geologlical Survey Open File Report 96-677  [Full-text pdf ]

Glicken, H., 1996, Rockslide-debris avalanche of May 18, 1980, Mount St Helens volcano, Washington: U.S. Geological Survey Open-file Report 96-677

   

See also: John, D.A., Sisson, T.W., Breit, G.N., Rye, R.O., Vallance, J.W., 2008, Characteristics, extent and origin of hydrothermal alteration at Mount Rainier Volcano, Cascades Arc, USA: Implications for debris-flow hazards and mineral deposits: Journal of Volcanology and Geothermal Research, v. 175, p. 289-314.