The Development and Distribution of Surface Textures at the Mount St. Helens Dome
Recently acquired detailed topographic data covering the 6-year emplacement of the Mount St. Helens dacite lava dome, and hydrogen isotopic analyses of lava samples, have revealed relationships among lava surface texture, underlying slope, volatile content, and repose period. Two principal types of surface texture, smooth and scoriaceous, have formed on the Mount St. Helens dome during two different types of extrusive episodes. Type I extrusions begin with the emplacement of a small amount of smooth lava followed by a larger amount of scoriaceous lava, resulting in a predominantly scoriaceous lobe. Type II extrusions are dominated by large, smooth fractures, called crease structures. Type II lobes can be further subdivided into II-A types, in which the initially smooth crease structure becomes largely scoriaceous away from the vent during emplacement, and II-B types, whose surfaces remain entirely smooth.
The underlying slope and water content of the extruding lava appear to play the dominant roles in determining extrusion type and associated textural pattern. Crease structures, which form on shallow slopes, induce rapid cooling of the crack tip region, resulting in the formation of smooth lava. Volatile contents of around 0.3 to 0.4 wt% allow vesiculation of lava at the surface or slightly beneath the initially smooth crease structure walls, forming surface scoria. This vesiculation causes volatile contents to drop to around 0.1 wt%. Type I lobes are predominantly scoriaceous, possibly because large crease structures do not form on slopes of greater than 20°. Type II lobes form on flatter areas near the top of the dome or on the crater floor. Type II-A lobes occur when higher water contents cause the lava to slowly expand beneath and break apart the overlying smooth surface, resulting in a distal increase in scoriaceous lava. Type II-B lobes have lower water contents and fail to exhibit appreciable scoria at the surface.
The observed long-term increase in the percentage of smooth lava on the dome surface may be related to more thorough degassing of magma during ascent and emplacement, rather than to drying out of the parent magma body.
KeywordsSurface Texture Volatile Content Lava Dome Flow Front Extrusion Rate
Unable to display preview. Download preview PDF.
- Anderson SW (1988) The development and distribution of lava textures at the Mount St. Helens dome (MS Thesis). Arizona State University, Tempe, 187 pGoogle Scholar
- Anderson SW, Fink JH (1987) Modeling crease structures on silicic lava flows (abs). EOS 68: 1545Google Scholar
- Blake S (1989) Viscoplastic models of lava domes. IAVCEI Proc Volcanol 2: 88–126Google Scholar
- Cashman KV, Marsh BD (1986) Use of crystal size distributions to place constraints on magmatic crystallization models - Mount St. Helens as an example (abs). International Volcanological Congress, Auckland, 232Google Scholar
- Eichelberger JC, Westrich HR (1984) Degassing of magma in an obsidian flow and inferred degassing at depth. In: Proceedings of Workshop XIX: Active tectonic and magmatic processes beneath Long Valley Caldera, CA, Volume 1: U.S. Geological Survey Open File Report 84–939: 147–150Google Scholar
- Fink JH (1979) Surface structures on obsidian flows ( PhD dissertation ). Stanford University, 164 pGoogle Scholar
- Friedman JD, Olhoeft GR, Johnson GR, Frank D (1981) Heat content and thermal energy of the June dacite dome in relation to total energy yield, May-October 1980. In: Lipman PW, Mullineaux DR (eds) The 1980 eruptions of Mount St. Helens, Washington, US Geological Survey Professional Paper 1250: 557–568Google Scholar
- Holcomb RT, Colony WE (1987) Large-scale maps of a growing lava dome, Mount St. Helens, Washington (abs), IUGG Assembly XIX abstracts, 2: 417Google Scholar
- Loney RA (1968) Flow structures and composition of the Southern Coulee, Mono Craters, California - A pumiceous rhyolite flow. Geol Soc America Memoir 116: 415–440Google Scholar
- Manley CR, Fink JH (1987) Internal textures of rhyolite flows as revealed by research drilling. Geology 15: 549–552Google Scholar
- Moore JG, Lipman PW, Swanson DA, Alpha TR (1981) Growth of lava domes in the crater, June 1980-January 1981. In: Lipman PW, Mullineaux DR (eds) The 1980 eruptions of Mount St. Helens, Washington: US Geological Survey Professional Paper 1250: 541–548Google Scholar
- Rutherford MJ, Sigurdsson H, Carey S, Davis A (1985) The May 18, 1980 eruption of Mount St. Helens, 1. Melt composition and experimental phase equilibria. J Geophys Res 90: 2929–2947Google Scholar
- Sampson DE (1987) Textural heterogeneities and vent area structures in the 600 year old lavas of the Inyo volcanic chain, eastern California. In: Fink JH (ed) The emplacement of silicic domes and lava flows. Geol Soc America Special Paper 212: 89–102Google Scholar
- Swanson DA, Holcomb RT (1989) Regularities in growth of the Mount St. Helens dacite dome, 1980–1985 (this volume)Google Scholar
- Swanson DA, Dzurisin D, Holcomb RT, Iwatsubo EY, Chadwick WW Jr, Casadevall TJ, Ewert JW, Heliker CC (1987) Growth of the lava dome at Mount St. Helens, Washington. In: Fink JH (ed) The emplacement of silicic domes and lava flows. Geol Soc America Special Paper 212: 1–16Google Scholar
- Turcotte DL, Schubert G (1982) Geodynamics. John Wiley and Sons, New York, 450 pGoogle Scholar