The Development and Distribution of Surface Textures at the Mount St. Helens Dome

Abstract

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.