Volatile exsolution drives volcanic eruptions by producing a gas phase that causes the rapid expansion of magma through vesiculation and fragmentation. Both pyroclast densities and textural analysis of the groundmass provide insight into the process of magma ascent and eruption as volatile exsolution may also cause crystal nucleation and growth by increasing the liquidus of anhydrous mineral phases. Thus pre and syn-eruptive degassing and subsequent crystallization as recorded in groundmass textures may be shown to play an important role in eruptive behavior, particularly in the context of transitional eruptive sequences. These transitions include changes from sustained eruptions to short explosive bursts or from explosive to effusive eruptive style, both of which were observed in the eruptive behavior of Mt. St. Helens during the summer of 1980. Here we examine groundmass textures of representative pumice clasts produced during the eruptive events at Mt. St. Helens during the summer of 1980 and use the observed range in textures to develop an inter-eruptive model of the conduit system.

Clast characteristics were used to determine the evolution of the conduit and the relative depth of conduit evacuation for each eruption as the range of observed groundmass textures records crystallization and volatile loss over a range of conduit depths. Differences in clast characteristics such as vesicularity and crystallinity between the June 12th eruption and subsequent eruptions were shown to correlate with pre-eruptive seismicity and gas flux. We concluded that the transition from sustained eruptions to short eruptive bursts occurred with a narrowing of the conduit, possibly because of a shallow intrusion prior to the June 12 eruption.