A QUANTITATIVE TEXTURAL ANALYSIS OF PYROCLASTS FROM THE ERUPTIVE EVENTS OF 1980 AT MOUNT ST. HELENS

Volatile exsolution drives volcanic eruptions by producing a gas phase that causes the rapid expansion of magma through vesiculation and fragmentation. Volatile exsolution may also cause crystal nucleation and growth by increasing the liquidus of anhydrous mineral phases. Thus, degassing and subsequent crystallization may play an important role in transitions in eruptive behavior. These transitions include changes from sustained eruptions to short explosive bursts or from explosive to effusive eruptive style. Here we examine pumice textures of representative clasts produced during the eruptive events at Mount St. Helens during the summer of 1980 and connect the range of representative textures to eruptive parameters such as repose interval and eruptive style. The character of the 1980 summer eruptions transitioned from sustained events to short bursts, and later, to effusive activity.

Both pyroclast densities and textural analysis of the groundmass provide insight into the process of magma ascent and eruption. The range of observed groundmass textures records crystallization over a range of conduit depths. Differences in clast characteristics between eruptions were shown to correlate with pre-eruptive seismicity and gas flux. Clast characteristics were used to determine the evolution of the conduit and the relative depth of conduit evacuation for each eruption. 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, 1980 eruption.