BALLISTIC BLOCK TEXTURES: INDICATORS OF A DYNAMIC AND COMPLEX SHALLOW CONDUIT DURING THE VULCANIAN PHASE OF THE 1912 ERUPTION OF NOVARUPTA, ALASKA

The study of a diverse range of juvenile and lithic blocks from a Vulcanian phase of the1912 eruption of Novarupta provides new insight to the state of the magma as an eruption passes from sustained Plinian to passive dome growth. Blocks that were predominantly ballistically ejected were measured and sampled within an ~2-3 km radius from vent and supply a picture of a dynamic and very complex shallow conduit prior to fragmentation. Extreme conduit heterogeneity is expressed in the diverse range of dacitic block types, including pumiceous, dense, mingled, and variably-welded breccia clasts, all with variable degrees of surface breadcrusting. Vesicularity, porosity/ permeability, and micro-textural data suggest considerable heterogeneity on length scales down to mm. Broad ranges in vesicularity and permeability values are present in the samples, with the breadcrusted rinds (which preserved syn-fragmentation vesicularities and volatile contents) representing the greatest variability (Table 1). Mingled blocks contain cm- to mm-scale banding with highly contrasting textures. These data indicate major physical and chemical heterogeneity reflecting very local variations in the extent to which magma was degassed or outgassed. We suggest that the shallow conduit architecture involved both the juxtaposition of meters-wide domains of contrasting texture and vesiculation state and the intimate mingling of different textures on short vertical and horizontal length scales at the contacts between these domains. This contrasts with the current models for repetitive Vulcanian explosions associated with more stable dome growth, and suggests that the Vulcanian phase at Novarupta was a complex, disequilibrium, and dynamic event rather than a simple progressive top-down evacuation of a simple horizontally stratified conduit.

Table 1. Vesicularity and permeability data for the homogeneous block types.
Lithology	Vesicularity (%)		Darcian Permeability (m2)
	Minimum	Maximum	Minimum	Maximum
Breadcrust rinds	0	55	10-16	10-12
Pumiceous dacite	40	82	10-14	10-12
Dense dacite	0	37	10-16	10-14