THE INFLUENCE OF SUBSURFACE GEOLOGY ON ERUPTIVE STYLES AT THE GUFFEY BUTTE MAAR, WESTERN SNAKE RIVER PLAIN, IDAHO

Guffey Butte is a dissected compound basaltic maar that erupted about 1 Ma near the southern margin of the western Snake River Plain in southwestern Idaho. Explosive eruptions of this volcano produced two adjacent and partially intersecting craters, each with its own distinctive sequence of ring deposits. Sections through the crater rim of the earlier and larger of the two maars expose a single wet-to-dry cycle of deposits that begins with massive lapilli tuffs and grades upward through coarsely bedded lapilli tuffs, fine-grained dune-bedded tuffs and welded spatter. Accidental clasts in the massive tuffs include arkosic sands and rounded cobbles of rhyolite, suggesting that explosions began when rising magma encountered a water-saturated buried channel deposit. Ring deposits associated with the younger crater consist mainly of massive tuff breccias and contain at least two layers of welded basaltic spatter. Blocks of siltstone and pre-Guffey Butte basalt make up most of the accidental clasts in these deposits.

Differences in the nature of the tephra deposits produced by these two closely related vents may be due in part to differences in the subsurface geology at the sites of magma-water mixing. The first series of eruptions originated in water saturated sands and gravels which appear to have mixed with the rising magma in progressively smaller proportions. Accidental blocks in the younger ring deposits indicate the second series of explosions was focused in a sequence of siltstones and basalts, rocks that were more strongly consolidated and less permeable than the channel deposits. We suggest that mixing of magma and groundwater would have been intermittent in the second case, which may explain why ring deposits at the younger maar alternate between massive tuff breccia, indicating high water-magma ratios, and welded spatter, indicating relatively dry conditions.