Stratigraphy and Origin of the Phreatomagmatic Deposits at Sinker Butte Volcano, Western Snake River Plains, Idaho

Publication Date


Type of Culminating Activity


Degree Title

Masters of Science in Geology



Major Advisor

Craig White


The products of basaltic hydrovolcanism (maars, tuff rings, tuff cones) are second only to scoria cones as the most abundant volcanic landforms on Earth (Cas and Wright, 1988, p.376). However, the origin of these deposits remains poorly understood. This project integrates both volcanology and sedimentology to study Sinker Butte, an erosional remnant of a very large basaltic tuff cone located at the southern edge of the western Snake River Plain. Subaerial and subaqueous deposits from this center are well exposed on both sides of the Snake River Canyon, creating an unusual opportunity to study the entire eruptive sequence. The deposits at Sinker Butte were studied by measuring 23 detailed stratigraphic sections from 20 to 100 m thick proximal to distal from the vent.

Prior to the eruption the bottom of Lake Idaho was fairly uniform and flat with small topographic highs in the northern areas, and had a depth of ~15-20 m. The lake gradually deepened to the south to a depth of ~60-70m. The magma once in contact with the lake water fragmented, causing violent explosions through the lake bed. Explosive pulses dominated this eruption, with each explosion ejecting more material out of the vent. This material included gas, liquid water, accidental clasts, and small juvenile cinders. This created a broad, asymmetrical platform of tuff that began growing around the vent towards the surface of the lake.

The lowest units are generally massive, poorly sorted deposits. As the platform grew towards the surface of the lake the deposits grade into better sorted sediments with large scale cross-stratification. The primary depositional mechanism at this time was subaqueous density flows.

The transition from the subaqueous to subaerial deposits is marked by an abrupt sharp contact into well-bedded palagonite and accretionary lapilli tuffs. The planar, laterally continuous beds suggest deposition by airfall. Present proximal to the vent are large regressive cross-strata, which suggest deposition by wet base surges. Sections intermediate and distal from the crater rim contain well-bedded tuffs with interbeds of fine cross-stratification. However, the cross-strata further from the vent tend to have smaller wavelengths and wave heights. This suggests co-emplacement of airfall and deflating base surges.

The deposits intermediate to distal from the vent grade into layers consisting of 75-95% juvenile clasts. This indicates that the magma/water ratio was decreasing and the eruption drying out. Overlying the "dryer" units are massive tuff breccias that introduce new accidental clasts of rounded river gravels. The introduction of new accidental clasts suggests that the volcano was coring downward deeper into the stratigraphy. The massive deposits indicate a renewed source of external water and; hence, a ''wetter'' phase of the eruption.

As the eruption continued the deposits graded back up into wet airfall tuff, then again into dryer airfall deposits. Finally the volcano effectively sealed itself off from external water, producing a magmatic cap and several radial dikes that intrude through the walls of the tuff cone.

At the time the eruption ceased the volcano had built up a broad asymmetrical platform ~330m above the lake bottom, with the bulk of the tephra deposited south of the vent in the deeper part of the lake. The width of the crater was approximately 1500-2000 m across. Post-depositional erosion has shaped the butte as it is seen today.

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