Emplacement and Compositional Variations of the Dikes at Sinker Butte Volcano, Western Snake River Plain, Idaho

Publication Date


Type of Culminating Activity


Degree Title

Master of Science in Geology



Major Advisor

Craig M. White


Understanding the evolution of shallow magma plumbing systems at short-lived, basaltic volcanoes such as Sinker Butte, in the western Snake River Plain of Idaho, gives insight into the potential behavior and hazards at current and future volcanic centers. Its extensive outcrops are conducive to examining how the lava flows and tephra units shaped the magma plumbing system of the volcano. Fifteen dikes and several additional intrusive sheets are exposed around the eastern and southern sides of Sinker Butte. Twelve of the dikes are oriented in a radial pattern around the capping mesa that is interpreted to have been a lava lake within the tuff cone's crater. This pattern is indicative of outward extrusion from one or more central magma chambers. The dikes are exposed for up to 450 m along strike and are between 28 and 133 cm in width.

All of the dikes and lava flows at Sinker Butte are tholeiitic and contain phenocrysts of plagioclase and olivine. The asymmetrical patterns of a few suggest that they had enough time to solidify before another magma pulse intruded. Most of the dikes have symmetrical patterns of vesicles and phenocrysts across their widths, indicative of a single continuous magma pulse. Greater quantities of phenocrysts in the dike interiors suggest the grains were concentrated by flow differentiation. Chemical differences in the dikes mostly result from the modal percentages of phenocrysts in a sample. Samples with more phenocrysts contain more of the elements included in plagioclase and olivine. Subsequently, specimens with higher total phenocryst contents have lower concentrations of elements excluded from both minerals.

The magma that formed dikes J, M, and N has a different magmatic history than that of the other dikes, because they have unique excluded trace element ratios. These dikes are also distinguished from the others by geographic region and relation to sills. Dikes J, M, and N do not continue stratigraphically below the sills they merge with indicating that either the dikes fed the sills or visa-versa.

The preferred intrusion hypothesis for the remaining radial dikes is based on flow directions derived by anisotropy of magnetic susceptibility analyses. Flow directions within the dikes indicate sources at various depths beneath the lava lake at the summit of the volcano. It is assumed that the lava lake existed prior to dike propagation and had crystallized to the point that magma could no longer be extruded. Mounting pressure caused fractures to radiate from several places along the vertical central magma conduit that had fed the lava lake, filling with magma. As dike propagation continued, the tops of the dikes eventually reached the surface causing flank eruptions and the bench lava flows. The breakout point at the distal end of Dike D was likely formed in this manner. Once the source of the magma was tapped, gravity would have caused the final flow directions within the dikes to tilt downward, especially in the sections directly beneath the lava flows. This is confirmed by the steep maximum AMS axes inclinations at the drill sites closest to the bench basalts. Also, cavities were found where the tops of dikes K and G meet the bench basalts.

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