Big Southern Butte

Big Southern Butte looking to the southwest.

Big Southern Butte is the largest and youngest (300,000 years old) of three rhyolite domes formed over the ~million years near the center of the Eastern Snake River Plain. In fact, it is one of the largest volcanic domes on earth. Formation of the dome is illustrated in the following series of panels.

How did Big Southern Butte form?

1. Grey color represents layers of Quaternary basalt lava flows and interlayered sediments; pink color represents Tertiary rhyolite; red represents magma associated with the formation of Big Southern Butte. In frame (1) rhyolite magma is rising through a fracture formed in the older (solidified) Tertiary rhyolite. Ascent of the rhyolitic magma is largely driven by its buoyancy (i.e. it is ~5% less dense than the surrounding rocks).
2. In frame (2) the rhyolite magma stagnates at a depth of about 900 meters near the boundary between basaltic lava flows and older Tertiary rhyolite. The basalts are highly fractured and porous (relative to the rhyolite) and as a result have an average density which is lower than the ascending rhyolitic magma. Continued supply of rhyolitic magma through a dike (i.e. magma which occupies a planar fracture) results in inflation of a pond of magma (called a sill). Continued inflation uplifts the overlying basaltic rocks forming a laccolith. Ferry Butte, and perhaps Blackfoot dome, located further to the southeast on the Snake River Plain from Big Southern Butte, are probably both examples of intrusions of magma that stopped at this stage.
3. Continued addition of viscous rhyolitic magma inflates the "roof" of overlying basalts to the point at which they fracture (break), allowing escape of some of the rhyolitic magma to the surface where it piles up as a steep sided volcanic dome. Part of the roof of basaltic rock caves in and sinks either piecemeal or as a flap down through the rhyolite. This is what has given rise to an odd asymmetry to the surface-rock geology of Big Southern Butte, i.e. the southern side is underlain by rhyolite, while the northern side is underlain by older, northward-tilted basalt. This asymmetry is easy to see from a distance because the rhyolite is much lighter colored than the basalt.

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