Formation of Big Southern Butte, eastern Snake River Plain, Idaho
Big Southern Butte is a rhyolite dome complex which formed about 300,000 years ago. The rhyolite magma forming Big Southern Butte fractionated from an olivine tholeiite parent magma. This rhyolite magma was ~5% less dense than surrounding rocks and rose through a fracture in the Tertiary rhyolite. Since the SRP basalts are highly fractured and are overall less dense than the ascending rhyolite magma, the magma stalled out at a depth of ~900 m and ponded at the interface between the Tertiary rhyolite and the Quaternary basalt to form a sill. Continued supply of magma inflated the sill into a laccolith, pushing up the ~900 m of basalt cover rock. Inflation progressed until the basalt cover rock fractured. As rhyolite extrusion began, the basalt cover on the south side of the laccolith began to sink either as a flap or broke into several large pieces and sank. A dome began to form from endogenous growth. The basalt cover on the north side of the dome remained intact and on top of the rhyolite. A massive white crust formed below a crumble breccia on the surface of the slowly growing dome. Later, rapid endogenous growth broke up the white crust, forming an autoclastic breccia. Minor extrusions of rhyolite reached the surface and chilled as obsidian. Small explosions produced a graded explosion breccia and lapilli tuffs near the top of Big Southern Butte. Near the end of the growth of the southeast dome, the intrusive center shifted slightly to the northwest forming another dome. Cessation of the magma supply during the final growth stages of this second dome resulted in a small depression near its summit.
For more information see:
Spear, D. B., and King, J. S., 1982, The geology of Big Southern Butte, Idaho, in Bonnichsen, B., and Breckenridge, R.M., eds., Cenozoic geology of Idaho, Idaho Bureau of Mines and Geology Bulletin, p. 395-403.
McCurry, M., Hackett, W. R., and Hayden, K., 1999, Cedar Butte and Cogenetic Quaternary Rhyolite Domes of the Eastern Snake River Plain, Guidebook to the Geology of Eastern Idaho: Pocatello, Idaho, Idaho Museum of Natural History, p. 169-179.
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