Formation of Big Southern Butte, eastern Snake River Plain, Idaho


Big Southern Butte is a complex of two 300,000 year old rhyolite domes older basalt flows. The domes formed when rhyolite magma rose through a fracture in the existing Snake River Plain rhyolite. The magma rose until it encountered the overlying basalt flows which were highly fractured and overall less dense than the ascending magma. The magma ponded below the basalts, creating a sill. Continued supply of magma inflated the sill (building a laccolith), pushing up the ~900 m thick section of overlying basalt. Inflation progressed until the basalt cover rock fractured. As rhyolite magma began to slowly flow from under the overlying cover of basalt, the basalt flap on the south side of the laccolith began to sink. The basalt flap on the north side of the dome remained intact and on top of the rhyolite. A dome began to form from slow growth from within, much like muffins rise and expand as they bake in the oven. Minor extrusions of rhyolite reached the surface and chilled to form volcanic glass known as obsidian. Small explosions at the surface of the growing dome layered material near the top of Big Southern Butte. Near the end of the growth of the southeast dome, the center of growth shifted slightly to the northwest forming another dome.

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