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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