Big Southern Butte EarthCache
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Seen when traveling on Highway 20 between Arco and Idaho Falls. Big Southern Butte rises 760 meters above the plain,is 6.5 kilometers across, and comprises 2 coalesced cumulo domes of 0.3 million-year-old rhyolite and an elevated section of older basalt.
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.
The area supports vegetation such as lodgepole pine, aspen, Douglas fir, and manzanita (not common to this region). This impressive formation is one of the largest volcanic domes in the world, yet is one of the youngest (300,000 years old). After a hot flow of molten rhyolite (acidic rock) boiled up through older lava, a second rhyolite dome pushed up a block of earlier basalt on its northwest side. They took many thousands of years to reach their present shape, but geologically they are very recent structures. Hikers who trek to the top of this 7,550-foot high summit are rewarded with spectacular panoramic views stretching for miles.
Rising above this level plain of lava flows and windblown soils, these high landmarks are recent additions to Idaho’s landscape. East Butte (farthest east) flowed up and cooled quickly about 600,000 years ago, while Big Southern Butte emerged about 300,000 years ago.
To log this please post a picture of you/your group with your log and email the answers to the following questions: How many buttes are there and what are their names? What is the difference in how they were formed?
If you have been to the site, have pictures and can answer the questions, we will consider you grandfathered in. You may log the cache.
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