Big Southern Butte

October 22, 2009


Photographer: Stu Witmer
Summary Author
: Stu Witmer

You literally can’t miss it as you drive along US 20 between Arco and Atomic City, Idaho. The Snake River Plain here is as flat and featureless as you might expect a massive river of basalt to be. The Plain was created as the North American plate was dragged very slowly over the Yellowstone hotspot, creating copious amounts of lava. There are three buttes south of the highway: East Butte, Middle Butte and Big Southern Butte. The folks who named this bit of geography were not much for hyperbole.

Big Southern Butte reaches about 7,500 feet (2,280 meters) above sea level and about 2,500 feet (762 m) above the surrounding landscape. It is about as wide as it is high. Big Southern is the youngest and most significant of the three. About 300,000 years ago when rhyolite magma drifted up from the hotspot it began to form a pool below the existing basalt. The rock expanded and cracked as if it were the biggest, hardest soufflé ever. Obsidian flowed from some of the fissures. This was collected many years later by humans who used it for arrowheads and sharp tools. Some years after that, travelers on their way to the Oregon country used Big Southern as a road marker for the Goodale Cutoff.

Big Southern Butte may be one of the most massive volcanic domes on Earth but it is not alone in the Solar System. The Moon has a larger formation similar to Big Southern named Gruithuisen dome for Franz von Paula Gruithuisen, a professor of astronomy at Munich in 1826 whose magnum opus is titled "Discovery of Many Distinct Traces of Lunar Inhabitants, Especially of One of Their Colossal Buildings." And on Mars, there’s Olympus Mons. The Martian volcano is not morphologically similar to Big Southern Butte, but it sure impresses the tourists! If you imagine a red tint to the sky in the above photo and ignore the clouds, you might think it was taken on Mars.

Photo Details: Nikon E5700; 1/1800 second at f /3.6; ISO 100; focal length 93mm. Picture taken August 30, 2007, at 1:55 p.m.

Big Southern Butte Coordinates: 43.4, -113.016667