Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week's contribution is from Zach Lifton, geologist with the Idaho Geological Survey.

The path of the Yellowstone Hot Spot across southern Idaho left behind a volcanic lowland called the Eastern Snake River Plain (ESRP). Volcanism along the hot spot track was bimodal, consisting of both basalt and rhyolite. Rhyolite rocks are generally older and covered by younger basalt. Most of the surface of the ESRP is composed of low-profile basaltic shield volcanoes that coalesce into a low-relief plain. But this plain is in fact littered with dozens of volcanic domes, craters, cinder cones, and fissures. Perhaps the most prominent of these are three massive volcanic domes, Big Southern Butte, Middle Butte, and East Butte, that stand tall above the surface of the plain.

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Big Southern Butte, Middle Butte, East Butte, and several other volcanic eruptive features are located along the Axial Volcanic Zone (AVZ), a northeast-trending line of volcanic surface features along the center axis of the ESRP. The mechanism for concentrating volcanic activity along this axis is not well known, but elevation and volcanic activity are higher here. Several perpendicular rift zones extend across the ESRP, oriented northwest-southeast. These include the Great Rift Zone, Arco Rift Zone, and Howe-East Butte Rift Zone. These rift zones accommodate northeast-southwest extension of the ESRP and may be connected at depth to the Basin and Range normal faults on either side of the ESRP.

Big Southern Butte and East Butte are steep domes where rhyolitic magma punched through the ESRP basalt to erupt and form dramatic, isolated buttes. Middle Butte is similarly caused by upward movement of rhyolitic magma, but it never breached the surface and is still covered by an uplifted block of layered basalt flows. At approximately 2,500 feet (760 meter), Big Southern Butte is the largest of the three rhyolite domes. It consists of two lobes that erupted and coalesced approximately 300,000 years ago.

Nineteen miles (30 km) northeast of Big Southern Butte is Middle Butte, which is approximately 1,200 feet (365 meters) tall. Because the rhyolite pushing Middle Butte up is not exposed at the surface, it cannot be dated and its age is unknown.

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East Butte is 3 miles further to the east-northeast, stands approximately 1,200 feet (365 meters) tall, and erupted about 600,000 years ago.

The rhyolite forming these buttes is chemically distinct from the older rhyolite of the Yellowstone hotspot that underlies the ESRP basalts. This implies that the rhyolite domes have a unique source that is not directly related that which formed the trail of calderas across the ESRP 5–15 million years ago.

Why do rhyolite lava flows stand up so steeply compared to basalt lava flows? It comes down to chemistry: silica content affects the viscosity of lava. Basalt has relatively low silica content (~50%) and can flow more easily. Basalt flows can travel far and tend to be flat and thin. Rhyolite, on the other hand, has relatively high silica content (~75%) and has higher viscosity. Rhyolite often erupts explosively because of its high viscosity and high dissolved gas content; however if it erupts as a lava (which it has done many times within Yellowstone National Park, most recently about 72,000 years ago)  it can build steep-sided domes like Big Southern Butte. Other notable rhyolite domes include the few-hundred-year-old Inyo Craters in eastern California, and at Chaitén, Chile, where a dome formed during an eruption in 2008.

In addition to their geological significance, the rhyolite domes of the ESRP are also culturally significant. Artifacts and archeological evidence suggest that humans have inhabited the ESRP since the end of the last ice age, ~13,000 years ago. Big Southern Butte was an important source of obsidian, and artifacts sourced from there have been found throughout central and southern Idaho. More recently, travelers along the Goodale’s Cutoff spur of the Oregon trail in the mid-late 1800s and subsequent stagecoach and train lines utilized freshwater springs along the base of Big Southern Butte.

The next time you are on the Eastern Snake River Plain, keep an eye out for these big, beautiful buttes. They are hard to miss and tell a spectacular geological and cultural story!