The shallow arc of Idaho's Snake River Plain spans southern Idaho, gently rising from west to east. Current theories suggest that the plain marks the path of continental movement over a deep hotspot now lying beneath the Yellowstone Plateau. As the continent drifted southwestward over millions of years, calderas--super-volcanoes 10 - 40 miles (15 - 64 km) wide--erupted over the hotspot. When Yellowstone Caldera erupted 640,000 years ago, it released about 240 cubic miles (1,000 km3) of material, covering half of North America in 6 feet (2 m) of debris.
In the past 17 million years, there have been about a dozen catastrophic eruptions releasing huge volumes of rhyolitic magma and ash. Between these super-eruptions were long periods when more fluid basaltic lava flowed from more than 8,000 shield volcanoes and numerous lava cones. Remnants of these dot the Eastern Snake River Plain today. Layer upon layer of basalt flows extend 3,000 - 6,000 feet (1,000 - 2,000 m) below the surface, completely covering the rhyolite "basement."
1. Sinking Rivers and a Flowing Aquifer
Streams that flow here are indirect tributaries to the Snake River. The aptly named Lost River flows to an area known as "the sinks" where it soaks into the ground, becoming part of an aquifer the volume of Lake Erie. The aquifer flows through pores and fractures in the rock hundreds of feet beneath the surface, eventually emerging from springs along the Snake River Canyon at Thousand Springs about 100 miles (160 km) to the southwest.
2. Big Southern Butte
Big Southern Butte, rising 2,500 feet (760 m) above the Eastern Snake River Plain, is a prominent reminder of the region's volcanism. About 300,000 years ago, the butte intruded through surrounding layers of basalt, rising to an elevation of 7,560 feet (2,300 m). It is one of the largest composite rhyolite domes in the world.