Idaho’s Snake River Plain forms a shallow arc across southern Idaho, rising gently from west to east, and current theories hold that it traces the continent’s movement over a deep hotspot now beneath the Yellowstone Plateau. As the continent drifted southwestward over millions of years, calderas, super-volcanoes 10 to 40 miles wide, erupted over the hotspot. In the past 17 million years, about a dozen catastrophic eruptions released huge volumes of rhyolitic magma and ash, and between these super-eruptions, more fluid basaltic lava flowed from more than 8,000 shield volcanoes and numerous lava cones. Remnants of these volcanoes still dot the Eastern Snake River Plain, where layer upon layer of basalt flows extend 3,000 to 6,000 feet below the surface and completely cover the rhyolite basement. Streams in this region are indirect tributaries of the Snake River, and the Lost River flows to an area called the sinks, where it soaks into the ground and becomes part of an aquifer with the volume of Lake Erie. That aquifer moves through pores and fractures in the rock hundreds of feet beneath the surface and eventually emerges from springs along the Snake River Canyon at Thousand Springs about 100 miles to the southwest. Big Southern Butte, which rises 2,500 feet above the Eastern Snake River Plain to an elevation of 7,560 feet, intruded through surrounding basalt about 300,000 years ago and is one of the largest composite rhyolite domes in the world. Ancient calderas along the plain record the North American continent’s movement over the Yellowstone hotspot, and when Yellowstone Caldera erupted 640,000 years ago, it released about 240 cubic miles of material, covering half of North America in 6 feet of debris.