Millions of years ago, the North American continent was dragged over the world's largest continental hotspot, a huge column of hot and molten rock rising from the Earth's interior that traced a 50-mile wide, 500-mile-long path northeastward across Idaho. Generating cataclysmic volcanic eruptions and large earthquakes, the hotspot helped lift the Yellowstone Plateau to more than 7,000 feet and pushed the northern Rockies to new heights, forming unusually large glaciers to carve the landscape. It also created the jewel of the U.S. national park system: Yellowstone. Meanwhile, forces stretching...