The dramatic scene was caused by one of the Earth's grandest volcanic events. "Around 1.1 billion years ago," says Esther Stewart, a geologist for the Wisconsin Geological and Natural History Survey, "the continent we call North America started to rip apart." The Midcontinent Rift caused a nearly 1,300-mile horseshoe-shaped gash from Lake Superior west to modern-day Kansas and east toward Ohio. Driving along Lake Superior, from Michigan through Wisconsin into Minnesota, visitors can retrace the results of this billion-year-old story via cascading waterfalls, red sandstone sea caves, and towering cliffs. It's a hidden landscape for most travelers to the Midwest?"unless you know what to look for. A molten histo