When continents try, and fail, to break apart‎

Great things can come from failure when it comes to geology. The Midcontinent rift formed about 1.1 billion years ago and runs smack in the middle of the United States at the Great Lakes. The rift failed to completely rupture, and had it succeeded it would have torn North America apart. Under immense pressure from receding tectonic plates, the weakened lithosphere instead created a basin in the crust eventually filled by Lake Superior, and it also exposed a 3000-km-long band of deeply buried igneous and sedimentary rocks.