About 1.1 billion years ago, the continent that had been building for billions of years began to split apart across what is now Minnesota. The “Midcontinent rift,” as it is called, is where the crust began to separate to form a new ocean basin. The same process is currently underway between Africa and Saudi Arabia. The rifting process stopped short of producing a new ocean basin in central North America, but the abundant dark red-brown basaltic rocks now exposed along Lake Superior's north shore are a testament to the massive outpouring of lava through fractures or cracks along the rift. Gooseberry Falls State Park is an ideal place to explore these ancient lava flows.

Shown here is an example of amygdaloidal basalt. The grey-brown basalt is riddled with vesicles, or holes, created when gas was trapped within the lava flow as it cooled. The cavities have since been filled by minerals to form zeolites or agates (light colored nodules in the above picture).