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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). |
