The Mystery of the Midcontinental Rift1.1 billion years ago, a rift opened under what is now Lake Su
The Mystery of the Midcontinental Rift1.1 billion years ago, a rift opened under what is now Lake Superior, creating a 2,000-kilometer gash that extends as far south as Oklahoma and Arkansas. Had this rift continued to develop, it would have torn what is today North America into two pieces. Instead, the rift stopped, leaving one of the largest basalt rock deposits in the world and a mystery as to why the rift failed.We’ve covered some of the mechanics of continental rifting before (http://on.fb.me/1HSzj3x essentially it occurs when the crust is pulled apart and thinned, resulting in extensive magma flows. The Atlantic Ocean began as a rift 200 million years ago and it is still growing. So why didn’t North America pull apart and become separated by an ocean?According to a 2014 study, the rift is actually a geological hybrid of an aulacogen and a large igneous province (LIP). Aulacogens are failed rift arms, meaning the Midcontinental Rift was just one part of a larger rift system. LIPs are extremely large accumulations - greater than 100,000 square kilometers - of lava that occur within a few million years or less; a relatively short amount of time in geologic terms. In this case, the LIP filled the gash to 32-kilometers thick, mostly with basalt.As to why the rift failed to break apart North America, researchers proposed that the larger rift system did succeed in pulling apart the crust, but in doing so may have added forces to the Midcontinental Rift arm that caused it to fail. The remaining deep gash was then filled with magma by the LIP. Eventually, the cooled lava was uplifted to the surface. Most of the rift is covered by sediments, but the uplifted volcanic rock is readily visible in outcrops along Lake Superior and the cliffs of the St. Croix River.REPhoto Credit: Greg Gjerdingenhttp://www.flickr.com/photos/greggjerdingen/9338206217Read More:http://phys.org/news/2014-10-mysterious-midcontinent-rift-geological-hybrid.htmlhttp://www.futurity.org/2000-mile-long-rift-split-continent-two/http://www.earth.northwestern.edu/public/seth/research/mcr.htmlhttp://www.nature.com/news/geology-north-america-s-broken-heart-1.14281 -- source link
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