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Smithsonian National Museum of Natural History


Olivine, Gabbro
Photo by Penland. Click to zoom.
Catalog Number NMNH 116647-5
Locality Minnesota, United States

This olivine-gabbro is part of the Duluth Complex, a package of plutonic rocks that formed over one billion years ago when North America began to rift apart.

From the exhibit

1.1 Billion Years Ago: A Near Rift

North America narrowly escaped a fate that has befallen other continents: splitting apart, or rifting.

About 1.1 billion years ago, from what is now Kansas to the shores of Lake Superior, the craton began to split. Had this rifting continued, North America would have become two continents separated by a new ocean basin. But after 20 million years, the rifting ceased.

The failed rift is 2,000-km (1,200-mi) long. Huge volumes of basaltic magma welled up, creating a stack of igneous rocks 25-km (15.5-mi) thick. Their strong magnetic signals enabled scientists to determine the extent of the rift.


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