Second, the MCR failed to split the continent because compression from a mountain-building episode ended the extension and volcanism. First, the MCR formed within Laurentia, the core of North America that assembled in the Precambrian era. The traditional view of the formation and evolution of the MCR involved two premises. Why was the MCR so magma rich? Scientists are beginning to tease out answers. The MCR’s igneous rocks tell of magma pools far more vast than what one would expect in a failed rift. It stands as one of the best examples of a failed rift (Figure 1). Called the Midcontinent Rift (MCR), this 3000-kilometer-long feature, made of 1.1-billion-year-old igneous and sedimentary rocks, extends underground across the central United States. The cliffs on the shores of Lake Superior-and the lake itself-are part of such a fossilized rift. Some rifts, however, fail to develop into seafloor spreading centers and instead leave major relict structures within continents-“fossils” preserving the geologic environments in which they formed. When a rift succeeds, the continent splits, and a new ocean basin forms between the two parts of the continent. Rifts are linear features along which continents stretch.
Croix River cuts through a series of lava flows. One of the best exposures of the Midcontinent Rift’s 1.1-billion-year-old volcanic rocks is in Interstate Park, along the border between Minnesota and Wisconsin, where the St.
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