The Greenstone Flow is one of the world's largest known lava flows, estimated at a volume of around of mafic lava. In places, the lava pooled to depth of . The flow was generated by a flood basalt eruption during the formation of the Midcontinental Rift, which occurred 1.1 billion years ago. Where the solidified lava is exposed, it forms much of the Keweenaw Peninsula and Isle Royale in Lake Superior. At Isle Royale, the flow forms the backbone of the island and the Greenstone Ridge trail runs along its length. The flow is the typical source for the semiprecious Isle Royale greenstone.
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