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The Greenstone Flow is one of the world's largest known lava flows, estimated at a volume ~1650 to ~6000 cubic kilometers of mafic lava.[2][3] In places, the lava pooled to depth of 487 meters.[4] The flow was generated by a flood basalt eruption during the Midcontinental Rift, which occurred 1.1 billion years ago.[5] Where the cooled lava is exposed, it forms much of the Keweenaw Peninsula and Isle Royal in Lake Superior. At Isle Royal, the flow forms the backbone of the island and the Greenstone Ridge trail runs along its length.[1] The flow is the typical source for the semiprecious Isle Royale greenstone.[6]