12/03/2025
Michigan’s ancient lava flows are some of the oldest exposed rocks on Earth, quietly hiding in the western Upper Peninsula. About 1.1 billion years ago, the Midcontinent Rift tried to split North America apart. It failed—but not before unleashing massive basaltic lava flows that stacked up thousands of feet thick. Over time, those layers tilted, cracked, and eroded into the dramatic spine of the Keweenaw Peninsula and the Porcupine Mountains.
These lava flows are what made Michigan copper country: super-heated fluids moved through the fractured basalt and left behind some of the world’s purest native copper deposits. That’s why the Keweenaw became the birthplace of America’s copper boom. Today, you can still see the flows in towering cliffs, shoreline outcrops, and trail cuts—especially the spectacular, tilted “ribs” of basalt at Brockway Mountain, Copper Harbor, and along the Porkies’ Lake of the Clouds overlook.
Geologically, these rocks are part of the Portage Lake Volcanics, and they record dozens of eruptions over millions of years. The UP’s distinctive north–south ridges, rugged coastlines, and even some inland lakes trace directly back to these ancient lava sheets. In short: Michigan’s most dramatic landscapes were forged in fire long before the Great Lakes ever existed.
The Keweenaw Peninsula and Isle Royale are basically built from these cooled lava sheets that were later tilted and uplifted so they stick up above Lake Superior. Erosion has carved cliffs, ridges, and waterfalls where the hard basalt stands up and softer surrounding rocks have worn away, giving the western U.P. its rugged scenery