GRAND RAPIDS, Mich.— When people think of volcanoes, they rarely think of Michigan. But volcanic activity was the driving force behind some of Michigan’s most important resources and landmarks.
The Upper Peninsula’s abundance of minerals is due to ancient volcanic activity. The evidence is hidden in plain sight.
Isle Royale and the Keweenaw Peninsula are part of one of the world’s largest known lava flows.
The Midcontinent Rift System is a massive break in North America’s continental plate. It formed around 1.1 billion years ago, with arms stretching roughly from metro Detroit up through Michigan, Lake Superior, and northern Ontario, then west to Minnesota, and finally south through parts of Iowa, Missouri, and Kansas.
According to William Rose, an emeritus professor of geological and mining engineering and sciences at Michigan Technological University, the formation of the rift was one of the first “significant Earthwide events.”
“(It was) a little more than a billion years ago, when we had a massive continent that was extremely thick. “And it piled up thicker and thicker across the Earth’s mantle,” Rose told News 8.
“The rocks underneath became hot. They became so hot that they released lavas unlike any we have now. It was so hot that it split apart, forming a rift basin.
Rose claims that geologists do not know the exact boundaries of the rift basin due to its size.
“We’re not sure exactly how far it extends, but at least it extends from Kansas up through Lake Superior and then down again, probably as far as Alabama,” Rose informed the crowd.
“However, it is mostly covered by much younger rocks. Only around Lake Superior and closer to Minneapolis can you see the lavas.
Greenstone Flow refers to the lava flow caused by the Midcontinent Rift System. The lava eventually cooled, forming a massive swath of basalt that lines the majority of Lake Superior’s floor and serves as the foundation for Isle Royale and the mountains on the Keweenaw Peninsula.
Researchers estimate that the Greenstone Flow contained 1,650 to 6,000 cubic kilometres of iron and magnesium-rich lava.
“It is very hard to make a volume estimate because (the flow) is only exposed in certain places,” Rose said. “There’s still no concrete proof that there is any other lava flow on Earth that is bigger than the Greenstone Flow.”
Parts of the Keweenaw show basalt up to 1,200 feet thick, indicating that the molten rock sat there as an ocean of magma for up to a thousand years before solidifying.
“It was a great, big, ponded bunch of basalt that covered an enormous area,” he told me. “And the lavas accomplished something no other lava flow on Earth has done. Geologists refer to it as differentiated: the rock’s composition has changed.
Rose claims that in the middle of the Keweenaw basalt is a layer of pegmatoids, a type of igneous rock with coarse grains that form giant crystals.
“Geologists come from all over the world to see that, because in no other place can you see such a thick lava flow that lasted on the Earth for such a long time,” according to him. “We have field trips from colleges and universities all over the country that come here every year and look at these kinds of things.”
Blake Point on Isle Royale has a significant amount of pegmatoids. Rose says they can also be found in Keweenaw County’s Estivant Pines Wilderness Nature Sanctuary, as well as near the Cliff Mine and Delaware Mine, which are now closed.
