Mysteries of the Midcontinent Rift

By Sahand Farivar

During the summer of 2023, I had the privilege of accompanying a team of geologists as they ventured through Lake Superior National Marine Conservation Area to collect field samples. In the interest of mining some knowledge for a geology program, I donned my journalist’s hat and joined them.

We travelled down Nipigon Bay out towards St. Ignace Island and spent the day making stops on various shorelines and islands. Throughout the trip, each of the three experts offered insights into what exactly we were looking for.

On the way out to the islands I asked one of my pre-packaged questions: how do geologists go from immediate observations in the field to a story about events that happened millions or even billions of years ago? The answer I received was far more complicated than I’d originally anticipated. In short, that exact leap, from direct observation to geological narrative, is the whole game.

We were after the story of the Midcontinent Rift, the 1.1-billion-year-old volcanic event that created the enormous basin of Lake Superior. Using field observations about certain rock formations – such as the type of rock, features on the rock like smearing or intrusions, and the way certain rock structures overlay others – and laboratory identification and dating techniques, some of which involved newer scientific developments of the 20^th and 21^st century, our task was to create a fuller picture, a geological map, of how the Midcontinent Rift must have occurred.

But, almost from the moment we stepped foot on the shores of Agate Point, it became apparent how many mysteries still shroud the story of the Great Rift. We were looking at thunder eggs, little egg-like rock intrusions spotting the coastal rhyolite (a volcanic rock rich in silica). At first, it was explained that smearing in the rhyolite meant the eggs were included in the rock after it had risen to surface. Then I heard another theory, that the eggs themselves were smeared in places and so must have been included in the magma before it cooled into rock. This was only one example of how field observations yielded different theoretical origin stories; the truth somewhere among them.

And yet, there was plenty to be sure of, like how those giant cliffs typical of the high rock in this region are formed. They are columnar basalt, flows of lava which cooled on the surface of the earth and, upon solidifying, contracted into columns like a giant accordion.

It was a sort of epiphany for me: those thirty to forty-foot columns are the lava from a billion years ago. This great mystery, the Midcontinent Rift itself, is right there before our eyes.

If you would like to explore Lake Superior National Marine Conservation Area through Interpretive programs, please join us during the summer season! Find out more on our website or follow us on Facebook.