Our planet can be very violent at times. Its spasms are wonderful to watch from a safe distance and may leave behind areas of great scenic beauty, but they seldom make things easier for us. The Great Lakes of North America are an exception because of their great beauty and conveniently interlinked connection to the St. Lawrence River that allows ocean transport to reach inland almost to the middle of the continent.
This area’s geological upheavals are over for now. However, Lake Superior, in particular – born of volcanism and continental glaciers – still holds billion-year-old evidence that has given geoscientists a much better idea of just how extreme things once were in this part of the world.
♦ Fire and Ice
All the Great Lakes as we know them today were formed when mile-high glaciers advanced and receded over the area during the last Ice Age, their paths dictated by very hard, ancient bedrock in some areas that the moving ice would go around, and relatively softer and much younger Paleozic sedimentary rocks that the thick ice could plow and scour away, forming basins that later filled up with water as the ice retreated.
As millennia passed, lakes were temporary because the glacier surged and then retreated, and then surged again. The earliest lake recognized in the Superior basin, Lake Keweenaw, formed around 12,000 years ago but only lasted about 200 years before the ice came back again. After this glacier front retreated far enough to expose the western end of the basin, about 11,000 years ago, Lake Duluth appeared there. In another 1,000 years or so, the ice returned, filling most of the basin, although it didn’t totally eradicate Lake Duluth and it also left a part of the eastern Superior basin open for a new lake, Minong.
Starting around 9,500 years ago, the continental ice sheet started retreating for good, and Lake Minong expanded, eventually absorbing Lake Duluth and filling the entire basin. After this, there were many local-scale alterations of the region’s surface and its water drainage patterns, but that all stabilized about 2,000 years ago, apart from a rising of the land, still going on today, after it had borne the weight of the big glaciers. The modern shape of Lake Superior was now in place.
The ice sheet had scrubbed off a lot of top soil, exposing many rock outcrops throughout the region. Scientists have found that those along the north short of Lake Superior are mostly volcanic and old enough to make the experts very interested.
As an example of this extreme age, here is a Canadian geologist, telling his young audience in an online slide show of “rocks … of Proterozoic age, around 1.1 billion years old, the same age as the Coldwell rocks, but much younger than the rocks we saw near Hemlo and Geraldton.”
The 1.1-billion-year-old Coldwell rocks are “much younger” than other rocks exposed around the lake’s shore! After decades of research into this during the 19th and early 20th centuries, geologists realized all the very old rocks belonged to a group that they named the Superior Province. This geological province of rock sits at the center of the North American continent and is some 2.7 billion years old, or almost half the age of Earth itself.
When modern science finally developed instruments that could “see” through the ground by measuring minute changes in gravity and magnetism, and also when they learned how to do an “Earth CT” by studying reflections of man-made seismic waves, geologists got a big surprise. Today’s Lake Superior sits atop the deepest known failed rift on the planet (see Reeves, T.K., and Carroll, H. B. 1999. Geologic Analysis of Priority Basins for Exploration and Drilling, p. 6-26. Prepared for United States Department of Energy. DOE/PC/91008-0062, Distribution Category UC-122) (PDF file).
Since the 1950s, Earth scientists have been painstakingly piecing together the geological events that formed this deep rift underneath the Superior basin. They now believe, that a little over 1 billion years ago, the continent that contained those Superior Province rocks started to break apart, much like the African continent today is splitting apart in the East African Rift, and on the same large scale. The process went on for about 100 million years and then for some reason it stopped before the continent could complete break apart (see Van Schmus, W.R., and Hinze, W.J. 1985. The Midcontinent Rift System. Annual Review of Earth and Planetary Sciences, 13, p. 345-383).
Evidence of the ancient break extends from the western end of Lake Superior south to Kansas and Oklahoma. It also goes in a arc through Lake Superior’s basin and then south across Michigan and Ohio at least as far south as Kentucky. Another, minor arm may extend north into Canada.
Reeves and Carroll say, that some 1.1-1.2 billion years ago, heat from the Earth’s mantle region rose up underneath what is now the Great Lakes-Central US, causing deep fractures in what is now 2.7-billion-year-old rock. Lava poured out in basalt floods as a three-way split developed in the continental bedrock. The rifting zone was centered roughly in what today is the center of Lake Superior. The land there subsided at least 20 miles, according to modern seismic profiling data, almost all the way to the mantle itself. Then the rift failed, possibly because another continental collision began a bit further away, in what geologists refer to as the Grenville Orogeny.
Stony debris and sediments, and eventually a great lake, not an ocean, filled the deep depression that had been left on the land. Today the actual rift valley floor is buried deeply under miles of surface rock, but modern Lake Superior’s shape still shows the directions followed by two of the three ancient splits in the continental rock.
The Earth’s surface is restless and constantly changing, though on our human time scale it often seems fixed and peaceful. Lake Superior also has many moods; some are very gentle and beautiful, but when wracked by storms, Superior does indeed “sing in the rooms of her ice-water mansions.” Just as Gordon Lightfoot memorialized the wreck of the Edmund Fitzgerald in 1975 during a storm on Lake Superior, the Earth has left a memorial of its own in the Lake Superior area.
As powerful and deadly as the lake’s wrath can be at times, it’s insignificant when compared to the great tectonic, volcanic and glacial forces that formed it.
