PAUL SUNDBERG
Volcanic activity created the rouge-colored rhyolite cliffs along Tettegouche State Park’s coastline.
Lake Superior was almost an ocean, not just a sweetwater sea.
Back 1.1 billion years ago, the earth began a shimmy, shake and split along a 1,800-mile arch that looked to be well on its way to creating a continental divide that literally would have divided into two continents. What we know of today as Lake Superior basin might have become an ocean, perhaps like the Atlantic, which also emerged from such a continental split.
Then our split quit.
We were left with the Midcontinent Rift that never finished its rifting.
“We don’t really know for sure why it stopped. The evidence is not all there,” says John Green, a University of Minnesota Duluth professor emeritus and still UMD’s Midcontinent Rift go-to-guy. “The normal model of a continental rift is to have it continue and to build an ocean.”
As we speak, geologists are debating theories about geologic pressures from a continental mountain building in the east or the petering out of excess heat from a volcanic plume, but for thee and me, what we really need to know is that this failed rift laid the base for much of the Lake Superior terrain above and below the waters with a few precious metals thrown in. The evidence of that ancient past can be easily seen on our treks today around the Lake.
CAROLYN ROCK / GOOSEBERRY FALLS SATE PARK
Three separate lava flows formed the three falls along the Gooseberry River in Gooseberry Falls State Park, Minnesota. The dark rocks exemplify the basalt of ancient volcanic activity.
But first, let’s learn a little more about rifts that do and rifts that don’t.
“Over geologic time, continents collide and fuse together. They also split apart, along rifts,” explains the feature “New Insights into North America’s Midcontinent Rift” on Eos.com. “Rifts are linear features along which continents stretch. When a rift succeeds, the continent splits, and a new ocean basin forms between the two parts of the continent. Some rifts, however, fail to develop into seafloor spreading centers and instead leave major relict structures within continents – ‘fossils’ preserving the geologic environments in which they formed. The cliffs on the shores of Lake Superior – and the lake itself – are part of such a fossilized rift.”
Our rift took place within a mega continent called Laurentia, the ancient land template for North America that also contained modern Greenland and pieces of which floated off to become northern Ireland and Scotland. Recent theories suggest that rather than a split within Laurentia, our Midcontinent Rift may have involved a “microplate” within the larger plate or continent.
Whether at the edges of a microplate or in the middle of a larger one, during the rifting process – which can happen over tens of millions of years – the molten rock came from below the crust (upper mantle), flowing onto the surface, taking advantage of fissures in the thinned and stretched upper crust.
Volcano cones are the spectacular spouts of the earth, but this hot basaltic magma fluidly flowed out, occasionally squirting in great fire fountains, to cover a vast area, layer upon layer. The result was what geologists call a “large igneous province” that includes all the igneous rocks from lava flows and intrusions formed as part of the Midcontinent Rift System. The lava created “an unusually large amount and volume of new rock,” says John.
Today, that layering can be seen at Gooseberry Falls State Park, with its trifecta of falls – Upper, Middle and Lower. “Each of the falls is formed by a single lava flow,” John explains the three individual flows.
The color of our Lake Superior rocks and cliffs, especially along the Minnesota and Ontario shores, also echoes those ancient flows. The dark basalt rock that identifies our northern cliffs and the reddish rhyolite of Palisade Head or Iona’s Beach all are igneous, or volcanic, rock, the color variation coming from “lava with a different chemical makeup,” says John.
“Once those rocks are formed, many, many different things happen to them,” explains Pete Hollings, chair of the Lakehead University geology department in Thunder Bay. The rocks can become deformed, mangled and metamorphosed by heat and pressure from other geologic activity. They can be eroded by the elements or buried under sand and sediment from rains or dumping of rocks carried by glaciers.
When geologists talk about the age of rocks, they mean when rocks last formed (or reformed). One of the things that turns geologists’ cranks about the Lake Superior basin is how “old” our rocks are with many of the billion-year-old rocks formed by the Midcontinent Rift still visible.
“It's amazing how pristine they are - almost nothing has happened to them except burial since their eruption in the rift,” says John. Our Midcontinent Rift rocks at 1.1 billion years are quite old. Much older still, says John, are the greenstones and granites exposed around Ely and Tower,
Minnesota and the Boundary Waters (2.6 billion years) and on the Biwabik Iron Range from Hoyt Lakes to Grand Rapids (1.85 billion years).
The Midcontinent Rift itself probably took about 20 million years to form and seems to have spread about 100 kilometres, Pete says. He hails originally from England, arriving at Thunder Bay by way of Australia. He saw the chance to hang out, so to speak, with the remnants of the rift as one inviting attribute of this region.
“It’s a research interest of mine. … Geologically, this is an amazing area.”
Pete is not alone in his geologic enthusiasm for this region. He is secretary of the Institute on Lake Superior Geology, a loosely formed binational professional group that gathers annually somewhere around the Lake for a series of field trips. It’s 64th gathering will be in May at Iron Mountain, Michigan.
The basic view of the Midcontinent Rift (top) is that it extends from an arch at Lake Superior down to Kansas on its western leg and to Ohio on its eastern leg, but some researchers believe that new gravity mapping – variations in the Earth’s gravity from density differences in rock types – show rift remnants as far as Oklahoma and Alabama. The graphic, inset, shows gravity mapping done of the Midcontinent Rift System by the U.S. Geological Survey: “MRS volcanic rocks (shown as warmer, reddish colors) that occupy the center of the rift are more dense than the MRS sedimentary rocks (shown as cooler bluish colors) that flank both sides of the volcanics.” At left, a diagram from a Thunder Bay Geotour brochure shows a simplified story of the evolution of Lake Superior.
The group also gives out scholarships and annually awards the Sam Goldich Medal to a geoscientist with a substantial interest in and contribution to Lake Superior Geology. (John Green earned the honor in 2000.)
Knowing even a little bit about the Midcontinent Rift gives all of us the chance to be geotourists, noting structures with a volcanic past. You know now, for instance, how those three levels of waterfalls came to be at Gooseberry Falls. The High Falls on Pigeon River at the border between Minnesota and Ontario, cascade 120 feet down along a diabase (dark igneous rock) dike that formed when lava filled a large fissure in the thick layer of a shale-and-sandstone deposit left there earlier by an ancient sea. The diabase intrusion remains more resistant to erosion than the shale and sandstone around it.
PAUL SUNDBERG
Waves have a harder time eroding old lava, John Green says, enabling our basalt cliffs to withstand Lake Superior storms like this one pounding Tettegouche State Park in Minnesota. Also, John and Pete Hollings both note that the Lake’s cliffs along the Minnesota and Ontario shores tend to lean toward the water … actually leaning in toward the old center of the Midcontinent Rift. These lava flows became tilted because the rift axis sank as the crust was being pulled apart and the huge volume of lava was erupted from beneath the crust.
Pete can view an impressive rift remnant from his city’s waterfront. “Standing on the harbor in Thunder Bay looking out to Sleeping Giant, the shape is the diabase sill on top, protecting the sediments underneath.” The giant’s silhouette, rising 240 metres (nearly 800 feet) with the highest cliffs in Ontario, eroded from the flat-lying layer of diabase. A similar layer of diabase sill caps the lower shale layers of Mount McKay and the rest of the Nor’Wester Mountains outside the city.
Most of Isle Royale, Michipicoten Island in the northeast part of the lake, and the backbone of the Keweenaw Peninsula are also held up by resistant basaltic lavas of the Rift.
While evidence of the rift can be found around the Lake, deep beneath Lake Superior’s waters there also lies a base far different from the other four Great Lakes – 20 to 30 kilometres of basalt, the dark, fined-grained volcanic rock left from the time of the rift.
“We know that there is sort of an expression of the rift that runs all the way down to Kansas,” says Pete, “but the other Great Lakes aren’t rift related.”
The critical component that links all the Great Lakes, though, John points out, are their colder origins. “The other Great Lakes, as well as Lake Superior, owe their existence to Ice Age glaciation,” he says. The repeated return of massive glaciers, the last of which receded about 10,000 years ago, both carved out the softer rocks in each area to create the basins that filled with water as the ice melted away.”
The rift and the glaciers give Lake Superior a history of fire and ice that could make eager geonerds of us all.