Gary McGuffin
Slate Island rises as an impressive reminder of the Earth’s ancient life that included meteorite strikes, which may have formed the Slates archipelago now found in Lake Superior. Adventurer/author Joanie McGuffin of Goulais River, Ontario, (seen here) lists her Slate Islands favorites as “the world’s tallest shatter cone and the rare woodland caribou.”
The strike of the Slate-making meteorite likely took place while the Earth’s land mass was still all or part of a “supercontinent,” shown in this illustration by Italian artist Massimo Pietrobon and indicating the future geopolitical divisions, such as North America and the future Great Lakes. The second map locates the Slates on the Lake.
Once upon a time some 450 million years ago, a nearly 1-mile-wide meteorite crashed into the Earth just south of the equator, sending out ripples through liquefied rock and creating a sort of giant bundt-pan indentation in the land.
And that, dear readers, may be how Lake Superior came to inherit the Slate Islands.
“Hold on there,” I hear you saying. “How does a meteorite hitting Earth some 4,000 miles (or 6,500 kilometres) south of here explain a unique collection of inland sea islands?”
Well, this Lake Superior basin bedtime story was a long time in the making.
The land base of Lake Superior is believed once to have been on the edge of the North American proto-continent called Laurentia, which was itself a breakaway portion of an even larger supercontinent concentrated near the equator.
That land mass was surrounded by shallow, tropical seas filled with coral reefs and teeming with marine life like squid and octopi. Sediments in these shallow seas became limestone and other sedimentary rocks still found in our region.
Back about that time – as the theory proposes – this land was hit by a large meteorite, perhaps 1.5 kilometres (nearly a mile) in diameter, traveling at speeds of 15 to up to 50 kilometres per second (about 9 mps). Upon impact, the meteorite itself vaporized. Most of its kinetic energy transferred to the surrounding rocks, which were instantaneously heated and vaporized or liquefied.
High-pressure shock waves traveled through the rocks, forming shatter cones (more on these later) and causing fracturing and fragmentation of the rocks.
As solid material was ejected by the impact, a large crater formed. At the center of the crater, the elastic rock bounced back up, giving rise to high points of land.
Fast forward (really forward, about 450,000 millenia) and Laurentia has shifted north and morphed into the present-day continent of North America, which about smack dab in its center holds Lake Superior, the freshwater gift of glaciers that helped to carve out and fill up the Lake basin. Yet geologic echoes of that early meteorite strike may remain, a pretty exciting prospect for geologists.
“We think of things happening over millions of years,” says Pete Hollings, professor of geology at Lakehead University in Thunder Bay, “but impacts like this happen in minutes. To capture a point in time like that is a very cool thing.”
That “point in time” generated the base for an eight-isle archipelago known as the Slate Islands about 8 kilometres (5 miles) south of Terrace Bay, Ontario.
Kayakers, campers and hikers visit the Slates for the scenery, the caribou, the rustic venue and the rare plants usually found only in the Arctic.
Geologists travel there for one of the most spectacular assemblages of visible meteorite-impact features on the planet, making the Slate Islands a field trip back in time where visitors can view the results of a single moment from hundreds of millions of years ago.
“The Slates are now regarded as the best-preserved, medium-sized impact structure in the world,” says geologist Mark Smyk of the Ontario Geological Survey in Thunder Bay.
Topographical maps or satellite images indicate that bundt-pan indent. “If you look out into the water around the Slate Islands, you’ll see a slight rise in the floor of the Lake,” Mark says of such images. “That’s presumed to be the crater rim.”
The crater is about 30 kilometres (18.6 miles) in diameter, Mark explains, with the edge close to the mainland.
It’s easy to imagine the whole scenario because it’s played out on a small scale every summer day on the shores of the Lake when a child tosses a rock into the water. The impact can cause a rise in the water right where it hits, followed by ripples of water surrounding it.
“With a meteor this size,” Mark says of the probable Slate Islands impact, “a lip of excavated rock is formed around the edge of the crater, and the center of the crater rebounds.” This gives rise to what is called a complex crater, with this central uplift. Most of the meteorite-impact crater lies under water, but this uplifted area became the Slate Islands themselves.
Much of the evidence for this theory comes from impact-related geologic features sprinkled around the Slates.
Richard Main Photography
Two impressive structures on the Slate Islands count as “tallest.” On Patterson Island, the 1903 Slate Islands Lighthouse, left, is 36 feet tall and stands elevated 224 feet above the Lake, making it the highest-placed light on the Great Lakes.
Tourism that rocks
In 2006, the Institute on Lake Superior Geology published the Field Trip Guidebook to the Slate Islands, and since then has offered one-day field trips to the Slates. The institute is a non-profit organization of Canadian and American geologists with the goal of promoting better understanding of the geology of the Lake Superior region.
Gary McGuffin
Meanwhile, also on Patterson, one of the two main Slate Islands along with Mortimer, a 10-metre (32-foot) tall remnant of an ancient meteorite strike is believed to be the world’s tallest known shatter cone
Mark and Pete have also offered trips to the islands at the request of academic institutions, researchers and others intrigued by the archipelago’s geology.
Field-trip stops outlined in the guidebook provide the best-exposed and most accessible examples of many impact features on the islands’ shores.
There are two main features indicating a meteorite impact.
First are breccias, fragmented rocks that abound in the Slates. Breccias are created in the instant after a meteorite impact, when the host rock fractures and liquefied, vaporized rock is injected into it.
All this creates a kind of rock stew, so when the main molten rock finally chills and hardens, it traps the foreign fragments in it, cementing larger pieces together.
Mark compares breccias to a log jam on the river, when logs moving in the water are suddenly frozen in ice and suspended.
The 1-mile-wide meteorites that likely created the Slate Islands probably made a less spectacular splash than the asteroid in this painting by Don Davis for NASA, though it gives you the idea.
“One of my predecessors said that they should have called them the breccia islands,” Mark says, referencing breccia’s variety of shapes, sizes and colors, ranging from grey, red, black to orange. “You can’t go anywhere without seeing breccia of some sort.”
Geologists have named and classified distinct breccias on the Slates, studying them in minute detail to determine the mechanisms that formed them.
This graphic (left) shows the geologic rebound that eventually became the Lake Superior archipelago over many millenia. The graphic, used in the Field Trip Guidebook for the Slate Islands, was adapted from one in a 1999 study of the islands led by Burkhard O. Dressler.
And then there are the megabreccias, areas where blocks of rock more than 10 metres (about 32 feet) across have been moved around and cemented back into place as a result of the impact. These breccias, too, are found throughout the Slates, and it is possible that some of the islands themselves might be giant blocks thrown around by the impact.
Shatter cones are the other important impact feature found throughout the Slates.
Although their formation is still poorly understood, shatter cones are associated with more than half of the known meteorite impact sites on Earth.
These distinctive multi-conical features, explains Mark, usually have horse tail-shaped grooves on their margins that are formed by the passage of very high-pressure shock waves spreading through the host rocks immediately after impact. The kind of rock through which the shock waves travel may determine the size and shape of the shatter cones. Basically, shatter cones call to mind a fusing together of upside-down ice cream cones (without the ice cream).
NASA scientists who have studied shatter cones in the Slates believe they’ve found one that rises from the Lake and measures 10 metres (about the height of a three-story building), making it the world’s largest example of a shatter cone.
One reason geologists flock to these islands is because the Lake’s frequent, expansive waves clear the shores of vegetation and give visitors unimpeded views of these impact features.
“You usually don’t get the chance to see these types of rocks that often,” says Pete. “It’s spectacular the way the waves have exposed these rocks.”
Pete describes how glaciation probably shaved some rocks down, and the waves continue to keep them smooth and eroded.
“So the outcrop (exposed bedrock) is just beautiful.”
Other outcrops are underwater, Mark says, “but because Lake Superior is so clear, we can still see them really well. Vegetation inland would obscure them.
“You usually don’t get that on land,” Mark adds. “It’s part of the allure of the Slates. People are blown away by how well they’re exposed. It gets you thinking about the force required to move rocks like that.”
Gary McGuffin
No matter their origins, the beauty of the Slate Islands is unearthly.
From the sky, or pie in the sky?
Geologic field trips to the island almost always stop at the site considered to be the world’s largest shatter cone.
It’s a good place for a pause, because it’s also part of an ongoing debate about both the shatter cone itself and about whether a meteorite really created these islands.
Mark C. Smyk
Megabreccias rest beneath the waters at Honeymoon Bay near Cove Island.
“We’ve gone back and forth as to if it is a shatter cone or not,” says Pete.
He himself fluctuates, sometimes convinced it is a shatter cone, and then, looking at a different angle, he’s certain it’s not.
Small shatter cones appear as a two-dimensional cut through a cone, making it easy to identify them as a shatter cone, says Pete. “For the mega cone, there is an outline of a cone which could be 10 metres tall in the right light, but when I look at it from a different angle
I can convince myself that it is just the way the rocks are fractured.”
According to Mark, because shatter cones in the Slates are typically small (3 to perhaps 25 centimetres long), it is usually easy to identify a partial cone, nested cones, or visualize an entire cone.
With the purported world’s largest shatter cone, says Mark, it is more difficult to visualize the entire cone structure because of its size and relative inaccessibility. “It’s a tough outcrop to get up-close and personal with,” he says. Fractures and folded local rocks can sometimes intersect to produce curved surfaces that may mimic the conical outline of a shatter cone.
“What you see and visualize is hugely dependent on light conditions and sun angle,” Mark says. “Although the debate at McGreevy Harbour persists, we are obliged to report what the NASA experts have interpreted in encouraging discourse on the subject.
“It’s geology,” he adds. “A lot of it is very subjective. There is always a right answer, but it’s not always obvious.”
The extraterrestrial influence on formation of the Slates also remains a topic of debate. Some scientists believe ancient volcanic processes were responsible for the unusual formations, but the more generally accepted theory is that these special rock formations are indeed related to a meteorite impact.
Some researchers have recently attributed the Slate Islands impact to what has been termed “the Middle Ordovician breakup event,” which occurred during the Ordovician Period about 470 million years ago. It’s thought an asteroid, some 200 kilometres (120 miles) in diameter was thrown out of its orbit by a collision in the Main Asteroid Belt, producing fragments that fell into Earth-crossing orbits. This would have resulted in a high number of impacts and cratering on Earth over the next several million years. Researchers have suggested that the Slate Islands structure is one of 10 known impact sites of similar geologic age in present-day North America and northern Europe likely related to that breakup.
“Researchers continue to visit the Slate Islands,” says Mark, “to look at specific features and test rocks with ever-more sophisticated and precise analytical techniques in order to better constrain the age and nature of this impact.”
The potential extraterrestrial origin of the Slate Islands will continue to capture the imagination. In fact, according to Mark, there’s been renewed interest in asteroids and meteors in general since a connection was drawn between the extinction of the dinosaurs and the impact near today’s Yucatan Peninsula of a gigantic asteroid remnant. Public interest has been ignited by both the scientific findings and the subsequent Hollywood disaster movies speculating on meteorites falling today.
From a scientific perspective, the discovery just a decade ago of rock thrown from an impact crater near Sudbury in the area surrounding Lake Superior has inspired growing research and interest in this subject.
“The idea of meteor impacts resonates with geologists of all kinds, as well as with the public at large,” says Mark. “It may be that it dramatically demonstrates our connection with the cosmos. Everyone is awestruck when they imagine the forces and processes that had to be involved in the creation of such features. It staggers the imagination!”
Hurtling Objects in a NASA Nutshell
Just to note, what is “small” to NASA in the universal scheme of things may be quite large to the rest of us Earthlings.
Asteroid: A relatively small, inactive, rocky body orbiting the Sun.
Meteoroid: A small particle from a comet or asteroid orbiting the Sun.
Meteor: The light phenomenon that results when a meteoroid enters the Earth’s atmosphere and vaporizes; a shooting star.
Meteorite: A meteoroid that survives its passage through the Earth's atmosphere and lands upon the Earth’s surface.
SOURCE: NASA
Good to Know
The Slate Islands are a provincial park, only accessible by boat or kayak, either on your own or part of a tour. There are no visitor facilities, though boaters, kayakers and hikers can enjoy the park if they are prepared for a rugged environment. Removing any material is prohibited without a permit.
Visitors should keep in mind that weather conditions on the Lake can shift suddenly, making it difficult to return to the mainland. “It is Lake Superior, so people have to be careful,” says Lakehead University Professor Pete Hollings. “We’ve heard stories from colleagues who ended up spending three days on the island before they got back.”
Find more about the geology and a PDF of the Field Trip Guidebook to the Slate Islands at www.lakesuperiorgeology.org.
For park details, go to ontarioparks.com/park/slateislands or call 807-825-3403.
There are charter services or guided kayak tours to the Slates: Discovery Charters (www.discoverycharters.ca) in Rossport, Bluebird Charter (www.bluebirdcharterboat.com) in Schreiber or for kayaking, Naturally Superior Adventures (www.naturallysuperior.com) in Wawa and Wilderness Inquiry (www.wildernessinquiry.org) out of Minneapolis.
Felicia Schneiderhan is a Duluth-based writer who dreams of traveling to the Slates with her family aboard their 38-foot trawler, Mazurka.