Origins Of The Great Lakes: Lake Superior

We live right next to some of the largest sources of freshwater on the planet — the Great Lakes!

But how did these massive lakes get here, and what kinds of processes went into creating these gorgeous lakes?

A recap on how the Great Lakes formed

Head over to our previous article in this series to find out exactly what went into the formation of Lakes Huron, Erie and Michigan, as we’ll touch on that more later in this article, too. 

But for a quick recap, we’ll give you the rundown. 

Around 385 million years ago, this continent was located below the Earth’s equator and, as such, had a vastly different climate. The region around these three great lakes was covered by a warm, shallow sea that, over time, deposited organic material onto the sea floor. That organic material turned into limestone, and that limestone formed a type of basin in which future sediments and deposits would accumulate.

Flash forward to 25 million years ago when the continent had moved to its current position. Three large rivers flowed from this limestone basin area into the Atlantic Ocean, leaving behind large valleys that had been gouged into the landscape by the powerful flowing waters. 

The last ice age lasted between 115,000–11,700 years ago, during which a large ice sheet called the Laurentide Ice Sheet covered most of the upper half of the continent. As this ice sheet began to recede, it scraped away the surface of the crust and left large pools of melted water accumulated behind sandy deposits.

Because of the limestone basin formed from the shallow sea, water was trapped in the non porous rock and, thanks to the river valleys, collected in large lakes. That’s why there are so many lakes around the upper Midwest and not as many around the rest of the country.

But Lake Superior has a totally different origin story — one that dates much farther back than a mere 385 million years ago!

How Lake Superior formed

Lake Superior is the world’s largest freshwater lake by surface area and holds 10% of the planet’s fresh water supply. It has a fiery and catastrophic history that is all tied to one especially unique piece of the world’s history called the Midcontinent Rift. 

The Midcontinent Rift system

The Midcontinent Rift is a band that stretches from Kansas up through Nebraska, Iowa, Wisconsin, Michigan, Minnesota, through Lake Superior and down into central Michigan and Detroit. This band is the ancient evidence of when the North American continent failed to tear itself apart almost 1.1 billion years ago, but let’s not get ahead of ourselves!

The world back then

Around 1.1 billion years ago, the continents as we know them today were still in their early stages of forming. The continent Laurentia was what would eventually become the central and oldest part of the North American continent (hence the name of the Laurentide Ice Sheet!), and it was attached to a smaller continent to the south known as Amazonia. 

Spoiler alert, that continent would eventually become Brazil would contain the Amazon River!

Anyway, Laurentia was located somewhere around the equator at that point, with many of the other continents surrounding it in one supercontinent known as Rodinia. The world was still coming together, and not a whole lot was happening yet, either. The dinosaurs were still millions of years away, and many scientists call this period of the Earth’s history “the boring billion” because not much of import was happening.

Except for the fact that Laurentia was in the process of tearing itself in two, of course. 

Trying to make a new continent

Continents tearing in half is not anything new, and it’s something that other continents had already been doing for a long time.

Tectonic activity sometimes splits continents along weak points so that the continent separates into smaller pieces. When the land does split, magma flows up from the mantle and volcanic debris fills in the gaps. Over time, water fills the space and is kept in place by the dense volcanic rock, forming new oceans or seas. 

A great example of this is the Red Sea that was formed after the East African Rift caused the Arabian Plate, African Plate and Indian Plate to move away from each other, breaking the crust and filling the space with ocean. This formed the Red Sea and the Gulf of Aden, and it separated modern-day Saudi Arabia and Yemen from Sudan, Eritrea, Djibouti and Somalia.

This continental split and resulting ocean-formation is what Laurentia was trying to do, so why do we not have an ocean in the middle of the continent?

Almost two continents

Back in the Supercontinent of Rodinia, the Amazonia continent was trying to break apart from Laurentia. This put an incredible amount of stress on Laurentia (after all, breakups are hard, right?) and Amazonia’s separation was creating passive rifting in Laurentia's crust, which occurs when a force pulls the crust sideways and it starts to thin in weak areas. Eventually, the crust tore and magma began flowing back and forth under the broken crust, spouting out at any opening. It’s kind of like tearing a Snickers bar, actually, where the chocolate breaks but the caramel is all stretchy and connected. 

For the next 15 million years, volcanoes exploded constantly along this rift, spouting fire and fury onto the landscape. There is evidence of these volcanoes all over, especially in the lake beds where basalt rocks are found right and left all around the Great Lakes.

Why don’t we have volcanoes there now? Well, scientists and geologists believe that once Amazonia finally broke free from Laurentia (the worst and ugliest break up in history, for sure) all the stress that had been pulling the continent apart finally ceased. The Midcontinent Rift stopped spreading and exploding, and Laurentia finally had the chance to sit back and think about its future without its toxic ex constantly bothering it. 

This period of volcanic activity left behind thick layers of basalt and other volcanic rock that were eventually covered by sedimentary rock and other debris. In the end, this rift is the largest one that we know of that never became an ocean. It was probably the world’s biggest failure up to that point. 

At least, it was until the newest Star Wars movies came out. But we won’t talk about that right now. 

Creating the lakes

Flash forward to the end of the ice age around 12,000 years ago. The Laurentide ice sheet was retreating, scraping away the crust below and leaving massive amounts of water in its wake. The sediment above the rift eroded away and the hard basalt made from all those years of volcanic activity trapped the glacial water in its dense walls. 

Thus, Lake Superior was formed! It's the deepest of all the Great Lakes and is incredibly rich in minerals and natural resources, thanks to its long and violent history. And all it took was a billion years of constant stress, painful breakups and a couple million years of terrifying volcanic activity!

Now, we get to enjoy Lake Superior in its most peaceful state, free from magma and continental tears. If you ever head over there from our luxury Chicago apartments, keep an eye out for all the smooth, black basalt pebbles that litter the lake bed — the reminder of an ocean that never came to be and another continent that almost got to exist. 

Enjoy!

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Featured photo courtesy Pixabay/havenshell