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1.1 billion years ago, long before the dinosaurs,
there wasn’t much going on in the world.
Many geologists even call this “The Boring Billion”, because
when it comes to the evolution of life and the conditions
on our planet, not a lot happened for a billion years!
Except...
In Laurentia, the continent that became central North America,
this time was anything but boring: The continent was busy trying —
and failing! — to rip itself in half.
Like, if it had succeeded, what is now Lake Superior would have been an ocean.
But instead, we are left with a huge scar across the land called
the Midcontinent Rift — and a story about how Earth changes,
and what might’ve been.
Looking at a map, it’s not obvious that what’s now
North America almost split in half.
Even scientists didn’t realize anything was suspicious until the 1940s.
Around then, geophysicists were mapping the strength of gravity
across the U.S.
We perceive gravity as being pretty much the same everywhere,
but really, it’s ever-so-slightly different from place to place.
A lot factors into that, but one big culprit is the density
of the rocks under your feet.
Denser rocks have more mass, so they have
a slightly stronger gravitational pull.
This is not something you would ever notice, but geophysicists
can measure this with an ultra-sensitive instrument called a gravimeter.
And it can give them some insight into what’s beneath the ground.
This is actually a pretty common practice, and these maps
are especially useful if you’re looking for oil and gas,
or for minerals to mine.
But… when the first detailed gravity maps of the middle
of the continent emerged, scientists noticed something was really off:
There was this huge region of stronger gravity extending from Ontario
all the way to Kansas.
There were some very dense rocks down there.
They called this the Midcontinent Gravity High.
And they realized it was connected to rocks they already knew about:
some dense, volcanic rocks at the surface around Lake Superior.
Soon, scientists figured out that these rocks actually extended
underground across the U.S.
In fact, with more research, they learned that the rocks went from
Oklahoma up to Lake Superior, then from Lake Superior
as far south as Alabama!
And the rocks themselves?
They were 1.1-billion-year-old flood basalts.
Flood basalts form when a massive amount of magma comes up through
the Earth’s mantle and erupts, flooding the surface with lava.
And eventually, that lava cools into a dark, dense rock — basalt.
In this case, they estimated that nearly two million cubic kilometers
of magma were responsible for these flood basalts, which, you know,
brings up a very good question:
What happened in North America a billion years ago?!
Well, since the 1950s, geologists have thought this strange formation
was a rift.
That’s a place where one of the tectonic plates making up
the Earth’s surface thinned and began to break into two.
But that still left more questions than answers.
Like, why did the rift start in the first place?
And why did it stop?
After all, when a rift starts, it can keep going until it forms a new ocean.
And there obviously isn’t one of those in the middle of the U.S. and Canada.
Also, where did all that basalt come from?
Most of the other rifts we know about aren’t filled with such thick
layers of volcanic rock, so why is the Midcontinent Rift so different?
Today, this is still an active area of research, and there are
many pieces of this puzzle we don’t quite know yet.
I mean, looking a billion years into the past is kind of a big project.
But by studying other rifts around the world and the flood basalts
in North America, geologists have put together a pretty good picture.
To understand what happened here, you have to know a bit about
what happens as continents move.
When continents break up and form oceans, they tend to follow
a fairly predictable pattern called the Wilson Cycle.
First, a rift forms, and a continent is pulled apart.
Hot material from the mantle rises and fills in this gap.
Today, the most famous example of this is the East African Rift,
which will likely form the Earth’s next ocean.
Second, if the rift is successful, it properly becomes an ocean!
So in addition to a bunch of water and fish showing up,
this means a mid-ocean ridge forms.
This is a line of undersea volcanoes near the center of an ocean
where a new seafloor — made of basalt — is created.
And it works a bit like a conveyor belt: Once the molten rock cools
into new ocean crust at the mid-ocean ridge,
it slowly moves outward to each side.
This is called seafloor spreading.
Third, a subduction zone forms at the edge of the ocean.
This is when the oceanic plate starts slipping beneath a continent
and gradually disappears into the mantle.
Fourth, the continents on each side of the ocean close in.
And finally, the continents smash together and close the ocean
entirely, often forming huge mountain ranges as they go.
1.1 billion years ago, the continents overall were at the end of this cycle
and were in the process of coming together into a huge landmass called Rodinia.
And in the midst of the continents smashing into one another,
one called Amazonia — which is now much of what we know as Brazil —
broke away from Laurentia.
This breakup happened right next to the Midcontinent Rift,
so geologists think this might be the key to understanding
why Laurentia almost split in two.
The current thinking is that, as Amazonia broke away,
it pulled and stretched Laurentia from the side.
That’s what’s called passive rifting, and it happens when
a sideways force pulls a continent apart.
This thins the crust and brings up hot material from below.
Rocks near the surface break, and those deeper down stretch and flow —
kind of like pulling apart a candy bar.
By looking at modern rifts like the one in Africa, geologists
have learned that this usually doesn’t happen cleanly:
Instead of breaking in two, a tectonic plate will often fracture
into smaller sections called microplates.
And the two arms of the Midcontinent Rift form the boundary
of the Illinois Microplate, which was directly connected to
the larger breakup of Laurentia and Amazonia.
But that still doesn’t explain the huge amount of flood basalts.
Instead, those are the remnants of an active rift.
Those happen when a hot plume from the mantle rises, heats,
and melts the crust above it.
This causes the crust to thin and rise up, weakening it
and causing it to break.
And in this case, it suggests that a mantle plume was
in the right place at the right time.
So as the plate stretched, magma filled the cracks from below —
and it continued to fill up even after the stretching stopped.
That means that, 1.1 billion years ago, if you were standing
in the middle of what’s now Minneapolis or Detroit,
you would not recognize anything about it.
Volcanoes extended across the continent, erupting lavas
that cooled into basalt — similar to what you see today in Hawai’i
or at the East African Rift.
And this wasn’t just, like, a dangerous year: This went on
for around 15 million years until, for some reason... it stopped.
If you do a quick Google search about this, you might see the ending
blamed on a collision with another landmass — one that might have
collided with Laurentia and, essentially,
pushed the continent back together.
And this collision did happen.
It’s called the Grenville orogeny — “orogeny” meaning
a collision between two plates that deforms them
and builds mountain ranges.
And it was part of the formation of Rodinia.
There’s even a line of deformed rocks called the Grenville Front,
which stretches through Ontario and Quebec and even matches up
with rocks in Scotland, since the Atlantic Ocean formed
much later than all this.
The Grenville Front actually intersects the Midcontinent Rift
in Michigan, too.
But in the last ten years, geologists have realized this old story
isn’t quite right.
After remapping the area and realizing they misinterpreted
some of the rocks, they now believe the Grenville Front likely formed
after the Midcontinent Rift stopped, and all that compression
happened much later.
So now, a more recent hypothesis suggests that when Amazonia
finally broke away, and an ocean started forming between it
and Laurentia, all that stress on Laurentia was relieved,
and the rifting ended.
We’re not sure exactly why this happens, but we do know
that it happens elsewhere.
Like, around 130 million years ago, the West Central African Rift System
cut right through the continent as Africa and South America
began to separate.
Except, once seafloor spreading began in the Atlantic,
rifting in central Africa stopped, and the continent remained intact.
Still, however it happened, the story of this rift was not over yet.
Because even after the drama was over, the Earth kept changing
— like it’s still changing all the time today.
After the volcanos stopped erupting, rivers gradually eroded the rock
into sedimentary layers that buried the basalts.
Then, much later, the entire area was compressed, and pushed upwards.
Glaciers eroded the sediments off the top of the volcanic rocks,
and carved out what is now Lake Superior!
So, even though we say it failed, the Midcontinent Rift
has a lot to be proud of!
It’s created some amazing landscapes in the eastern U.S.
and southern Ontario, and in fact, it is the deepest known rift
on Earth that didn’t form an ocean.
In the end, it’s a rare snapshot of what happens in the midst
of continents trying to break apart — and what happens
when they almost succeed.
In geology — and really, in just about everything else —
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