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  • Thanks to Brilliant for supporting this episode of SciShow.

  • Go to Brilliant.org/SciShow to check out their new course,

  • Knowledge and Uncertainty.

  • [♪ INTRO]

  • 1.1 billion years ago, long before the dinosaurs,

  • there wasn't much going on in the world.

  • Many geologists even call thisThe 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 Riftand 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.

  • Butwhen 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 rockbasalt.

  • 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 seafloormade of basaltis 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 Amazoniawhich 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 basaltsimilar 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 landmassone that might have

  • collided with Laurentia and, essentially,

  • pushed the continent back together.

  • And this collision did happen.

  • It's called the Grenville orogeny — “orogenymeaning

  • 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 apartand what happens

  • when they almost succeed.

  • In geologyand really, in just about everything else

  • there's a lot we don't know.

  • That's just part of this whole human experience thing.

  • But if you want to make your life feel a little more certain,

  • you might like Brilliant's brand-new Knowledge and Uncertainty course.

  • It gives you mathematical tools for figuring out things

  • I did not realize you could figure out with math.

  • Like, you can put a number on how much you don't know.

  • Or maybe you want a formula for figuring out how new information

  • should change the things you believe.

  • Apparently that exists!

  • You can learn more at Brilliant.org/SciShow.

  • And if you're one of the first 200 people to sign up there,

  • you'll also get 20% off an annual Premium Subscription.

  • [♪ OUTRO]

Thanks to Brilliant for supporting this episode of SciShow.

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