Thanks to Brilliant for supporting this episode
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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 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 — 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
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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
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Subscription. [♪ OUTRO]
