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Time English (United States) Language 2 Action
00:00
In 1844, a group of government surveyors discovered something… weird.
00:05
They were trudging through Michigan’s Upper Peninsula, toward the shore of Lake Superior,
00:09
when suddenly, their compasses went haywire.
00:12
Now, I’ve seen enough horror movies for this to raise some red flags.
00:14
When the compasses get funky, there’s something funkier afoot.
00:20
What the surveyors found was a big surprise, though not a supernatural one.
00:24
It was a whole lot … of iron.
00:28
And that iron would change what we know about the history of the world.
00:32
Hi! I’m Sage, and this is Crash Course Geology.
00:35
[THEME MUSIC]
00:39
To get started, we have to turn back the clock.
00:41
And I don’t mean back to 1844.
00:44
I mean way back— before compasses, or Michigan, or human beings existed at all.
00:49
We’re going back in geologic time.
00:52
Geologic time, also called deep time, is the history of Planet Earth.
00:56
You know, no big deal.
00:57
It started about 4.6 billion years ago,
01:00
when a cloud of dust and gas smushed in on itself so hard it turned into a planet.
01:05
I’ll say that number again: 4.6 billion.
01:09
It’s hard to wrap your head around, right?
01:11
Like, if you started counting right now, you’d get to 4.6 billion in about 147 years.
01:19
Well, if you could magically live that long.
01:21
"The human brain isn’t built to understand numbers that big.
01:25
[Dwayne] Sage, you are far too young to explain geologic time.
01:29
Allow me.
01:30
[Sage] Whoa! Okay! You’re using your Big Voice!
01:33
So, um... alright!
01:35
Take it away, Dwayne.
01:36
[Dwayne] Let’s dig a little deeper…
01:39
The geologic timescale is broken down into several, different-sized portions.
01:45
The largest are eons, which last from hundreds of millions to billions of years.
01:51
Our story starts in the Hadean Eon,
01:54
when the earth was born and covered in an ocean of molten rock.
01:59
Today, we’re living in the Phanerozoic Eon.
02:02
Eons are broken down into eras, each hundreds of millions of years long.
02:08
And really hard to get tickets to.
02:11
For example, inside the Phanerozoic Eon, the Mesozoic Era was the famous “age of reptiles”
02:18
when dinosaurs, crocodiles, and pterosaurs roamed the Earth.
02:23
Eras are further broken down into periods, which last tens of millions of years,
02:28
and mark major geologic events,
02:31
like the extinction of the dinosaurs at the end of the Cretaceous Period.
02:36
In fact, all of Earth’s five major extinctions have marked the end of a geologic period.
02:43
Then there are epochs, lasting several million years.
02:47
Epochs capture shifts in characteristics like climate and biodiversity.
02:52
For instance, we’re now in the Holocene epoch,
02:56
characterized by human civilization and climate change.
03:00
Back to you, Sage.
03:02
[Sage] Wow, I think you have a career in voiceovers, Dwayne.
03:05
[Dwayne] Thanks pal.
03:06
[Sage] For the vast majority of geologic history, humans weren’t here—
03:10
the earliest members of our species only arrived on the scene some 300,000 years ago.
03:15
So how do we know all of this?
03:17
In our last episode, we talked about how layers of rock can help be a window into Earth’s past.
03:22
Geologists study those rocks to piece together the major events that shaped our planet.
03:26
To get an idea of how that works,
03:28
let’s take another look at what those guys dug up in the U.P.
03:30
That’s Michigan-speak for “Upper Peninsula.”
03:32
Shout-out yoopers.
03:34
What those government surveyors had stumbled on was this:
03:36
a banded iron formation, or BIF,
03:38
which was what threw off the magnets inside their compasses.
03:41
But it took decades to come up with a solid guess for where it came from.
03:45
The standard hypothesis these days is that, billions of years ago,
03:49
water-soluble iron floated throughout early oceans,
03:52
possibly because of volcanic activity on the seafloor.
03:56
At that point, there likely wasn’t much oxygen around.
03:59
But then, somewhere around two and a half billion years ago,
04:01
we got a lot of oxygen, all at once.
04:04
Possibly from bacteria in those early oceans photosynthesizing:
04:07
eating up carbon dioxide and spitting out oxygen.
04:10
That event is known as “The Great Oxidation.”
04:12
And oxygen loves to bond with iron— it’s why exposed iron gets rusty.
04:17
When dissolved iron is oxidized,
04:19
it changes color to a shiny black or reddish solid.
04:22
And it precipitates out of, or separates from, water.
04:26
So, all that new oxygen would have bonded with that dissolved iron
04:29
and sunk it down onto the ocean floor.
04:32
As for their distinctive zebra stripes,
04:34
those came from super iron-rich layers alternating with more silica-rich layers.
04:39
But, to this day, we don’t fully know why.
04:43
Maybe those layers formed because of bacterial growth cycles, or some other reason.
04:47
But BIFs stopped forming on a large scale around 1.85 billion years ago.
04:52
That means we don’t have modern versions of BIFs
04:55
to compare them to and help us answer those questions.
04:57
What we do know is that during the Great Oxidation,
05:00
so much oxygen eventually built up in the oceans,
05:03
that it rose out of the water and into the atmosphere.
05:07
This set the stage for increasingly complex life on earth—
05:11
from oxygen-breathing bacteria to multicellular plants, to human beings.
05:16
Banded iron formations have shown up all across the globe,
05:19
but there are still many mysteries about them.
05:21
Like, not all BIFs seem to have formed during the same period in exactly the same way,
05:26
including some that formed before the Great Oxidation.
05:29
Still, they’ve made a huge impact on the planet.
05:32
Most of the iron we use today, especially to make steel, comes from BIFs.
05:37
And they’re an exciting example of how
05:38
rock records can reveal our billions-of-years-long geologic history.
05:42
But we haven’t always known the earth was billions of years old.
05:46
In fact, we didn’t have an accurate age for our planet until the 1950s.
05:51
And it was a major shake-up.
05:53
Before that, most people believed—
05:54
for a variety of reasons, both scientific and religious—
05:57
that the planet was only a few thousand years old and was formed quickly,
06:01
as the result of a single event.
06:03
This belief is part of a school of thought called catastrophism,
06:06
which promotes the idea that sudden, violent events— like floods, volcanic eruptions,
06:11
or asteroid impacts— are what ultimately shaped our planet.
06:14
But in the 1700s, a Scottish geologist named James Hutton developed a new theory.
06:18
He said: “the past history of our globe must be
06:21
explained by what can be seen to be happening now.”
06:23
This theory became known as uniformitarianism:
06:26
the idea that the processes happening on Earth today
06:29
are the same processes that happened throughout its history.
06:32
In other words, Earth's many features were crafted by slow,
06:35
continuous processes, not one-time, dramatic events.
06:39
So, for many years, the pendulum swung from catastrophism to uniformitarianism—
06:44
especially thanks to an 1830 book called “Principles of Geology,”
06:47
written by Hutton stan Charles Lyell.
06:50
And uniformitarianism did help geologists learn about our planet.
06:54
Like the principle of faunal succession—