In 1844, a group of government surveyors discovered something… weird.

They were trudging through Michigan’s Upper Peninsula, toward the shore of Lake Superior, when suddenly, their compasses went haywire. Now, I’ve seen enough horror movies for this to raise some red flags.

When the compasses get funky, there’s something funkier afoot. What the surveyors found was a big surprise, though not a supernatural one. It was a whole lot … of iron.

And that iron would change what we know about the history of the world. Hi! I’m Sage, and this is Crash Course Geology. [THEME MUSIC] To get started, we have to turn back the clock.

And I don’t mean back to 1844. I mean way back— before compasses, or Michigan, or human beings existed at all. We’re going back in geologic time.

Geologic time, also called deep time, is the history of Planet Earth. You know, no big deal. It started about 4.6 billion years ago, when a cloud of dust and gas smushed in on itself so hard it turned into a planet.

I’ll say that number again: 4.6 billion. It’s hard to wrap your head around, right? Like, if you started counting right now, you’d get to 4.6 billion in about 147 years.

Well, if you could magically live that long. "The human brain isn’t built to understand numbers that big. [Dwayne] Sage, you are far too young to explain geologic time. Allow me. [Sage] Whoa! Okay!

You’re using your Big Voice! So, um... alright! Take it away, Dwayne. [Dwayne] Let’s dig a little deeper… The geologic timescale is broken down into several, different-sized portions.

The largest are eons, which last from hundreds of millions to billions of years. Our story starts in the Hadean Eon, when the earth was born and covered in an ocean of molten rock. Today, we’re living in the Phanerozoic Eon.

Eons are broken down into eras, each hundreds of millions of years long. And really hard to get tickets to. For example, inside the Phanerozoic Eon, the Mesozoic Era was the famous “age of reptiles” when dinosaurs, crocodiles, and pterosaurs roamed the Earth.

Eras are further broken down into periods, which last tens of millions of years, and mark major geologic events, like the extinction of the dinosaurs at the end of the Cretaceous Period. In fact, all of Earth’s five major extinctions have marked the end of a geologic period. Then there are epochs, lasting several million years.

Epochs capture shifts in characteristics like climate and biodiversity. For instance, we’re now in the Holocene epoch, characterized by human civilization and climate change. Back to you, Sage. [Sage] Wow, I think you have a career in voiceovers, Dwayne. [Dwayne] Thanks pal. [Sage] For the vast majority of geologic history, humans weren’t here— the earliest members of our species only arrived on the scene some 300,000 years ago.

So how do we know all of this? In our last episode, we talked about how layers of rock can help be a window into Earth’s past. Geologists study those rocks to piece together the major events that shaped our planet.

To get an idea of how that works, let’s take another look at what those guys dug up in the U. P. That’s Michigan-speak for “Upper Peninsula.” Shout-out yoopers.

What those government surveyors had stumbled on was this: a banded iron formation, or BIF, which was what threw off the magnets inside their compasses. But it took decades to come up with a solid guess for where it came from. The standard hypothesis these days is that, billions of years ago, water-soluble iron floated throughout early oceans, possibly because of volcanic activity on the seafloor.

At that point, there likely wasn’t much oxygen around. But then, somewhere around two and a half billion years ago, we got a lot of oxygen, all at once. Possibly from bacteria in those early oceans photosynthesizing: eating up carbon dioxide and spitting out oxygen.

That event is known as “The Great Oxidation.” And oxygen loves to bond with iron— it’s why exposed iron gets rusty. When dissolved iron is oxidized,  it changes color to a shiny  black or reddish solid. And it precipitates out of, or separates from, water.

So, all that new oxygen would have bonded with that dissolved iron and sunk it down onto the ocean floor. As for their distinctive zebra stripes, those came from super iron-rich layers alternating with more silica-rich layers. But, to this day, we don’t fully know why.

Maybe those layers formed because of bacterial growth cycles, or some other reason. But BIFs stopped forming on a large scale around 1.85 billion years ago. That means we don’t have modern versions of BIFs to compare them to and help us answer those questions.

What we do know is that during the Great Oxidation, so much oxygen eventually built up in the oceans, that it rose out of the water and into the atmosphere. This set the stage for increasingly complex life on earth— from oxygen-breathing bacteria to multicellular plants, to human beings. Banded iron formations have shown up all across the globe, but there are still many mysteries about them.

Like, not all BIFs seem to have formed during the same period in exactly the same way, including some that formed before the Great Oxidation. Still, they’ve made a huge impact on the planet. Most of the iron we use today, especially to make steel, comes from BIFs.

And they’re an exciting example of how rock records can reveal our billions-of-years-long geologic history. But we haven’t always known the earth was billions of years old. In fact, we didn’t have an accurate age for our planet until the 1950s.

And it was a major shake-up. Before that, most people believed— for a variety of reasons, both scientific and religious— that the planet was only a few thousand years old and was formed quickly, as the result of a single event. This belief is part of a school of thought called catastrophism, which promotes the idea that sudden, violent events— like floods, volcanic eruptions, or asteroid impacts— are what ultimately shaped our planet.

But in the 1700s, a Scottish geologist named James Hutton developed a new theory. He said: “the past history of our globe must be  explained by what can be  seen to be happening now.” This theory became known as uniformitarianism: the idea that the processes  happening on Earth today are the same processes that happened throughout its history. In other words, Earth's many features were crafted by slow, continuous processes, not one-time, dramatic events.

So, for many years, the pendulum swung from catastrophism to uniformitarianism— especially thanks to an 1830 book called “Principles of Geology,” written by Hutton stan Charles Lyell. And uniformitarianism did help  geologists learn about our planet. Like the principle of faunal succession— the idea that fossils of various organisms follow one after another in a specific order throughout geologic time.

That information came from the observations of fossils, especially by English surveyor William Smith. Because the uniformitarian view said geological processes had always worked the same, Smith could assume that the deepest-down layers of fossils were probably the oldest, and the ones toward the top were pretty new. Which meant that, if a rock contained a fossil, you could match it up with other fossils like it, and figure out when it formed.

But as time wore on, it became clear that catastrophism wasn’t entirely bunk science. Huge, one-time events do shape Earth’s geology, like the floods that formed the Channeled Scablands in Washington State. Earth’s landscape was shaped by both continuous processes and major catastrophes.

Now, proving those theories was the tricky part. Faunal succession gave us some early clues to the ages of rocks, but it could only tell us so much. It’s a type of relative dating, which involves looking at the relationships between rocks or fossils.

Like, it can tell you if  rocks are older or younger  than other rocks based on  which layer appears on top. But it can’t tell you how old they actually are. To do that, geologists now use absolute dating, a way of figuring out the age of rocks themselves based on how they change over time.

It’s a method of geochronology, or the study of when past geologic events occurred. The main type of absolute dating geochronologists use is called radiometric, or isotopic, dating. It’s about to get real science-y in here.

Here’s how it works. Isotopes are elements with different numbers of neutrons in their nuclei. And some of them are radioactive, which means their nuclei are unstable— over time, they decay.

The “parent” isotope gets replaced by its “daughter products,” the decayed form of the isotope. And thanks to lots of physics research, we know that replacement happens at a constant rate for each element, until the parent is totally gone. So when a rock contains radioactive isotopes, we can figure out how long it’s been decaying.

If there’s tons of the parent isotope in there, it must be pretty new. But if it’s mostly the daughter isotope, it must have been decaying for a while. And since we know exactly how fast stuff decays, we can figure out exactly how long.

Absolute dating is what lets us be absolutely sure about the age of rocks. But not all rocks have the elements required for absolute dating. So both relative and absolute dating can be used to figure out how old a rock is, or when a geologic event happened.

Radiometric dating was what allowed American geochemist Clair Patterson and his colleagues to accurately estimate the earth’s age for the first time in 1955. They compared the radioactive breakdown of uranium on Earth's surface and in meteorites. Super chill stuff.

Knowing that Earth is around 4.6 billion years old completely shifted the way we think of time and our place within it. ‘Cause like I said at the  top, it’s almost impossible  for humans to conceive of such a number. To the earth, we’re just… a blip. What was I saying?

But some scientists say that actually thinking about deep time and your relationship to it can be kind of… good? Not in a “99 percent of   doctors say considering your own mortality decreases your chance of heart attack” kind of way. More like: thinking about deep time can provide us some much needed perspective.

Like, think of your neighborhood. Imagining what it might have looked like millions of years ago— or what it’ll look like millions of years from now— can provide you a sense of awe and wonder that you can carry into the rest of your life. From banded iron formations to radioactive meteorites, rocks have a lot to teach us about the past 4.6 billion years on Earth.

Looking back into deep time can be kind of spooky, but it’s one of the coolest ways we can understand the planet’s past, present, and future— and our place in it. Next time, we’ll check out some of the best records of life on

Earth: fossils. See you then. Thanks for watching this episode of Crash Course Geology, which was filmed at our studio in Indianapolis, Indiana, and was made with the help of all these nice people. If you want to help keep Crash Course free for everyone, forever, you can join our community on Patreon.