In short · Summary by AI
This Crash Course Geology episode explains how geologists determine Earth's age, using banded iron formations and radiometric dating as examples. The main conclusion is that Earth is about 4.6 billion years old, a figure first accurately estimated in 1955 by Clare Patterson and colleagues, and that rock records reveal this deep history.
- In 1844, government surveyors in Michigan's Upper Peninsula found their compasses disrupted by banded iron formations, which later helped reveal how the early Earth changed.
- Earth formed about 4.6 billion years ago, and the geologic time scale is divided into eons, eras, periods, and epochs.
- Banded iron formations likely formed when oxygen from early bacterial photosynthesis, during the Great Oxidation about 2.5 billion years ago, bonded with dissolved iron and sank it to the ocean floor.
- Before the 1950s, many people believed Earth was only a few thousand years old and shaped mainly by sudden catastrophes, a view called catastrophism.
- In the 1700s James Hutton proposed uniformitarianism, the idea that slow processes seen today shaped Earth's history, which Charles Lyell later popularized in his 1830 book Principles of Geology.
- William Smith's use of fossil succession allowed relative dating of rock layers, but it could not give exact ages.
- Radiometric dating, which measures the decay of radioactive isotopes at constant rates, allowed Clare Patterson and colleagues to estimate Earth's age accurately in 1955.
- The episode notes that thinking about deep time can give useful perspective and a sense of wonder.
- 0:00Surveyors and Mysterious Compasses
- 0:46Introducing Geologic Time
- 1:30Eons, Eras, Periods, and Epochs
- 3:10How Rocks Record the Past
- 3:46Banded Iron Formations Explained
- 5:45Catastrophism Versus Uniformitarianism
- 7:02Relative Dating and Fossil Succession
- 8:16Radiometric Dating Explained
- 9:31Earth's Age and Deep Time
Surveyors and Mysterious Compasses
0:00In 1844, a group of government surveyors discovered something weird. They were treging 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.
0:19A foot. 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. [music] To get started, we have to turn back the clock. And I don't mean all the way back to 1844.
0:42I mean way back before compasses or Michigan or human beings existed at all. We are going back in geologic time. Geological 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 smooshed in on itself so hard it turned into a planet.
Introducing Geologic Time
1:05I'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. Sage, you are far too young to explain geologic time.
1:26Allow me. >> Whoa. Okay. Uh, using your big voice. So, um, all right. Take it away, Dwayne. >> Let's dig a little deeper. The geologic time scale 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 Hadian Eon when the Earth was born and covered in an ocean of molten rock.
Eons, Eras, Periods, and Epochs
1:57Today we're living in the Fannerizoic 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 phenorizoic Don, the Mesazoic era was the famous age of reptiles when dinosaurs, crocodiles, and terasaurs roamed the earth. Eras are further broken down into periods which last tens of millions of years and mark major geological events like the extinction of the dinosaurs at the end of the Cretaceous period.
2:34In fact, all of Earth's five major extinctions have marked the end of a geologic period. Then there are epochs lasting several million years. Epoch capture shifts in characteristics like climate and biodiversity. For instance, we're now in the holene epoch characterized by human civilization and climate change.
2:58Back to you, Sage. >> Wow. Uh, I think you have a future in voiceovers, Dwayne. Thanks, pal. >> For the vast majority of geological 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.
How Rocks Record the Past
3:21Geologists 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 UP. That's Michigan speak for upper peninsula. Shout out Yupers. What those government surveyors had stumbled on was this abandoned iron formation or Biff, which is what threw off those magnets inside their compasses.
3:40But 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.
Banded Iron Formations Explained
3:57But 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.
4:17When dissolved iron is oxidized, it changes color to a shiny black or a reddish solid. And that 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 the super ironrich layers alternating with more silicar layers.
4:38But to this day, we don't fully know why. Maybe those layers formed because of bacterial growth cycles or for some other reason. But biffs stopped forming on a large scale around 1.85 billion years ago. That means we don't have modern versions of biffs to compare them to. And to 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 water and into the atmosphere.
5:05This set the stage for increasingly complex life on Earth. From oxygen breathing bacteria to multisellular plants to human beings. Bandit iron formations have shown up all across the globe. But there are still many mysteries about them. Like not all biffs seem to have formed during the same period in exactly the same way, including some that formed before the great oxidation.
5:28Still, they've made a huge impact on the planet. Most of the iron we use today, especially to make steel, comes from biffs, and they're an exciting example of how rock records can reveal our billions of years long geological history. But we haven't always known the Earth was billions of years old. In fact, we didn't even have an accurate age for our planet until the 1950s.
Catastrophism Versus Uniformitarianism
5:51And it was a major shakeup. 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 the 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.
6:14But 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 processes happening on Earth today are the same processes that happened throughout its history.
6:30In 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 principles of geology written by Charles Lyall himself a huge Hutton stan and uniformitarianism did help geologists learn about our planet like the principle of fondal succession the idea that fossils of various organisms follow one after another in a specific order
Relative Dating and Fossil Succession
7:01throughout geological 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 deep down layers of fossils were probably the oldest and the ones toward the top were pretty new.
7:18Which meant that if a rock contained a fossil, you could match it up with 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 form the channeled scablands in Washington State.
7:35Earth's landscape was shaped by both continuous processes and major catastrophes. Now, proving those theories was the tricky part. Final succession gave us some early clues to the ages of the 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.
7:56like 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 use absolute dating, a way of figuring out the age of rocks themselves based on how they changed over time. It's a method of geocchronology, or the study of when past geological events occurred.
Radiometric Dating Explained
8:16The main type of absolute dating geocchronologists use is called radiometric or isotopic dating. It's about to get real sciency 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.
8:35The parent isotope gets replaced by its daughter products the decayed form of the isotope and thanks to lots of physics research we know that the 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.
8:55If 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.
9:16But 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 geological event happened. Radiometric dating is what allowed American geochemist Clare Patterson and his colleagues to accurately estimate the Earth's age for the first time in 1955.
Earth's Age and Deep Time
9:38They compared the radioactive breakdown of uranium on Earth's surface and in meteorites. You know, super chill stuff. Knowing that the Earth is around 4.6 6 billion years old completely shifted the way we think of time and our place within it. Cuz like I said at the top, it's it's almost impossible for humans to conceive of such a number.
9:54To the earth, we are just a a blip.
10:06What 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% 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.
10:21Like think of your neighborhood. Imagining what it might have looked like millions of years ago or what it might 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.
10:41Looking back into deep time can be kind of spooky, but it's one of the coolest ways we can understand our planet's [music] 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, was made with the help of all these nice people.
11:04If you want to help keep Crash Course free for everyone forever, you can join our community on Patreon.
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