Archaeological Dating

The Earth is 4.6 billion years old. How did scientists come up with this number?

This story was created for the Google Expeditions project by Vida Systems, now available on Google Arts & Culture.

Archaeological Dating by Vida Systems

And how do we know that dinosaurs showed up 300 million years ago or that the continents have moved repeatedly since they first formed? Let's take a look at archaeological dating.

Introduction

When we think about Mother Earth, we imagine a pretty stable world of static continents, with ice at the poles, and a pretty steady environment otherwise. But this hasn't always been the case. Over the past 4.6 billion years, our continents have broken apart and merged to form new, bigger continents.

At times, most of the land on Earth was one gigantic desert and at other times, it was almost completely covered in ice!

Ice Crust

About 716 million years ago the Earth was covered in ice, even at the equator. This wasn’t just a thin sheet of ice but was almost half a mile thick. 

Pangaea

When the continents merged to form Pangaea 300 million years ago, it was such a giant landmass that the water cycle could not take place in the interior of the continent. Therefore, most of the land on Earth was desert. 

The Shape of Life

Throughout the Earth’s history, many species have been preserved as fossils in rocks, which reveals information about our past. Scientists use 3 main dating methods to learn about the past through rocks and fossils: relative age dating, absolute age dating, and magnetism. 

Relative Age Dating and Absolute Age Dating

There are 2 basic methods we use to find out the age of rocks or fossils: absolute age dating and relative age dating. The difference between these 2 methods is simple; relative age dating is simply looking at how old something is in relation to another thing and absolute age dating finds out the exact or near age of something.

Relative vs. Absolute

If we look at you and your grandfather, we can say, using relative dating, your grandfather is older than you. If we were using absolute age dating, we would say your grandfather is 88 years old and you are 16. 

Relative Is Easier

It is obviously a lot simpler to date things using relative age than absolute age dating. Initially, relative age dating was the only way scientists had to date ancient substances.

Mixed Is Better

However, these days, scientists use a mixture of both dating methods. This way we are able to discover the near absolute age of many items or fossils that are millions or even billions of years old.

Relative Age Dating — The Principle of Superposition

Did you ever wonder why, when you look at dramatic mountains, you can see many, many layers of horizontal stripes? Well, these layers are called strata and they exist because mountains are in fact layers and layers of rock, built up over millions of years. 

Rock Layers

Strata is incredibly useful for us to measure the ages of sedimentary and volcanic rock. Rock layers are built up over time by the movement of Earth’s crust, constantly depositing more material onto the Earth. 

Old Rocks

Over time, more and more rock is thrown upon the Earth, therefore we know that the layers at the bottom of rock formations are much older than the layers at the top.

Grand Canyon

The Grand Canyon strata ranges between 200 million and 2 billion years old. At the top we see the youngest rock, formed a mere 200 million years ago, and at the bottom, rock that has existed for half the life of Earth itself!

Relative Age Dating — Correlation By Fossils

Dinosaurs and trilobites were not friends. That makes sense: dinosaurs were large, mostly lived on land, and trilobites were small and lived in the sea. These are definitely conditions for an unlikely friendship. 

But that’s not the reason; the real reason is they never actually met. The trilobite became extinct 450 million years ago and the dinosaurs only appeared 300 million years ago. 

Species Extinction

Every species that has ever roamed the Earth has only existed for 2–3 million years before it became extinct. Reasons include a mass extinction event and environmental factors, such as a food source being removed or temperature changing dramatically. 

Relative Dating

Scientists have a general idea of what species existed and when. If we find a rock layer with a certain type of fossil and then find another rock layer with the same type of fossil, we can correlate it and say it is from the same age.

Index Fossils

Fossils that are used to tell one rock layer from another are called index fossils. The best index fossils are common, easy to spot, and have lived for a short time, so they appear in only one rock layer, like rodents and pigs.

Absolute Age Dating Radiometric Methods

Absolute age dating is an incredible method scientists use to find out almost precisely the age of a certain fossil or rock. By measuring how much of a certain radioactive material is remaining in something, we can accurately determine how long ago it existed.

Radioactive Transformation

Radioactive materials consist of unstable atoms, meaning they don’t have enough energy to bind their nucleus together. Unstable atoms want to become stable, so they get rid of neutrons or protons. Sometimes, when substances decay and lose protons, they transform into entirely different elements!

Uranium

Uranium contains 92 protons and 146 neutrons, for a combined total of 238 protons and neutrons. This unstable element gives off alpha particles (made up of 2 protons and 2 neutrons) to become more stable.

Alpha Particle

An alpha particle consists of 2 protons and 2 neutrons. This is identical to a Helium nucleus, which is also made up of 2 protons and 2 neutrons.

Thorium

The element now has an atomic number of 234 and it is the element thorium. It is composed of a total of 234 protons and neutrons (this is also the atomic mass). 

Parent and Daughter Atoms

When isotopes decay, we call the atom that was originally present the parent atom and the atom that it becomes, the daughter product. Once we know how long it takes for half of the parent atoms in a sample to transform into daughter products, we are then able to measure how old something is by measuring how much parent and daughter product is left in one sample.

Potassium

Potassium 40K breaks down into its daughter product Argon 40Ar. It has a very long half life of 1.251 billion years, which means it has the ability to help us mark a specific time within the range of billions of years! 

Parent/Daughter Ratio

If we measure the ratio of the parent product, Potassium, to the daughter product Argon, we can know how much time has passed since the rock first crystallized until today. This helps us get a better idea of how old things are on Earth.

Radiocarbon Dating

Another important isotope we use for dating is Carbon 14. Carbon dioxide is essential to life on Earth; plants take it in, animals eat the plants, and we eat the animals and the plants, so there are carbon molecules bouncing around everywhere. 

Most of the carbon we have inside of our bodies is the stable atom of Carbon 12, but we also have a steady ratio of unstable or radioactive Carbon 14 within us. 

Carbon 14

Carbon 14 is useful for radiometric dating. All living things take it in during their lifetime, and stop taking it when they die. Since Carbon 14 has a half-life of 5,730 years, we can only use it to record dates up to about 50,000 years.

Carbon 14

Carbon 14 comes from the stable molecule of Nitrogen 14. Cosmic rays from the sun send an energetic neutron to hit stable Nitrogen 14. This neutron turns Nitrogen 14 into Carbon 14.

Plants absorb Carbon 14

Plants absorb Carbon 14 through the process of photosynthesis, converting light energy from the sun into food. Animals and people then absorb Carbon 14 by eating plants.

When plants and animals decay

When plants and animals decay, any radioactive Carbon 14 that is present sheds protons and neutrons. Over time, the Carbon 14 turns back into its stable daughter product, Nitrogen 14.

Magnetism Dating

Where is the North Pole? Well, obviously it’s north, right? Well, that is true, today, but North has not always been north and South has not always been south. 

Deep inside the Earth, the Earth’s solid inner core rotates a bit faster than the liquid outer core and this is thought to create the Earth’s magnetism. But over time, about every 200,000 years, the direction of this magnetism has flipped.

Crystallized North

About 800,000 years ago, a compass would point South but still say North! Minerals inside rocks point to wherever North is at the time of their crystallization. Knowing the direction of magnetic poles in certain time periods help determine a rock’s age.  

Magnetic Time Scale

The time scale shows how the Earth’s magnetic field has changed through time. The black bands show when the magnetic field was normal and pointed north and the white bands indicate when it was pointing south.

Today

Here, we can see our magnetic field as it is today. This magnetic field is also responsible for protecting our planet from the Sun’s harmful rays.

The Ratio Mass Spectrometer

We all know that atoms are so small they can’t even be seen with a microscope. How is it possible then to actually count the ratio of radioactive to stable atoms when all that makes them distinct is a few protons or neutrons at best? 

Ratio Mass Spectrometer

We use a fancy machine, called the isotope ratio mass spectrometer (IR-MS). It uses ions to charge the sample and then separate the different ions by measuring the mass to charge ratio of the ions.

How Does It Work?

It dеflеcts atoms and molecules by magnetic fields — providеd thе atom or molеculе is first turnеd into аn ion. Electrically charged particles are affected by а mаgnеtic fiеld аlthough еlеctricаlly nеutrаl onеs аrеn't.

Deflection

If something is moving аnd you subject it to а sidewаys force, insteаd of moving in а strаight line, it will move in а curve — deflected out of its originаl pаth by the sidewаys force.

The Less the Deflection, the Heavier the Particle

If you knew the particle’s speed аnd the size of the force, you could cаlculаte the bаll’s mаss if you knew the curved pаth it wаs deflected through. Less deflection means a heаvier bаll. The same principle applies to аtomic-sizеd particles.

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