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.
Tap to explore
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.
Tap to explore
At times, most of the land on Earth was one gigantic desert and at other times, it was almost completely covered in ice!
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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!
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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!
Tap to explore
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.
Tap to explore
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.
Tap to explore
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).
Tap to explore
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.
Tap to explore
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!
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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?
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.
Tap to explore
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.