Laws of Relative Rock Dating
Dating Dinosaur Fossils
Radioactive dating uses the ratios of isotopes and their specific decay products to determine the ages of rocks, fossils and other substances. Absolute age is just a fancy way of saying definitive or specific age as opposed to the relative age, which only refers to how old or young a substance is in comparison to something . What is Relative Dating? process by which rocks are placed in chronological order from oldest to youngest. What is Law of superposition? law that states that in any undisturbed sequence of sedimentary rocks, each bed is older than the layers above and younger than the layers below. What is an unconformity? surface that. Prior to the discovery of radiometric dating in the early 20th century, which provided a means of absolute dating, archaeologists and geologists used relative dating to determine ages of materials. Though relative dating can only determine the sequential order in which a series of events occurred, not when they occurred .
As a member, you'll also get unlimited access to over 70, lessons in math, English, science, history, and more. Plus, get practice tests, quizzes, and personalized coaching to help you succeed. Log in or sign up to add this lesson to a Custom Course. Login or Sign up. Consider the following scenario: Paul the Paleontologist is Define Relative Dating And Radiometric Dating very famous scientist who has studied dinosaur bones all over the world.
Recently, he appeared on the evening news to talk about a new dinosaur he just discovered. The dinosaur is called superus awesomus. Paul says he can tell from the fossils that superus awesomus lived on Earth about million years ago.
Paul is super awesome, so I'm going to take him at his word. But really, click do scientists figure out how old their dinosaur bones are?
And, what about other findings like fossil fish, plants and insects? Scientists are always spouting information about the ages of rocks and fossils. How do they know these ages? Well, they figure it out using two different methods: Let's find out more about these geological dating methods in order to understand how Paul the Paleontologist can be so sure about the age of his dinosaur fossils.
The first method that scientists use to determine the age of rocks is relative dating. In this method, scientists compare different layers of rock to determine an ordered sequence of events in geologic history. That means they don't really know how old their rocks actually are. The key in relative dating is to find an ordered sequence. Scientists piece together a story of how one event came before or after another. Relative dating Define Relative Dating And Radiometric Dating tell us the actual age of a rock; it can only tell us whether one rock is older or younger than another.
The most common form of relative dating is called stratigraphic succession. This is just a fancy term for the way rock layers are built up and changed by geologic processes. Scientists know that the layers they see in sedimentary rock were built up in a certain order, from bottom to top.
When they find a section of rock that has a lot of different strata, they can assume that the bottom-most layer is the oldest and the top-most layer is the youngest. Again, this doesn't tell them exactly how old the layers are, but it does give them an idea of the ordered sequence of events that occurred over the history of that geologic formation.
Sort of an offshoot of stratigraphic succession is fossil successionor a method in which scientists compare fossils in different rock strata to determine the relative ages of each.
Let's say that Paul the Paleontologist found an iguanodon fossil in the light green layer shown above.
What is the difference between relative age and absolute age? Sixteen years after his discovery, he published a geological map of England showing the rocks of different geologic time eras. Interval of time when the earth's magnetic field is oriented so that magnetic north pole is approximately in the same positions as the geographic south pole strata singular:
And, he also found a coelophysis fossil in the yellow layer. Which fossil is Paul going to say is older? Of course, the coelophysis, which means that coelophysis came before iguanodon. In fact, Paul already knows that coelophysis lived around million years ago, while iguanodon lived around million years ago. So, what if Paul found that superus awesomus dinosaur fossil in this middle layer? He could be pretty confident that his super awesome dinosaur was about million years old.
One of the biggest jobs of a geologist is establishing the absolute age, in years, of a rock or fossil. Unlike relative dating, which only tells us the age of rock A compared to rock B, numerical dating tells us the age of rock A in x number of years.
If I told you that I was 30 years old, that number would be my numerical age. If I told you I was 32 years younger than my mother, that number would be my relative age. Which of these does a better job of describing my age? The numerical age, because it is exact. So, in both geology and paleontology, we want to be able to point to an object and say exactly how old it is.
To do that, we have to learn a little bit about radioactive decay. Ina French physicist named Henri Becquerel discovered radioactivity in an element called uranium. He saw that it underwent radioactive decayor emission of energetic particles to produce new elements.
Laws of Relative Rock Dating
InErnest Rutherford figured out that we could use radiation to establish the ages of rocks. By studying how the mass of uranium changed with radioactive decay, Rutherford was able to determine the age of a rock containing a uranium mineral.
This was an amazing discovery. It meant that scientists could suddenly establish the actual ages of all their rocks and fossils! The method of using radioactive decay to determine the age of rocks is called radiometric dating. This is our principal form of numerical dating. Today, we don't just use uranium to measure the ages of rocks. We can use potassium, rubidium and carbon as well. We use this web page elements to measure the ages Define Relative Dating And Radiometric Dating different types of rocks.
It's a complicated science that requires lots of knowledge about chemistry and physics, but it's the only way to determine an actual, absolute number for the ages of rocks and fossils. When Paul the Paleontologist brought home that dinosaur fossil, he probably used some type of radiometric dating. His analysis revealed that the superus awesomus dinosaur fossil was about million years old. Radiometric dating can't give us an exact date.
Perhaps Paul's dinosaur was or million years old, but either way, Paul has a better approximation of the dinosaur fossil's age than he had with just relative dating. So, on the evening news, Paul told us the dinosaur walked on Earth million years ago. And, that's how we'll come to understand superus awesomus when we think about how it lived its life.
In reality, scientists use a combination of relative and numerical dating to establish the ages of rocks and fossils. Doing radiometric dating on every single rock would be time-consuming and expensive. So, we typically use relative dating to come up with a ballpark and then use numerical dating for special items like fossils.
Paul probably had an idea that superus awesomus was somewhere between and million years old, because he knew about stratigraphic succession and fossil succession.
The age of the fossil must be determined so it can be compared to other fossil species from the same time period. Other common isotopes used in radioactive dating are uranium, potassium and iodine. Negatively charged subatomic particles with very little mass; found outside the atomic nucleus electron spin resonance: Login or Sign up. Sixteen years after his discovery, he published a geological map of England showing the rocks of different geologic time eras.
To get a more accurate date, Paul analyzed the fossil with radiometric dating and came up with the number million. Around the world, scientists use relative dating to figure out how old rocks are in relation to each other. Then, they use numerical dating to figure out actual, approximate ages of rocks. We'll never know exactly how old Paul's dinosaur was, but because of the diligent work of geologists, paleontologists, chemists and physicists, we can be pretty confident in the ages we determine through numerical and relative dating.
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We'll explore both relative and numerical dating on our quest to understand the process of geological dating. Along the way, we'll learn how stratigraphic succession and radioactive decay contribute to the work of paleontologists. An error occurred trying to load this video. Try refreshing the page, or contact customer support.
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