Using Geological Layers & Radioactive Dating to Determine the Earth's Age

Using Geological Layers & Radioactive Dating to Determine the Earth's Age
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  • 0:02 How Old is the Earth?
  • 0:46 Geologic Layers
  • 2:04 Radioactive Dating
  • 3:00 Bringing It All Together
  • 4:16 Lesson Summary
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Lesson Transcript
Instructor: Kevin Newton

Kevin has edited encyclopedias, taught middle and high school history, and has a master's degree in Islamic law.

If you want to know how old a person is, you ask them. If you want to get an idea about a tree's age, you count its rings. But what if you want to know the age of the Earth? In this lesson, we'll get an idea of how scientists date our planet.

How Old Is the Earth?

One of the most basic questions in geology centers on how old the Earth is. For centuries, people were happy to simply accept the teachings of mythology or religion. These datings put the age of the Earth anywhere from around 7,000 to trillions of years old. However, in the past few hundred years many people have been more inclined to use the scientific method to date the Earth. That doesn't make the rest of those belief systems any less valid, if they are what you believe, but we are looking for facts in science, the 'how' of Earth's existence, not the 'why.' To that end, scientists have a number of ways to try to figure out just how old the world is. In this lesson, we will look at two of those methods in particular - radioactive dating and the use of geological layers.

Geological Layers

Imagine you were making a cake with layers of fruit filling in between. I know, with that kind of talk it may be hard to focus on science, but I'm going to ask you to focus anyway. So imagine you were building this cake - where would you start? Chances are unless you are a magician and can make things float in mid-air, you'd start at the bottom. By the time you finished the cake, the deeper you went into the cake from the top, the longer it has been since that part of the cake has been on the surface. In fact, if you were to assemble a layer of cake a day, the lowermost layers would be older. They'd probably be stale, which changes the taste and texture of the cake.

Okay, enough of the cake analogy; however, it holds pretty well. The deeper we go into the Earth, more and more towards the core, the older the rock gets. This is the basic principle of geological layers. In fact, as we go deeper, the qualities of the rock change to reflect this age. Sedimentary rock, rock formed by layers of sediment being squeezed together, becomes more and more compact. Metamorphic rock, rock changed by immense pressure and heat, becomes even more common. In fact, we can use the occurrence of certain rocks to help figure out just how old a layer of rock is.

Radioactive Dating

Still, those rocks are only useful if we have something to compare them to. Wouldn't it be nice to just ask a rock when it was created? Well, luckily we can. Through the process of radioactive dating, we can figure out just how old a particular rock is based off the relative intensity of some substances within it. Think about it like this. Imagine each rock has a lantern that loses half of its brightness every so many years. This brightness is radioactive isotopes, or substances, which we can detect with the right equipment. The amount of time it takes for the rock to lose half of its radioactivity from a given isotope is called its half-life. The half-lives for different isotopes is pretty well known, as is the amount of a given isotope that is present in a given rock. Therefore, we can tell within a range how many half-life cycles that a rock has been through and adjust its age accordingly.

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