# Radiometric Dating with Index Fossils

Instructor: David Wood

David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.

After completing this lesson, you will be able to explain radiometric dating, and index fossils, and how we use a combination of the two to figure out the ages of rocks and fossils. A short quiz will follow.

Radiometric dating is a method used to figure out how old rocks are by looking at the amount of certain radioactive isotopes present in the sample.

But that probably sounds a bit complex, so let's start with some basics. Everything in the universe is made of various elements, such as carbon, oxygen, iron and magnesium. There are at least 118 elements in total. Some of those elements are stable, and some are radioactive (unstable). Radioactive elements are ones that, after a while, will spontaneously break down and turn into other, more stable elements. The nice thing is that this happens in a predictable pattern. The amount that decays into other elements can be predicted; the more years pass, the more will be broken down. By looking at how much of each element there is in a rock, we can trace it back and figure out how old those layers of rock are.

Using this method, we've been able to figure out how old various rocks are and learn a lot about Earth's history. Using these methods, we've even been able to figure out the age of the Earth itself: around 4.54 billion years.

## Use of Index Fossils

A related method we use to date rock is the use of reference fossils. Reference fossils are remains of long-dead animals and plants that lived at a known time in history. If we know from radiometric dating that an animal only existed for a particular period of a few million years, we know that whenever we see that animal in a rock face, that part of the rock must have that particular age. This also tells us that any other animals in the same rock, or the same layer, must have died around the same time.

One thing that makes index fossils more useful is the way sedimentary rock is created. Many of the rock faces we look at were created layer by layer as sediments settled on a sea or lake bed. Because of this, when we look at a rock-face, we know that the bottom layers are the oldest, and that fossils parallel to each other along the layers of sediment are likely to be from the same era. These rock faces often look striped, because different layers contain different materials as local conditions changed over millions of years.

The most useful index fossils are from animals that existed for only short periods of time, and that are found over a wide area. They're useful because they give us a more exact date for a rock, and they can be used to date more rocks since they're common. Ammonites are a great example of an especially useful index fossil. Ammonites changed over time, so that ones from a few million years apart are never quite the same. These fossils are found all over the place!

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