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# How Old?: How Can you Model a Method for Dating Fossilized Bones?

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Source:
Author: Janice VanCleave

### Problem

How can you model a method for dating fossilized bones?

### Materials

• hand towel
• marking pen
• bowl
• small empty coffee can with a lid
• 100 pennies
• timer

### Procedure

1. Stretch the towel out on a table.
2. Use the tape and marking pen to label the bowl Changed and the can Unchanged.
3. Place all of the coins in the can.
4. Set the timer for 1 minute.
5. At the end of 1 minute, pour the coins out of the can and onto the towel. (The towel keeps the coins from rolling off the table.)
6. Transfer half of the coins to the bowl. Record this as being the first division.
7. Return the remaining coins on the towel to the can.
8. Again, set the timer for 1 minute.
9. Continue to separate the coins at the end of each minute, keeping track of how any divisions are made. Stop when the number of coins is so small that you cannot divide it (when only one coin remains in the can). Record this as being the last division. NOTE: When trying to divide odd numbers, such as 25 coins, place the closest even number back into the can. For 25 coins, return 12 coins to the can.
10. Count the number of divisions made.

### Results

The coins are divided seven times.

### Why?

Elements that are radioactive have undergone radioactive decay. In this experiment, each I-minute division of the coins represents one half-life(the time it takes for one half of the mass of a radioactive element to decay). At the end of 1 minute, half of the coins were placed in the bowl (changed) to demonstrate the change that occurs in radioactive elements, such as those found in fossilized bones. After another minute, half of the remaining coins were placed in the bowl, leaving only one-fourth, or 25 coins, in the can (unchanged) and a total of 75 coins in the bowl. As time passed, the number of coins in the bowl increased, but the number of coins in the can decreased, just as all radioactive elements will eventually change. The half-life for this experiment was 1 minute. While some half-lives are less than this, many are thousands, millions, and even billions of years.

The amount of original radioactive material is determined and compared to the amount of unchanged radioactive material in a fossilized (hardened trace of an organism from past geologic times) bone to determine its age. The original amount of radioactive material in a dinosaur bone is estimated by comparing the bone to that of a bone of a similar animal living today. The amount of original radioactive material left in a dinosaur bone indicates the age of the bone.

### Let's Explore

1. How does the half-life time affect the number of divisions? Repeat the experiment dividing the coins every 2 minutes.
2. How does the mass affect the number of divisions? Repeat the original experiment using 50 coins.

### Show Time!

1. Another way that scientists date fossilized bones is by determining their relative age. The relative age of an object or event is its age as compared with that of another object or event. Finding the relative age simply places things in order of occurrence. In a group of rock layers, each layer is usually younger than the one beneath itand older than the one above it (except when the Earth has been moved through natural or man-made events).
2. Model the way that deposited sediments form rock layers over a period of time. Using 1/2 cup (125 m!) each of three different colors of aquarium gravel, pour one color of gravel into each of three bowls. Add 1/2 cup (125 m!) of soil or sand to each bowl of gravel. Stir. Fill a 2-quart (2-liter) rectangular glass baking dish halfway with water. Use your hand to sprinkle the gravel-soil mixture from one of the bowls into the water. Wait 10 minutes and observe the appearance of the layer formed by the mixture. Sprinkle the gravel-soil mixture from one of the remaining bowls into the water. Again, wait 10 minutes and observe the appearance of the materials in the dish. Add the last gravelsoil mixture. After 10 minutes, observe the contents of the dish.

3. A drawing of the layers formed in the previous activity can be compared to fossils found in three time periods, such as Cretaceous, Jurassic, and Triassic. Design a diagram with a legend showing dinosaur fossils found in these three time periods such as the one shown here.

### Check It Out!

The dinosaur era is known as the Mesozoic era and is divided into three parts called the Triassic, Jurassic, and Cretaceous periods. How old are each of the time periods? How do these time periods fit into the Earth's geologic timescale? For information, see pages 41-49 in Janice VanCleave's Dinosaurs for Every Kid (New York: Wiley, 1994).