Classroom Copy
Radiometric Dating Activity
Background Information: Determining a fossils age can be done in a couple of ways. The first is relative dating. Scientists use relative dating to determine which fossils are older or younger. To understand how relative dating works imagine that a river has cut down through layers of sedimentary rock to form a canyon. If you look at the canyon walls, you can see the layers of sedimentary rock piled up one on top of another. The layers near the top of the canyon were generally formed most recently and are the youngest rock layers. The lower down the canyon wall you go, the older the rock layers are. Therefore, fossils found in layers near the top of the canyon are younger than fossils found near the bottom of the canyon. This method does not provide an actual age of the fossil and can only be used when rock layers have been preserved in their original sequence.
Actual Age or Radioactive Dating: A revolutionary technique called radioactive dating allows scientists to determine the actual age of a fossil. Using the knowledge that not all elements are eternal scientists came up with a method to date fossils and rocks. Radioactive isotopes fire off subatomic particles and energy. The new particle switches from one element to another in the process. For example, Uranium 238 (parent isotope) decays to thorium 234. Each radioactive element decays at a unique rate. The half-life of a radioactive element is the time it takes for half of the atom in a sample to decay. Thus, the rocks fossils are in or the fossils themselves contain radioactive elements allowing scientist to find the absolute or actual age of a fossil or age of the rock layer in which it is found. /The graph in Analyzing Data shows how a radioactive isotope breaks down over time. Look at the graph. When 50% of the parent isotope remains the element has gone through one half-life. When 25% of the parent isotope remains the element has gone through two half-lives.
A few ways how the fossil record and radiometric dating support the theory of evolution
1) Using the fact that many naturally-occurring elements are radioactive and they break down, or decay, at known predictable rates.
2) Many isotope pairs are useful in dating the Earth such as rubidium/strontium, thorium/lead, potassium/argon, argon/argon, or uranium/lead, all of which have very long half-lives, ranging from 0.7 to 48.6 billion years.
3) Subtle differences in the relative proportions of the two isotopes can give good dates for rocks of any age.
4) Geologists have made many radiometric age determinations, and continue to refine earlier estimates with new data. Dates are often cross-tested using different isotope pairs.
5) Results from different techniques, often measured in rival labs, continually confirm each other.
6) Repeatable results
7) Comparisons of the radiometric dating data to other methods of absolute dating.
Procedures
1) Read the information above and answer pre-lab questions.
2) You will use knowledge of radioactive decay and half-life properties to determine the age of five different “fossils”. The bag itself represents the fossil and the beads inside represent some of the millions of atoms that make it up. As a scientist, it is your job to count the number of parent and daughter isotope atoms in each bag, and from this data to determine how many half-lives the isotope has gone through and thus the age of the rock or fossil.
3) Count the number of parent isotopes and daughter isotopes. Ignore those of various colors – they represent other atoms within the sample.
4) Determine the % of the parent isotope remaining. Remember the original is the parent plus the daughter because the parent has decayed into the daughter element.
# of parent isotopes / (# of parents isotopes + # of daughter isotopes) this is the initial # of parent isotopes
5) Use the graph to find the number of half-lives the sample has gone through based on the % parent isotope remaining.
6) Use the table to fill in the half-lives and then calculate the age of the fossil by multiplying the number of half-lives by the specific half-life of the isotope listed below.
# of half-lives x length of half-life = age of sample
Each radioactive isotope has a specific half-life and is listed below
1. Half-life: 704,000,000 million years
2. Half-life: 704,000,000 million years
3. Half-life: 4.5 billion years or 4,500,000,000
4. Half-life: 4.5 billion years or 4,500,000,000
5. 1/5 half-life: 17 billion years
Data and Analysis for Radioactive Dating LabName Period
Pre-Lab Questions
1. Describe what relative dating is and how it works.
2. Is a fossil found in a lower layer of sedimentary rock older or younger than a fossil found in an upper layer? Explain.
3. What is half-life?
4. Describe radioactive dating.
5. If 50% of the parent-isotope remains, how many half-lives has the sample gone through?
6. If 25% of the parent-isotope remains, how many half-lives has the sample gone through?
Age of fossils determined by radioactive decay
"Fossil" Number / Number of parent isotope atoms / Number of daughter isotope atoms / % of daughter isotope remaining / Number of half-lives / Age of "fossil"1
2
3
4
5
Use the data and the lab information sheet to answer the follow-up Questions: RSQ the short answers.
- Rank the fossils from oldest to youngest.
- Which two were very close in age?
- In this activity, which "fossil" came from the time just after the formation of the earth? Do you think any real fossils could come from that time? Explain.
- How does the technology to determine the actual age of a fossil support the theory of evolution? (read)
- Why do scientists often use more than one radiometric dating pair to determine the age of fossils?