Part 1: Engage (Anchoring Phenomenon)

The Puzzle of the Oldest Rocks: The Earth formed approximately 4.6 billion years ago along with the rest of the solar system. However, the oldest known rocks found on Earth’s surface (in places like Canada and Australia) are only about 4.0 to 4.4 billion years old. Much of the Earth’s early crust has been destroyed by tectonic plate movement and erosion. To understand the true age of the Earth, scientists often turn to meteorites—ancient space rocks that have crashed onto our planet and remained unchanged since the dawn of the solar system.

But how do scientists actually know the age of a rock or meteorite? They can’t just count tree rings. Instead, they use a technique called radiometric dating.

Your Task: Think about what it means to determine the age of something that has been sitting around for billions of years. Write down two “need to know” questions you have about how scientists could possibly figure out the exact age of a meteorite.

Part 2: Explore (Simulation Investigation)

Materials:

• Computer with internet access
• Radiometric Dating Explorer Simulation

Instructions:

1. Open the Radiometric Dating Explorer Simulation.
2. You will see a microscopic view of a rock sample, starting with 100% Parent Isotopes (red dots). These parent isotopes are unstable and will undergo radioactive decay, transforming into stable Daughter Isotopes (blue dots) over time.
3. In the “Simulation Controls” panel, select the Uranium-238 → Lead-206 isotope system.
4. Set the Simulation Speed to a comfortable pace (e.g., 10x) and click Start Decay.
5. Watch the Rock Sample view and the Decay Curve graph. Pause the simulation at the intervals listed in the table below and record the percentage of Parent and Daughter isotopes. (Use the Reset Sample button if you miss a time point.)

Data Table 1: Uranium-238 Decay

1. Reset the sample and switch to the Potassium-40 → Argon-40 isotope system. Run the simulation and record data for its specific decay intervals.

Data Table 2: Potassium-40 Decay

Part 3: Explain (Sensemaking)

Use your data from Part 2 to answer the following questions.

1. Defining Half-Life: Look at the time it took for the Parent Isotope percentage to drop from 100% to exactly 50%. This time period is called the “half-life.” What is the half-life of Uranium-238? What is the half-life of Potassium-40? _____

2. Identifying Patterns: Look at the Decay Curve graph in the simulation. Does the number of parent isotopes decrease by the same amount (e.g., subtracting 10 particles) every billion years, or by the same percentage (e.g., dividing by half)? Explain your reasoning using evidence from your data tables. _____

3. Appropriate Tools: The simulation also includes Carbon-14, which has a half-life of 5,730 years. If a scientist found a meteorite from the early solar system (billions of years old), could they use Carbon-14 to date it? Why or why not? Make a claim and support it with evidence about exponential decay. _____

Part 4: Elaborate/Evaluate (Argumentation & Modeling)

Challenge Mode: Dating an Unknown Meteorite

1. In the “Simulation Controls” panel, locate the Challenge Mode section and click Generate Unknown Meteorite.
2. A new rock sample will appear. Look at the data provided for this specific sample (specifically, the remaining Parent %).
3. Hover your mouse over the Decay Curve graph to find the exact time that corresponds to that remaining Parent %.
4. Enter your estimate in the “Estimated Age” box and click Submit.
   • My Estimated Age: _____ Billion Years

Final Deliverable: Scientific Argument Imagine you are presenting your findings at a geology conference. Write a short scientific argument confirming the age of your unknown meteorite and explaining how it provides evidence for the early history of the solar system. Your argument must include:

• A Claim stating the age of the meteorite.
• Evidence citing the specific ratio (or percentage) of parent to daughter isotopes you observed, and the half-life of the chosen isotope system.
• Reasoning explaining why the exponential decay pattern allows scientists to determine the age of rocks, and how ancient meteorites provide clues about Earth’s formation that are missing from Earth’s own active, changing crust.

My Scientific Argument: _____

Teacher Notes (NGSS Alignment)

Performance Expectation: HS-ESS1-5: Evaluate evidence of the past and current movements of continental and oceanic crust and the theory of plate tectonics to explain the ages of crustal rocks. (Secondary PE focus for this simulation: using radiometric dating as the underlying mechanism for dating these rocks).

Science and Engineering Practice (SEP): Engaging in Argument from Evidence Students evaluate evidence from simulated radioactive decay to support claims about the age of rock samples. They synthesize their findings in Part 4 to construct a logical argument linking isotope ratios to the absolute age of a meteorite.

Disciplinary Core Ideas (DCI):

• ESS1.C (The History of Planet Earth): Students learn that while active geologic processes have destroyed early Earth rocks, meteorites have changed little, and their ages can be determined using radiometric dating.
• PS1.C (Nuclear Processes): Students observe that spontaneous radioactive decays follow a characteristic exponential decay law (the half-life), allowing nuclear lifetimes to be used to determine the ages of rocks.

Crosscutting Concept (CCC): Patterns Empirical evidence is needed to identify patterns. Students identify the exponential decay pattern in Part 3 and use it to predict and calculate ages in Part 4.

Evidence Statements Addressed:

• 1.a.ii (via HS-ESS1-6 context): Using absolute ages obtained by radiometric dating of meteorites to reconstruct early history.
• 2.a.i: Measurement of the ratio of parent to daughter atoms produced during radioactive decay as a means for determining the ages of rocks.
• 4.a: Describing patterns observed from the evidence to support explanations about rock ages.