Summary
How old is Earth? For much of human history, people had no way of knowing. But a few centuries ago, scientists started to realize that the key to solving the mystery of Earth's age is right beneath our feet! It started with using rock layers, or strata, to put together a timeline of Earth’s history. Scientist Nicolaus Steno’s principle of superposition showed that younger strata always form on top of older ones. This simple idea helps you determine which layers are older based on their positions, a method known as relative dating. Geologist James Hutton took the next step by estimating that each layer took thousands of years to form—which meant Earth must be much older than anyone realized at the time. Then came geologist William Smith, who used fossils to match up, or correlate, strata around the world. Finally, the invention of radiometric dating made it possible to determine rocks' absolute age, in years. Click "play" to dig deeper into this age-old question!
Viewer safety
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Teaching resources
Discussion questions
A few questions to talk through after watching Relative Dating:
- What is one new thing you learned about relative dating?
- Can you explain it to a partner in your own words?
- Where might you see this in the real world?
- What is one question you still have?
Quick work-a-long
Try it:
- Write one sentence explaining relative dating.
- Draw or give one example of your own.