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# Age of Earth

## Scientists have tried to determine Earth's age in various ways over time; we now know that Earth is about 4.6 billion years old.

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Practice Age of Earth
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Age of Earth

How old is Earth and how do scientists know?

4.6 billion years old. Arriving at this number wasn't easy but there are many lines of evidence that have allowed scientists to reach that conclusion.

### Indirect Estimates

During the 18th and 19th centuries, geologists tried to estimate the age of Earth with indirect techniques. What methods can you think of for doing this? One example is that by measuring how much sediment a stream deposited in a year, a geologist might try to determine how long it took for a stream to deposit an ancient sediment layer. Not surprisingly, these methods resulted in wildly different estimates. A relatively good estimate was produced by the British geologist Charles Lyell, who thought that 240 million years had passed since the appearance of the first animals with shells. Today scientists know that this event occurred about 530 million years ago.

In 1892, William Thomson (later known as Lord Kelvin) calculated that the Earth was 100 million years old. He did this systematically assuming that the planet started off as a molten ball and calculating the time it would take for it to cool to its current temperature. This estimate was a blow to geologists and supporters of Charles Darwin’s theory of evolution, which required an older Earth to provide time for geological and evolutionary processes to take place.

Thomson’s calculations were soon shown to be flawed when radioactivity was discovered in 1896. What Thomson didn't know is that radioactive decay of elements inside Earth’s interior provides a steady source of heat. Thomson had grossly underestimated Earth’s age.

### More Quantitatively

Radioactivity turned out to be useful for dating Earth materials and for coming up with a quantitative age for Earth. Scientists not only date ancient rocks from Earth's crust, they also date meteorites that formed at the same time Earth and the rest of the solar system were forming. Moon rocks also have been radiometrically dated.

Using a combination of radiometric dating, index fossils, and superposition, geologists have constructed a well-defined timeline of Earth history. With information gathered from all over the world, estimates of rock and fossil ages have become increasingly accurate. This is the modern geologic time scale with all of the ages.

All of this evidence comes together to pinpoint the age of Earth at 4.6 billion years. A video discussing the evidence for this is found here: http://www.youtube.com/watch?v=w5369-OobM4

The age of Earth is also discussed in this video: http://www.youtube.com/watch?v=lplcRdNDcps&feature=channel .

### Summary

• Early geologists estimated Earth's age in a variety of inaccurate ways like the amount of time it might take for a sediment layer to be deposited.
• Estimates of how long it would take for a molten Earth to cool were also too young since scientists didn't know about radioactivity.
• Radiometric dating of meteorites and Moon rocks indicate that Earth is 4.6 billion years old.

### Practice

Use this resource to answer the questions that follow.

1. What is comparative dating?

2. How is absolute dating determined?

3. What is carbon dating?

4. What is paleomagnetic dating?

6. What has been found with radiometric dating?

7. How old is the Earth?

### Review

1. How do scientists know that Earth is 4.6 billion years old?

2. Why were early estimates of Earth's age too young?

3. How does the modern geologic time scale differ from the original?