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# How Did Scientists Calculate the Age of Earth?

The examination and analysis of rocks on Earth’s surface, and of extraterrestrial rocks, have enabled scientists to determine the approximate age of the planet.

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### Subjects

Earth Science, Geology, Physics

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Earth has high mountains, deep oceans, and sprawling glaciers. But how old is Earth? Scientists have determined our planet is about 4.5-billion years old.

How did scientists determine that age? They used geology, math, and chemistry. First they learned about the rocks and elements that make up our planet. Then they used that knowledge to figure out a way to tell the age of the rocks.

Scientific Method Used, but It Yielded a Poor Estimate

In the 1800s, scientists tried to determine Earth's age. One of them was a famous Irish scientist named Lord Kelvin. He studied physics and math. Lord Kelvin thought that if Earth began as a mass of melted rock, it had to cool. He tried to figure out how long it would have taken to cool. In 1862, he said that Earth was between 20-million years old and 400-million years old.

His estimate was wrong. But the way he tried to solve it was right. He drew conclusions based on what he could see or observe. He also drew conclusions based on what he could figure out using math, and on what was known about physics at the time. Although his estimate was wrong, Kelvin used an accurate scientific method.

Getting Closer Using Relative Dating

Scientists also tried to compare layers of rock to figure out the age of the planet. This method is called relative dating. Scientists try to determine how old each layer is by comparing it to another layer. This method can show which layers are older or younger, or which events happened first.

Relative dating could not give scientists the exact age of those rock layers. However, it did tell them that Earth was most likely billions of years old.

Advances in science continued. In the early 1900s, scientists discovered radioactive elements. These elements decay, or break down, into other elements. Scientists figured out how each element breaks down, what it turns into and exactly how long that takes to break down. The amount of time it takes for half of the original element to break down is called its half-life. The decay is like a clock, counting down how much time has passed.

Scientists use a certain process to tell the age of a rock or mineral. First, they measure how much of the original element is left. Then they measure how much of the new element has formed. They use the half-life to tell how much time has passed. This method is called radiometric dating.

Studying Moon Rocks Produces Breakthrough

One problem with radiometric dating is the the rock cycle. During the rock cycle, rocks change forms. Old rocks are destroyed as they slide back into Earth. New rocks form when lava cools and solidifies. The rocks are constantly changing.

The first rocks that formed on Earth are no longer here. This makes finding an age for the planet difficult. The oldest rocks that have been found are about 3.8-billion years old. However, some tiny minerals have been dated at 4.2 billion years.

To get around the difficulty presented by the rock cycle, scientists have looked elsewhere in the solar system for even older rock samples. They have examined rocks from the moon and from meteorites, neither of which have been changed by the rock cycle. Radiometric dating has also been used on those rocks. Scientists have used all of this data to estimate the age of Earth at 4.5 billion years.

###### Media Credits

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###### Director
Tyson Brown, National Geographic Society
###### Author
National Geographic Society
###### Production Managers
Gina Borgia, National Geographic Society
Jeanna Sullivan, National Geographic Society
###### Program Specialists
Sarah Appleton, National Geographic Society, National Geographic Society
Margot Willis, National Geographic Society
Clint Parks
Roza Kavak

October 19, 2023