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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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How old is Earth? Scientists know our planet is about 4.5-billion years old. But how did they find out Earth's age?

The First Estimate Was Far Off

In the 1800s, scientists tried to find the age of the planet. One of these scientists was Lord Kelvin. He studied physics and math. Lord Kelvin knew that Earth was very hot when it first formed. He thought about how long it would take to cool down. He said that Earth was between 20-million years old and 400-million years old.

Lord Kelvin's estimate was wrong. That is too young for Earth. But the way he tried to answer the question was right. He used the scientific method.

From Millions to Billions of Years Old

Scientists also tried to compare layers of rock to get the age of the planet. This is called relative dating. They compare one layer to another layer. This can show which layers are older or younger. But it cannot give an actual age. However, it did tell the scientists that Earth was billions of years old.

Advances in science continued. In the early 1900s, scientists found elements that break down into other elements. They are called radioactive elements. The time it takes for half of the element to break down is called its half-life.

Scientists use a certain process to tell the age of a rock. They find radioactive elements in the rocks. They look at how much of the element is left. They look at how much of the new element has formed. They use the half-life to tell how much time has passed. This is called radiometric dating.

Looking Beyond Earth

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 melted rock cools and becomes solid.

The first rocks that formed on Earth are no longer here. This makes it hard to find Earth's age. The oldest rocks that have been found are about 3.8-billion years old. Some tiny minerals are 4.2-billion years old.

Scientists have looked at rocks from the moon and from meteorites. These rocks have not been changed by the rock cycle. Scientists used radiometric dating on those rocks. Scientists have gathered a lot of data from the rocks on Earth and beyond. They have used the data to estimate that Earth is 4.5 billion years old.

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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