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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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Take a look at a globe or a map of the world. You might see towering mountains, deep oceans, and sprawling glaciers. All of these features make Earth what it is today.

Even more interesting, by some people's standards, is the age of Earth. Scientists have calculated the age of our planet to be about 4.5 billion years. But how did scientists determine that age? The answer is complicated: It involves everything from observation to complicated mathematics to understanding the elements that make up our planet.

From Hot to Cold Rocks

In the 1800s, scientists tried to determine the age of the planet, but they made a few mistakes. In 1862, Lord Kelvin, a famous Irish scientist who studied physics and math, estimated that Earth was between 20-million years old and 400-million years old. Even an age of 400 million years would make the planet quite young compared to the rest of the universe.

Lord Kelvin thought that if Earth had started as a mass of melted rock, it had to cool. He tried to calculate how long it would have taken to cool. His estimate was wrong, but his idea of drawing conclusions based on observations and calculations was an accurate scientific method.

Relative Dating Pushed Earth's Age into Billions

Scientists also tried to use relative dating to determine Earth's age. Stratigraphy compares the configuration of layers of rock or sediment in order to determine how old each layer is in relation to one another. This technique can reveal which layers are older or which events happened before others if the layers of sediment have remained in sequential order. Layers can be rearranged, bent, or contain inconsistencies. However, stratigraphy yields no exact age for those layers or events.

Relative dating did not give scientists the exact number they were looking for. However, it did suggest that the Earth was most likely billions of years old, and not just millions as was previously thought.

Determining Absolute Age of Rocks

Advances in chemistry, geology and physics continued, and in the early to mid-1900s, scientists found a method to determine the absolute age of a rock or mineral sample. The absolute age of a sample is its age in years. This method of determining absolute age is called radiometric dating, and it involves the decay, or breakdown, of radioactive elements. Using radiometric dating, scientists can determine the actual age of a rock.

Radiometric dating requires an understanding of isotopes. Isotopes are different forms of the same element, which have a different number of neutrons. Neutrons are tiny particles inside the nucleus, or core, of an atom.

The isotopes of unstable radioactive elements are called parent isotopes. They decay, or break down, into other, more stable elements called daughter isotopes. They do this in a predictable way in a certain amount of time called a half-life. The half-life of an element is the amount of time required for exactly half of a quantity of that element to decay.

Scientists can measure the number of parent isotopes that are left in a sample. They compare this to the number of daughter isotopes that are in the sample. This comparison is called a ratio. Using the half-life, they can calculate how long it would take for that number of daughter isotopes to form. Using the ratio and the half-life, they can determine the age of a rock sample.

Radiometric Dating Zeroes in on Earth's Age

One problem with this approach to dating rocks and minerals on Earth is the presence of the rock cycle. During the rock cycle, rocks are constantly changing forms. Old rocks are destroyed as they slide back into the planet, and new rocks form when lava cools and solidifies.

The first rocks that formed on Earth are no longer here, and this makes finding an exact age for the planet difficult. The oldest rocks that have been found are about 3.8-billion years old, though 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. All of the data from this planet and beyond has led scientists to estimate Earth's age at 4.5 billion years.

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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