The scientific community estimates the age of the Earth using methods that determine the radioactive isotopes of rocks on the planet and asteroids from outer space.
The Earth is approximately 4.54 billion years old. During this time, the planet has witnessed the formation and disappearance of continents, the expansion and contraction of ice sheets, and the evolution of life from single-celled organisms to gigantic blue whales. So how do scientists know the age of the Earth?
“When you are an Earth scientist and look at a rock, it is not just a rock. That rock contains a story that you can try to decode,” said Becky Flowers, a geologist at the University of Colorado Boulder.
Many great scholars throughout history have attempted to find a method for determining the age of the Earth. In 1862, Lord Kelvin calculated the cooling time of the Earth from its original molten state and concluded that the planet we live on formed between 20 and 400 million years ago, according to Earth Sky.
Subsequently, scientists determined the age of the Earth through the layers of rock in the lithosphere, similar to geological cross-sections of a mountain. This method had low accuracy, and by the early 20th century, we still did not have a precise answer. However, researchers believed that the age of the Earth was not millions of years but billions of years.
The estimated age of the Earth is about 4.5-4.6 billion years. (Photo: NASA).
The method of determining age using radioactive isotopes emerged in the late 1940s and 1950s. Some heavy chemical elements can decay into lighter elements, such as uranium decaying into lead. Scientists can analyze the Earth’s crust to calculate the amounts of uranium and lead. They substitute these values along with the half-life into a mathematical equation to calculate the age of the rock.
During the 20th century, researchers conducted tens of thousands of radioactive isotope age measurements, resulting in the conclusion that the Earth formed around 3.8 billion years ago.
Later, scientists began determining the age of the Earth by identifying the radioactive isotopes of the oldest rocks on Earth. To increase accuracy, they also looked for asteroids that crashed into Earth to measure their radioactive isotopes. Scientists believe that these asteroids collided with Earth from primordial times, having ages comparable to our planet.
Thanks to these methods, geologists discovered minerals on Earth that are dated at 4.4 billion years, meaning the blue planet has existed for at least that long. But why do they believe the Earth is over 4.5 billion years old, an additional 100 million years?
The process of Earth’s formation in the Solar System. (Video: BBC).
The Earth has changed dramatically over billions of years, particularly through processes like plate tectonics, where the crust shifts, new landforms arise from magma, and older land is pulled back into the Earth. As a result, scientists find it very challenging to locate rocks from the early days of the blue planet. They have long since eroded or melted into raw materials.
However, scientists can apply carbon dating methods to determine the age of rocks from other places in the Solar System. Some asteroids contain material over 4.56 billion years old, and rocks from the Moon and Mars are also dated at around 4.5 billion years.
These age markers are quite close to the time that experts believe the Solar System began to form from a cloud of gas and dust surrounding the young Sun. Based on these relative ages, they can compile a timeline of the formation processes of Earth, the Moon, Mars, and other nearby celestial bodies.
The transition from a primordial dust cloud to the planet Earth did not happen instantly but took millions of years, according to Rebecca Fischer, an Earth and planetary scientist at Harvard University. This means that human understanding of the age of the Earth is not a specific year but rather a period when the blue planet began to take shape.
