The age of the Earth

earthPlanet Earth is 4.54 billion years old, but how do we know that?

For a long, long time, nobody really knew how old the Earth was. Then in 1650, Archbishop Ussher carefully calculated the date of creation as Tuesday October 25, 4004BC. Ussher’s work was methodical, precise and reproducible, but because he based it on a literal reading of the Bible he was hopelessly wrong. But it was still a big step forward in dating our home planet.

During the nineteenth century it became apparent to geologists and paleontologists that the Earth must be a lot older than this in order for there to be time for the many extinct species of creature they discovered to have lived. But nobody knew how long.

In the late nineteenth century the great scientist Lord Kelvin calculated that the age of the Sun and the planets was at most 24 million years. But that was because Kelvin assumed the Sun to be a great ball of burning gas, and had no idea about thermonuclear fusion, the process that actually causes the sun to generate heat and light.

Then, in the early twentieth century, the New Zealander Ernest Rutherford discovered the principle of radioactive half-life and realized that it could be used to measure the age of, well, anything. Because radioactive materials decline in radioactivity at constant rates that can be measured with great accuracy, measuring the radioactivity of a rock sample (using a Geiger counter) determines its age. When rock samples were measured in this way, they were found to be hundreds of millions of years old. In fact, the oldest samples were billions of years old.

None of this came as a big surprise to the geologists, who had long guessed that the geological processes that created mountains would take this long. Nor did it surprise biologists who had tracked the rise and extinction of countless animal species preserved as fossils in layers of rock.

Modern day measurements indicate that the Earth was created at the same time as the other planets in the solar system and that the Sun itself is hardly any older. All were formed from remnants of earlier star systems, making us and the world around us children of the stars.

15 responses to “The age of the Earth

  1. Since each rock was formed from remnants of earlier star systems, would measuring its half-life give its age on earth or the age of the earlier star system?

    • Excellent question! The method dates the rock from the time at which it became isolated from the surrounding environment. Before the rock was created, radioactive isotopes were in equilibrium with the surroundings. The relative abundance of radioactive isotopes in specific decay chains measures the time that has elapsed since then.

      For example, a common technique for dating rocks is the Uranium-Lead method, which measures the relative activity of uranium and lead isotopes. U235 decays into lead 207 with a half-life of approx 700 million years. Since no new uranium atoms can enter the rock after its formation, the relative abundance of U235 to lead 207 indicates the age of the rock layer. Also U238 decays into lead 206 with a half-life of about 4.5 billion years, and so that gives a second measurement as a cross-check.

  2. So interesting. Thank you for this information.

    Best regards,

  3. I like the current stories about how water got here to form the oceans and how the moon was formed… just a thought

    • Yes, the moon is thought to have been created by a collision between Earth and another planet-sized object. The moon has similar geology to the Earth, suggesting a common origin.

    • I’m a fan of Archbishop Ussher. His thinking was surprisingly modern. In essence he believed that we could discover truths about the world for ourselves by examining information sources and making calculations, instead of just relying on books and ancient philosophers. He used the best source of information that he had available to him.

    • From what I understand, our oceans come from comet impacts in the earliest periods of Earth’s history. Kind of interesting that there is far more water (or any other raw material you can think of) in the outer reaches of the solar system than on Earth.

      • true… I had a difficult time believing this when in the early 1990s it first was being seriously discussed as a probability – now, it is easy to find the news – then was different. It still is only a very likely theory – but it is clear now that a nebula that eventually form a solar system forms being hotter and denser toward the center and cooler and less dense away from the center. .. the central region must VERY probably have contained high concentrations of metals and silicates that can form rocky planets, whereas the more icy chunks and particles must have existed in far greater quantities away from the center. Of course in the center must mostly be hot hydrogen gases that form the star… Earth has really only a small amount of water by mass — only 0.02 percent in oceans and a little more on and below the continents.

      • about the materials orbiting too – yes there are vast quantities that dwarf what is on Earth… vast quantities – very vast

  4. Thank you for the additional information. I still am not quite sure what is the answer to my question. Would some of the rocks get isolated from its surrounding environment in a previous star system?

    • The isolation occurred when the rock layers were created on Earth, either as sedimentary layers from dead plants or when they cooled as igneous layers. Prior to this, the material was very thoroughly mixed.

      Similarly, in carbon dating of organic material, the decay of radioactive carbon isotopes is measured from the date the plant/animal stopped breathing. That’s when the isotopes started to move out of equilibrium compared with the background rates.

  5. Thanks. Now it’s clear to me. And it’s also good to know that we haven’t started to decay yet!

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