19.2 Age of the Earth

Modern geologists and geophysicists consider the age of the Earth to be around 4.54 billion years (± 1%). This age has been determined by radiometric age dating of meteorite material and is consistent with the ages of the

oldest-known terrestrial and lunar samples.

19.2.1 Development of Modern Geologic Concepts
Studies of strata, the layering of rock and earth, gave naturalists an appreciation that Earth may have been through many changes during its existence. These layers often contain fossilized remains of unknown creatures, leading some to interpret a progression of organisms from layer to layer.

Abū Rayhān Bīrūnī (11th century CE) discovered the existence of shells and fossils in regions that were once sea floor, but were later uplifted to become dry land, such as the Indian subcontinent. Based on this evidence, he realized that the Earth is constantly changing and proposed that the Earth had an age, but that its origin was too distant to measure. The principle of superposition of strata was first proposed by Avicenna (11th century). He outlined the principle while discussing the origins of mountains in “The Book of Healing” in 1027. Shen Kuo (11th century) also later recognized the concept of deep time.

Nicolas Steno (17th century) was one of the first Western naturalists to appreciate the connection between fossil remains and strata. In the 1790s, the British naturalist William Smith hypothesized that if two layers of rock at widely differing locations contained similar fossils, then it was very plausible that the layers were the same age.

In 1830, the geologist Charles Lyell, developing ideas found in Scottish natural philosopher James Hutton, popularized the concept that the features of Earth were in perpetual change, eroding and reforming continuously, and the rate of this change was roughly constant. This was a challenge to the traditional view, which saw the history of Earth as static, with changes brought about by intermittent catastrophes. Many naturalists were influenced by Lyell to become “uniformitarians” who believed that changes were constant and uniform.

19.2.2 Early Calculations: Physicists, Geologists and Biologists
In 1862, the physicist William Thomson (who later became Lord Kelvin) of Glasgow published calculations that fixed the age of Earth at between 20 million and 400 million years. He assumed that Earth had been created as a completely molten ball of rock, and determined the amount of time it took for the ball to cool to its present temperature. His calculations did not account for the ongoing heat source in the form of radioactive decay, which was unknown at the time.

Geologists had trouble accepting such a short age for Earth. Biologists could accept that Earth might have a finite age, but even 100 million years seemed much too short to be plausible.

19.2.3 Radiometric Dating
Rock minerals naturally contain certain elements and not others. By the process of radioactive decay of radioactive isotopes occurring in a rock, exotic elements can be introduced over time. By measuring the concentration of the stable end product of the decay, coupled with knowledge of the half life and initial concentration of the decaying element, the age of the rock can be calculated. Typical radioactive end products are argon from potassium-40 and lead from uranium and thorium decay. If the rock becomes molten, as happens in Earth’s mantle, such nonradioactive end products typically escape or are redistributed. Thus the age of the oldest terrestrial rock gives a minimum for the age of Earth assuming that a rock cannot have been in existence for longer than Earth itself.

19.2.4 Invention of Radiometric Dating
Radioactivity, which had overthrown the old calculations, yielded a bonus by providing a basis for new calculations, in the form of radiometric dating.

Ernest Rutherford and Frederick Soddy, working jointly at McGill University, had continued their work on radioactive materials and concluded that radioactivity was due to a spontaneous transmutation of atomic elements. In radioactive decay, an element breaks down into another, lighter element, releasing alpha, beta, or gamma radiation in the process. They also determined that a particular radioactive element decays into another element at a distinctive rate. This rate is given in terms of a “half-life”, or the amount of time it takes half of a mass of that radioactive material to break down into its “decay product”.

Some radioactive materials have short half-lives; some have long half-lives. Uranium and thorium have long half-lives, and so persist in Earth’s crust, but radioactive elements with short half-lives have generally disappeared. This suggested that it might be possible to measure the age of Earth by determining the relative proportions of radioactive materials in geological samples. In reality, radioactive elements do not always decay into nonradioactive (“stable”) elements directly, instead, decaying into other radioactive elements that have their own half-lives and so on, until they reach a stable element.

Soddy and Sir William Ramsay, then at University College in London, had just determined the rate at which radium produces alpha particles, and Rutherford proposed that he could determine the age of a rock sample by measuring its concentration of helium. He dated a rock in his possession to an age of 40 million years by this technique.

Rutherford assumed that the rate of decay of radium as determined by Ramsay and Soddy was accurate, and that helium did not escape from the sample over time. Rutherford’s scheme was inaccurate, but it was a useful first step.

19.2.5 Arthur Holmes Establishes Radiometric Dating
Arthur Holmes became interested in radiometric dating and continued to work on it after everyone else had given up. Holmes focused on lead dating, because he regarded the helium method as unpromising. He performed measurements on rock samples and concluded in 1911 that the oldest (a sample from Ceylon) was about 1.6 billion years old. These calculations were not particularly trustworthy. For example, he assumed that the samples had contained only uranium and no lead when they were formed.

More important, in 1913 research was published showing that elements generally exist in multiple variants with different masses, or “isotopes”. In that same year, other research was published establishing the rules for radioactive decay, allowing more precise identification of decay series.

Many geologists felt these new discoveries made radiometric dating so complicated as to be worthless. Holmes felt that they gave him tools to improve his techniques, and he plodded ahead with his research. Barrell’s research determined that the layers of strata had not all been laid down at the same rate, and so current rates of geological change could not be used to provide accurate timelines of the history of Earth.

Holmes’s persistence finally began to pay off in 1921, when the British Association for the Advancement of Science came to a rough consensus that Earth was a few billion years old, and that radiometric dating was credible. Holmes published “The Age of the Earth”, an “Introduction to Geological Ideas” in 1927 in which he presented a range of 1.6 to 3.0 billion years. In 1931, the National Research Council of the US National Academy of Sciences finally decided to resolve the question of the age of Earth by appointing a committee to investigate. Holmes was a committee member, and in fact wrote most of the final report. The report concluded that radioactive dating was the only reliable means of pinning down geological time scales.

19.2.6 Modern Radiometric Dating
Radiometric dating continues to be the predominant way scientists date geologic timescales. Techniques for radioactive dating have been tested and fine tuned for the past 50+ years. Forty or so different dating techniques are utilized to date a wide variety of materials, and dates for the same sample using these techniques are in very close agreement on the age of the material.

Possible contamination problems do exist, but they have been studied and dealt with by careful investigation. Hundreds to thousands of measurements are done daily with excellent precision and accurate results.