The Age of The Earth (Finally the Answer)

FAS Astronomers Blog, Volume 33, Number 5.

Radioactivity

Because of the finite speed of light, we can look back in time and see the cosmos as it was millions and billions of years ago. But, what about the Earth? The answer is found through radioactivity – the process with which heavy unstable atoms (elements) break down into lighter stable atoms. 

The discovery of radioactivity starts with Wilhelm Roentgen. In November 1895, he was passing cathode rays (electrons) through a glass tube. He noticed a glow emanating from a screen a few feet away. After some experimentation, he discovered that his experiment was producing unknown “rays” which would penetrate some material, but not others. He called these rays “X-rays”, which we now know is a form of electromagnetic radiation. 

A short time later, in February 1896, Henri Becquerel was experimenting with various materials to see if they would produce these X-rays after being exposed to sunlight. His analysis included uranium, which he found emitted some type of “ray”. However, when a few cloudy days caused him to pause his experiments, he noticed that uranium still spontaneously produced these mysterious rays even without sunlight. 

Marie and Pierre Curie continued with these experiments and eventually coined the term radioactivity to explain the phenomenon discovered by Becquerel. While experimenting with uranium, they investigated pitchblende (a mineral containing uranium and other materials) and found it was even more radioactive than pure uranium. This led to the discovery of two new radioactive elements polonium and radium. 

Ernest Rutherford, along with Frederick Soddy, demonstrated that radioactivity was produced during the decay of a heavy unstable element into a lighter element (Rutherford & Soddy 1902). Rutherford also identified three distinct types of radioactivity and discovered that unstable elements would lose a specific percentage of their mass over a specific time. This is known as the elements half-life. Allegedly, when they discovered radioactive decay, their conversation went something like this.

“Rutherford, this is transmutation!”
“For Mike’s sake, Soddy, don’t call it transmutation. They’ll have our heads off as alchemists.”

Bertram Boltwood extended Rutherford and Soddy’s work by tracing the decay of uranium through several stages and discovered that the final stage is the stable element lead. He also proposed that the ratio of lead to uranium could be used to determine the age of rocks. He put this to the test by studying the decay of uranium in rocks with one sample yielding 2.2 billion years. (Boltwood 1907). 

Arthur Holmes took the next step and confirmed Boltwood’s approach. Measuring the decay of uranium to lead, he calculated the age of some rocks at 1.6 billion years. He published his results in the 1913 book The Age of the Earth, although he only reported the age of the rocks, and not of the Earth itself. 

Radiometric Dating

We now know that many heavy elements have too many neutrons vs. protons and are unstable. Because of this, they undergo radioactive decay. These elements emit radiation in one of three forms (alpha, beta, and gamma). As they do, they decay into lighter elements. This process continues until more stable elements (e.g., lead) are reached. Different elements decay at different rates. The rate is measured by an element’s half-life, which is the time required for half of the mass to decay into something else. The rate can also be expressed by an element’s decay rate (see the appendix below). 

Not all radioactive elements are useful for dating the Earth. You might have heard of Carbon-14 dating (aka radiocarbon dating). It works, but only for one time living things. Because the half-life of carbon-14 is around 5,730 years, it’s only good for looking back about 50,000 years. This isn’t nearly enough time to measure the age of the Earth.

For the Earth, we need something with a much longer half-life and uranium (U) is just that. Uranium 238 has a half-life of around 4 ½ billion years and decays to lead 206. As a check, geologists also use uranium 235, which decays with a half-life of 704 million years to lead 207. 

Unfortunately, it is hard to find ancient rocks on the Earth. The Earth was a big molten ball for its first ½ billion years or so during the Hadean eon when few if any rocks were produced. Even later, rocks go through something called the Rock Cycle and are transformed from one type into another over time. This means, even if we find an ancient rock, we can only determine its age back to its last re-formation. 

We need something that is durable enough to maintain its chemical structure over a rather long period of time. It turns out that the mineral zircon (ZrSiO₄) works quite well. Zircon is widespread in all types of rocks and has a structure that doesn’t easily break down into other forms. It also tends to combine with uranium, but not with lead. So, the ratio of lead to uranium in a sample of zircon provides a good measure of its age. 

Not everything, particularly meteorites, contain uranium or zircon. Arthur Holmes’ work, along with similar efforts by Fritz Houtermans and E. K. Gerling, led (pun intended) to a method of radioactive dating referred to as the (Gerling)Holmes-Houtermans method. This approach involves the measurement of different isotopes of lead (Pb), where two are the decay products of uranium and one is not. 

• Pb₂₀₇ results from the decay of U₂₃₅.
• Pb₂₀₆ results from the decay of U₂₃₈.
• Pb₂₀₄ is independent of the decay of uranium.

By plotting the ratio of Pb₂₀₇/Pb₂₀₄ vs. Pb₂₀₆/Pb₂₀₄, samples end up falling on a line (called an isochron). The slope of the line gives the age of the material (and the Earth). Note that one needs multiple independent samples to plot a line. Also required is a sample that represent the “primordial/primeval” mixture of lead isotopes. The Canyon Diablo meteorite (Tatsumoto, Knight, and Allegre 1973) is often used for this. 

Rocks (and Zircon) point the way

In the 1940s, the (Gerling) Holmes-Houtermans method (aka lead-lead dating) came into play. Using this approach, Holmes placed the age of the Earth at 3 billion years old based on the analysis of lead in rock samples by Alfred O. Nier (Holmes 1946). Gerling and Houtermans develop similar estimates of the Earth’s age at around 3 billion years (Dalrymple, chapter 7).

The “definitive” result might have come from Clair Patterson. After an extensive analysis of the Canyon Diablo and other meteorites during the early 1950s, he set the Earth’s age at 4.55 billion years +/- 70 million years in his 1956 article (Patterson 1956). This estimate was later adjusted to 4.48 billion years based on revised Uranium decay rates. In any event, this estimate was a significant jump from the previous estimates of around 3 billion years or less. 

Arthur Holmes eventually revised his estimate of the Earth’s age upward to 4 ½ billion years based on the work by Clair Patterson and others (Holmes 1956).

Several recent studies of ancient rocks and minerals set the minimum age of the Earth to around 4 billion years.

• Isua Greenstone Belt in Greenland [3.8 billion years] (Nutman et al. 2002).
• Acasta Gneiss Complex in Northwest Canada near the Great Slave Lake [4.03 billion years] (Stern and Bleeker 1998).
• Nuvvuagittuq Greenstone Belt on Hudson Bay in northern Quebec [4.28 billion years] (O’Neil et al. 2008).

However, it is a few grains of zircon found at Jack Hills in Western Australia that provide the oldest terrestrial estimate of the Earth’s age. The Jack Hills zircon was initially dated at 4.276 billion years (Compston and Pidgeon 1986) and later revised to 4.404 billion years (Wilde et al. 2001). A 2014 analysis confirmed the 4.4-billion-year age (Valley et al 2014).

Lunar samples brought back by the Apollo astronauts have been dated at 4+ billion years. 

• Lunar Sample 67215 from Apollo 16 [4.46 billion years] (Norman et al. 2003).
• Lunar Sample 15415, ”The Genesis Rock” from Apollo 15 [4.1 billion years]. 
• Lunar/Earth Sample 14321 from Apollo 14 [4.0 to 4.1 billion years]. 

Meteorites also offer estimates of the solar system’s age, which by inference gives an approximate age for the Earth. The Alan Hills Mars meteorite ALH84001, found in Antarctica, is thought to be 4.091 billion years old. Several other meteorites have been found and dated back to around 4.6 billion years. Among them are:

• Allende Meteorite [4.567 billion years] (Amelin and Krot 2010).
• Murchinson Meteorite [4.6 billion years] (Heck et al. 2020).
• EC 002 [4.565 billion years] (Barrat et al. 2021).

The Hadean Boundary

In Geologic time, the Hadean eon marks the beginning of the Earth (or maybe of the Solar System – or maybe of both). 4.567 billion years seems to be the magic number for the Hadean upper bound. However, I’m not sure where this age initially came from.

As noted in part 1, the International Commission on Stratigraphy (ICS) tracks the various strata and determines boundaries for Geologic time including the upper bound for the Hadean period. 

• For their earlier charts, they listed this boundary as ~4.6 billion years. 
• A chart in A Brief History of Earth: Four Billion Years in Eight Chapters by Andrew H. Knoll shows the upper bound at 4.544 billion years and references the 2020 version of the ICS chart (although I couldn’t find this specific chart). 
• The 2024 edition of the ICS chart places the upper bound of the Hadean eon at 4.567 billion years ago.
• An article, from 2020, also references the same number (Strachan et al. 2020). 

Two studies of the Allende meteorite come close. 

• Audrey Bouvier and Meenakshi Wadhwa have 4.568 billion years (Bouvier and Wadhwa 2010). 
• Yuri Amelin and Alexander Krot have 4.5662 billion years, which rounds to 4.567 (Amelin and Krot 2010).

Amelin and Krot’s paper references an earlier paper that has 4.5672 billion years, which also rounds to 4.567 (Amelin, Krot, and Hutcheon 2002). 

Stephen O. Moshier (“The Genisis Rock”) suggests that 4.567 is an average of several studies. 

Conclusion

The most cited figures for the age of the Earth, at least that I can find, are between 4.54 and 4.57 billion years. The Age of the Earth by G. Brent Dalrymple is referenced by many authors as the source. The various rocks and lunar samples discussed above place the Earth’s age at least 4.0 to 4.4 billion years. The analysis of meteorites, including the Canyon Diablo meteorite, pushes this to the 4.5+ billion range. The International Commission on Stratigraphy and a couple of other sources place the beginning of the Solar System at 4.567 billion years. So, the most sensible conclusion is that the Earth is 4 ½ billion years old and I’ll leave it at that.