Potassium/Argon Dating: A Powerful Tool for Dating Volcanic Rocks

How do we know that a hominin skull dug out of an East African gorge is 1.8 million years old, when radiocarbon dating stops being reliable after about 50,000 years? The answer usually has nothing to do with the fossil itself. It comes from the volcanic rock lying just above or below it. This is the job of potassium/argon dating, one of the oldest and most trusted tools in a palaeoanthropologist’s kit.

What is potassium/argon dating? 🔗

Potassium/argon dating (often written as K/Ar dating) is a radiometric dating method used to determine the age of rocks, particularly igneous rocks that have solidified from molten lava. It is one of the most widely applied absolute dating techniques in geology, archaeology, and the study of human evolution.

The method is especially useful at sites where lava flows or layers of volcanic ash, called tuffs, sit directly above or below sediments containing tools, animal bones, or hominin remains. Since the age of the volcanic layer can be calculated directly, the age of the fossil-bearing sediment can be estimated by association, either as older than the tuff below it or younger than the tuff above it. This is exactly how the method was first applied to hominin fossils at Olduvai Gorge in East Africa, one of the most important sites for understanding early human evolution.

The chemistry behind the clock 🔗

Potassium is one of the most abundant elements in the Earth’s crust. It occurs naturally as three isotopes: 39K, which makes up the bulk of natural potassium, the rare 40K, and 41K. Of these, only 40K is radioactive, and it is the one that makes dating possible.

When 40K decays, it does not turn into a single product. Roughly 89 out of every 100 atoms decay into stable calcium-40, while the remaining 11 decay into argon-40, an inert, chemically unreactive noble gas. The calcium pathway is not useful for dating because ordinary calcium is already so common in rocks that the small radiogenic contribution gets lost in the noise. The argon pathway, however, is a different story.

Why argon behaves differently in molten and solid rock 🔗

Because argon is a gas, it cannot stay trapped inside molten rock. As long as lava or magma remains liquid, any argon-40 produced by decaying potassium simply bubbles out and escapes into the atmosphere. According to an overview of the method’s fundamental assumptions, this is treated as the starting point of the process: the rock is assumed to contain essentially no trapped argon-40 at the moment it solidifies.

Once the rock cools and crystallises, the situation reverses. The mineral lattice now traps the argon gas produced by ongoing potassium decay, and it steadily accumulates within the crystal structure. This moment of solidification is effectively when the geological “clock” is set to zero and begins ticking.

How the dating procedure actually works 🔗

To calculate an age, researchers measure the ratio of 40K remaining in a sample to the amount of 40Ar that has accumulated within it. Since the rate of decay of 40K is known precisely, this ratio can be converted directly into an age using the decay equation for radioactive isotopes.

Two conditions have to hold for the calculated age to be meaningful. First, all of the argon generated by decay must have stayed trapped inside the rock, with none lost through diffusion, weathering, or cracking. Second, the sample must not have undergone renewed heating or recrystallisation after it first formed, since this would reset the clock or partially reset it, producing an artificially young age. Geochronology labs, such as the one described by the Argon Geochronology Methods programme at New Mexico Tech, note that argon loss and excess argon contamination remain the two most common sources of error in this technique, and specialised checks are used to detect them.

Why the long half-life matters 🔗

The half-life of 40K, the time it takes for half of a given quantity to decay, is about 1,300 million years, or 1.3 billion years. This is an extraordinarily long span compared to something like carbon-14, whose half-life is only about 5,730 years.

This long half-life is precisely what makes potassium/argon dating so valuable for extremely old material. A detailed account of the method notes that it has been used to date meteorites at roughly 4.5 billion years old, essentially the age of the solar system itself, as well as volcanic rocks as young as around 20,000 years. Because the decay proceeds so slowly, measurable amounts of argon-40 only build up over very long stretches of time. This is a strength for dating ancient geological formations, but it is also the root of one of the method’s biggest limitations, covered further below.

Why this method matters for human evolution research 🔗

Potassium/argon dating transformed the study of human origins almost as soon as it was applied to the fossil record. In the early 1960s, researchers used the technique on volcanic tuff from Bed I at Olduvai Gorge in Tanzania, the layer that had yielded fossils of early hominins including specimens later assigned to Paranthropus boisei and Homo habilis. The dates that emerged were far older than most scientists had expected. A review of K/Ar dating’s role in reconstructing hominid chronology describes how this single application, dating Olduvai’s earliest hominin-bearing beds, effectively rewrote the timeline of human evolution, pushing back the antiquity of stone tool-making hominids by roughly a million years compared to earlier estimates.

This is the pattern that repeats across East African sites, whether at Olduvai, Koobi Fora, or Hadar. Volcanic ash layers interbedded with fossil-rich sediments give researchers reliable brackets for the age of the fossils and artefacts found between them, allowing them to build detailed timelines of when different hominin species existed and when key behaviours, such as tool-making, first appeared.

The limitations of potassium/argon dating 🔗

Despite its importance, the method has clear boundaries that anyone studying it needs to understand.

It dates rocks, not fossils 🔗

Potassium/argon dating cannot be applied directly to bone, wood, or other organic fossil material. It can only be used on the potassium-bearing minerals in the surrounding volcanic rock. Any age assigned to a fossil using this method is therefore indirect, based on its stratigraphic position relative to dated volcanic layers, rather than a direct measurement of the fossil itself.

It only works on volcanic material 🔗

The technique is suited to igneous rocks such as lava flows and volcanic tuffs. It has little use in regions without a history of volcanic activity, which limits its application to specific geological settings, chiefly rift zones and other areas with a volcanic past, such as the East African Rift Valley.

The closed-system assumption can fail 🔗

The whole method depends on the assumption that no argon has entered or escaped the rock since it solidified. In practice, this does not always hold. As the discussion of common sources of error in K/Ar dating explains, argon can be lost through diffusion, weathering, or later recrystallisation, which produces an age that is too young, while contamination by atmospheric or “excess” argon can push a calculated age to appear artificially old. For this reason, the method works best on rocks that show no sign of having been reheated or altered after their initial formation.

It has a lower age limit 🔗

Because the half-life of 40K is so long, very young rocks simply have not had enough time to accumulate a measurable amount of argon-40. As a rule of thumb, rocks younger than about 100,000 years cannot be reliably dated using this method, since the quantity of argon produced is too small to measure with confidence. For more recent samples, researchers turn to other techniques such as radiocarbon dating or thermoluminescence instead.

To address some of these limitations, particularly the uncertainty over argon loss, researchers developed a refinement called argon-argon (40Ar/39Ar) dating, which irradiates the sample and measures argon isotope ratios more precisely, reducing some of the sources of error present in the original K/Ar technique.

What do you think? Given that potassium/argon dating tells us the age of the rock rather than the fossil itself, how confident should researchers be when assigning an age to a hominin fossil based purely on its position between two dated volcanic layers? And in regions with little or no volcanic history, what kind of evidence would you look for instead to build a reliable chronology of early human sites?

References

1. https://www.ebsco.com/research-starters/history/leakeys-find-175-million-year-old-fossil-hominid
2. https://www.sciencedirect.com/topics/earth-and-planetary-sciences/potassium-argon-dating
3. https://geoinfo.nmt.edu/labs/argon/methods/home.html
4. https://www.britannica.com/science/potassium-argon-dating
5. https://link.springer.com/chapter/10.1007/978-1-4757-9694-0_4