The ways archaeologists use uranium, volcanoes, and trapped electrons for dating

By Josh Gross
volcano

We’ve covered several dating methods on StoneAgeMan, like dendrochronology and radiocarbon dating, but there are still more ways archaeologists can tell how old a site or artifact is. Three of the most clever techniques utilize uranium, volcanoes, and trapped electrons.

We’ll touch on each of them below, and provide examples of places where archaeologists have used them.

Uranium-series Dating

Did you know that scientists can tell how old a cave painting is by dating the rocks on top of or underneath it? Uranium-series (U-series) dating is the tool they use to accomplish such a feat, and here’s how it works:  

Like radiocarbon dating, Uranium-series (U-series) methods rely on radioactive decay. They track the breakdown of two isotopes of uranium – 235U and 238U – into a series of “daughter isotopes.”

238U, for example, decays into 234U and thorium-230 (230Th). 230Th then continues to break down, until it transforms into a stable isotope of lead called 206Pb.

Scientists have learned how long it takes the above isotopes to decay: 238U has a half life (the time it takes for half of a sample to decay) of 4.47 billion years, 234U 245,500 years, and 230Th 75,584 years.

Therefore, by measuring how much of the parent and daughter isotopes of uranium remain in a sample, scientists can determine how old it is.

When Would You Use U-Series Dating?

The two main types of U-series dating are U-Th (uranium-thorium) and U-Pb (uranium-lead) dating. Since the latter is for objects that are over 1 million years old, archaeologists mostly use U-Th dating.

U-Th dating is most effective on samples that are between 50,000 – 500,000 years old. It works on shells, teeth, bones, and more.

One fascinating application for U-series dating is determining the ages of cave paintings.

Here are two studies, one from China and the other from Spain, in which the authors collected samples of calcite (calcium-based rocks) that were either underneath a cave painting, or that had formed on top of one.

Dating the calcites underneath a painting allowed the archaeologists to determine its maximum age, because the painting couldn’t be older than the surface it was painted on. Conversely, dating calcites that had formed on top of a painting gave archaeologists a minimum age for it.

U-series dating also works on bones, and these scientists used it to date a controversial skull that was found in Ethiopia. Image by sbox from Pixabay.

Dating with Volcanic Rock: Potassium-Argon Dating

We humans seem to be drawn to geologically-active areas that want to kill us, like California, Japan, and the slopes of Mount Vesuvius.

Fortunately, our ancestors were no different. Scientists are now using a dating method called potassium-argon dating – which works exclusively on volcanic rocks – to learn about our earliest forebears.  

Potassium-argon (K-Ar) is another radiometric dating method, meaning that it relies on radioactive decay. More specifically, it requires the breakdown of potassium-40 (40K).

Volcanic rocks don’t contain the element argon when they’re formed, because the heat of the eruption forces it out of them. But over time, 40K in the rocks decays into argon-40 (40Ar).

The creation of volcanic rocks. Image by skeeze from Pixabay.  

As with U-series and radiocarbon dating, scientists know how quickly 40K decays. They can also measure how much 40Ar is in a volcanic rock, and thereby learn its age.

When archaeological remains lie above or below layers of volcanic rock, scientists can employ K-Ar dating to calculate maximum and/or minimum ages for those remains.

When Would You Use K-Ar Dating?

K-Ar dating comes with two catches.

The first is that 40K decays slowly: it takes about 1.3 billion years for half of the 40K in a sample to change into 40Ar. Consequently, K-Ar dating typically only works on rocks that are over 80,000 years old.

The second catch is that volcanic rocks aren’t found everywhere.

Thankfully, some crucial archaeological sites lie in volcanic areas. This includes Tanzania’s Olduvai Gorge, which boasts the most extensive record of human evolution of any site.

Scientists have found the fossils of multiple species of early hominids in Olduvai Gorge, along with their stone tools. They’ve also used K-Ar dating extensively in this location, and at other sites in East Africa.

Olduvai Gorge, sometimes called “The Cradle of Mankind” (“The Cradle of Humankind” is a different site). Image by Harvey Barrison, found on Wikimedia. CC BY-SA 2.0  

Luminescence Dating

The final two dating methods we describe here utilize electrons that have become trapped in crystalline minerals. They’re called thermoluminescence (TL) and optically-stimulated luminescence (OSL) dating.

Thermoluminescence Dating

Pottery is one of the most common types of artifacts that archaeologists find. But how do you date it? If you’re only interested in relative dating, then techniques like typology and seriation would suffice.

However, if you want to attach a specific year to pottery or similar items, then TL dating will help you do so.  

Certain materials – such as flint, pottery, and bricks – have “holes” that loose electrons can get stuck in.

For instance, if a pottery sherd was buried in the ground, then over time radiation in the surrounding environment would free some of the sherd’s electrons – a portion of which would become trapped in defects within the pottery.

Broken pieces of pottery are called “sherds.” Black Burnished Ware Pottery Sherds (exterior) by Frank Basford. CC BY-SA 2.0 

Heating an object to a high enough temperature (listed as 500º Celsius in most sources) will release its trapped electrons.

To continue the example from above, when ancient people fired our pottery sherd to harden it, they reset its “electron clock” to zero.

As such, if scientists could measure how much energy (electrons) had become trapped in the sherd, and if they knew the level of radiation it was exposed to, then they’d know how long ago it was fired.

That’s exactly how TL dating works.

Scientists take a sample, heat it, and record the energy that it emits as light (luminescence). In addition, they place devices that measure radiation at the spot where the sample was found.

With these two pieces of information, scientists can calculate when a sample’s electron clock was last reset.

It’s possible for natural fires to reset artifacts’ electron clocks, too. That’s why archaeologists need to carefully examine the contexts of all their finds, so they can identify complicating factors like wildfires. Image by Alexas Fotos from Pixabay.  

Optically-Stimulated Luminescence Dating

OSL dating functions in much the same way as TL dating, with one key difference: rather than heating a sample to release its trapped electrons, scientists expose it to light. This mainly works on materials that contain quartz or feldspar.

When Would You Use Luminescence Dating?

TL dating’s sweet spot is from 40,000 – 200,000 years ago. Archaeologists have used it successfully in several cases, but one example comes from a study in Malaysia, where they utilized TL dating to determine the ages of ancient bricks.

By contrast, OSL dating’s ideal time period varies depending on the mineral being tested. 

OSL dating is reliable for quartz up to 200,000 years ago, and potentially longer for feldspar. Unfortunately, electrons sometimes leak out of feldspar grains, generating results that are too young.

Professor Nick Barton and his colleagues conducted a noteworthy OSL-based study in Morocco. There, they drew on OSL dating to ascertain the ages of a series of artifacts that came from the Aterian industry.

What Barton and his coauthors learned was that these Aterian artifacts made it to Morocco before archaeologists thought possible, meaning that technological development spread faster throughout North Africa than scientists realized.

Wrapping Up

By now, you’ve probably realized that archaeologists can use a wide array of methods to figure out how old an object is.

These methods require different types of materials, work best for specific ages, and have their own strengths and weaknesses.

For this reason, the best practice is to use multiple dating methods at the same site. This will produce the most accurate results, because the various techniques will compensate for one another’s blind spots.

Sources on Dating Methods

The following list of articles were of great use for this article and provide a wonderful background on dating methods. 

About The Author
Josh Gross

Josh Gross

Josh Gross is a writer with education and experience in a wide range of fields, including multiple branches of social and ecological sciences. He is also an avocational archaeologist, having worked on excavations in Belize and Ohio. Josh is currently a volunteer in the Cleveland Museum of Natural History's archaeology lab, and a member of a Northern Ohio-based archaeological society called Firelands Archaeology.