When it comes to understanding the past, archaeologists have two powerful tools in their toolkit: relative dating and absolute dating – but how do they work? Let’s explore some of the methods that help us piece together human history, from traditional detective work of relative dating to the precise scientific techniques of absolute dating!
Relative dating versus absolute dating: what are they?
Archaeologists have two different ways of dating things: relative dating, and absolute dating. Relative dating is a bit like being an archaeological detective – we can use where artefacts or features were found or what they look like to determine which things came first, without pinpointing the exact years.
While relative dating can’t give us exact dates, it is a handy tool for determining the archaeological sequence – what happened in what order. For a long time, archaeologists had to rely on relative dating, until we got the gift of modern science!
With modern science, came absolute dating. Absolute dating uses different scientific techniques to provide a historical timestamp – telling us exactly when something was created, built or buried. With it, we can turn estimations into precise facts – give or take some error margins.
Let’s explore some of the different methods archaeologists use to date their sites!
Relative Dating
Stratigraphic layers on an archaeological site – each one is a slice of history (DigVentures)
1. Stratigraphic Dating: The Archaeological Layer Cake
Think of an archaeological site like a giant layer cake – layer upon layer of soil deposits that make up human history, each new one formed on top of the other. The deeper you go, the older things generally are. This simple but effective method, called stratigraphic dating, helps archaeologists understand what came first, second, and third – even if they don’t know the exact dates yet. When we excavate, we go one layer at a time, separating out artefacts from each to track the changes through time.
2. Typology Dating: Tracking the Changes
The oldest method of archaeological dating uses ‘typology’: literally, what type of artefact is it. All kinds of artefacts are divided up into types based on their characteristics – for instance, Samian ware pottery versus green-glazed pottery. In the past, archaeologists built big databases, tracking the changes in the forms and materials of different artefacts throughout the ages. By comparing an artefact uncovered on site with the characteristics of a certain ‘type’, we can make an estimate to how old it might be. A handy technique for dating on the go!
Absolute Dating:
Rate of carbon-14 decay (ExeterPaul, via Wikimedia Commons)
1. Radiocarbon Dating: The Time-Telling Isotope
The superstar of archaeological dating methods! Radiocarbon dating works by measuring how much carbon-14 (a radioactive form of carbon) is left in organic materials like wood, bone, or charcoal. Since we know how fast carbon-14 decays, we can calculate when that material was last alive. However, it only works on organic materials up to around 50,000 years old – so don’t try to carbon date your dinosaur fossils!
Carbon-14 has helped to date some famous discoveries, including the Turin Shroud, and Ötzi the Iceman!
2. Luminescence Dating (OSL): Trapped Light in Ancient Objects
Here’s a mind-bender: buried objects can trap light energy in their crystals. When archaeologists expose these objects to light in the lab, they release this stored energy, telling us how long they’ve been buried. It’s particularly useful for pottery and other fired materials. Think of it as measuring how long something’s been in the dark.
Tree rings in a piece of wood (James St. John, via Wikimedia Commons)
3. Dendrochronology: Tree Ring Time Travel
Trees add a new growth ring each year, creating a unique pattern based on climate conditions. By matching overlapping patterns from living trees, old buildings, and ancient logs, archaeologists can build a timeline stretching back thousands of years. It’s like nature’s barcode system!
Dendrochronology was used to date the Sweet Track in Somerset, an ancient wooden trackway built in 3807 BC.
Excavation of a medieval lime kiln on Lindisfarne, later dated using archaeomagnetic dating (via DigVentures)
4. Archaeomagnetic Dating: When Rocks Remember
Earth’s magnetic field changes over time, and some materials record these changes when they’re heated. When ancient kilns or hearths were last fired up, magnetic particles in the clay aligned with Earth’s magnetic field like tiny compasses. By comparing these alignments with known historical changes, we can determine when they were last used.
Archaeomagnetic dating helped to identify a medieval lime kiln uncovered by our community at Lindisfarne.
5. Thermoluminescence (TL) Dating: Heat, Light, and Time
Similar to luminescence dating, TL measures trapped electrons in fired materials like pottery or burnt flint. When these objects were originally heated, their “clock” was reset to zero. By measuring the accumulated electrons, archaeologists can calculate how long ago that heating happened. It’s particularly useful for dating pottery when there’s no organic material around for radiocarbon dating.
Support one of our crowdfunded digs today to see some of these dating methods in action! Will you find the artefact that unlocks the final clue in a stratigraphic sequence? Or help to fund the next groundbreaking radiocarbon date?
Take a look at our upcoming projects and be part of the next big discovery.
There are no comments on this post yet. Be the first!