Absolute dating
Geologists often need to know the age of material that they find. They use absolute dating methods, sometimes called numerical dating, to give rocks an actual date, or date range, in numbers of years. This is different to relative dating, which only puts geological events in time order.
Comparing radiocarbon dating methods
Dr Christine Prior is Team Leader of the Rafter Radiocarbon Laboratory at GNS Science. In this video, she compares conventional and accelerator mass spectrometry (AMS) radiocarbon dating. AMS is faster and needs a much smaller sample, but is more expensive. Also shown are views of bone preparation at the Waikato Radiocarbon Dating Laboratory.
Radiometric dating
Most absolute dates for rocks are obtained with radiometric methods. These use radioactive minerals in rocks as geological clocks.
The atoms of some chemical elements have different forms, called isotopes. These break down over time in a process scientists call radioactive decay. Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter. Each isotope is identified with what is called a ‘mass number’. When ‘parent’ uranium-238 decays, for example, it produces subatomic particles, energy and ‘daughter’ lead-206.
Absolute dating methods
Absolute dating methods give rocks an actual date or date range in numbers of years. This interactive explores four different methods used in absolute dating.
Isotopes are important to geologists because each radioactive element decays at a constant rate, which is unique to that element. These rates of decay are known, so if you can measure the proportion of parent and daughter isotopes in rocks now, you can calculate when the rocks were formed.
Because of their unique decay rates, different elements are used for dating different age ranges. For example, the decay of potassium-40 to argon-40 is used to date rocks older than 20,000 years, and the decay of uranium-238 to lead-206 is used for rocks older than 1 million years.
Radiocarbon dating measures radioactive isotopes in once-living organic material instead of rock, using the decay of carbon-14 to nitrogen-14. Because of the fairly fast decay rate of carbon-14, it can only be used on material up to about 60,000 years old. Geologists use radiocarbon to date such materials as wood and pollen trapped in sediment, which indicates the date of the sediment itself.
The table below shows characteristics of some common radiometric dating methods. Geologists choose a dating method that suits the materials available in their rocks. There are over 30 radiometric methods available.
| Dating method | Material dated | Age range dated |
|---|---|---|
Carbon-14 to nitrogen-14 (radiocarbon) | Organic remains, archaeological artefacts | Up to 60,000 years ago |
Luminescence | Tephra, loess, lake sediments | Up to 100,000 years ago |
Fission track | Tephra | 10,000 to 400 million years ago |
Potassium-40 to argon-40 | Volcanic rocks | 20,000 to 4.5 billion years ago |
Uranium-238 to lead-206 | Volcanic rocks | 1 million to 4.5 billion years ago |
Measuring isotopes is particularly useful for dating igneous and some metamorphic rock, but not sedimentary rock. Sedimentary rock is made of particles derived from other rocks, so measuring isotopes would date the original rock material, not the sediments they have ended up in. However, there are radiometric dating methods that can be used on sedimentary rock, including luminescence dating.
What is an isotope?
Dr Fiona Petchey explains what an isotope is, and then focuses on the isotopes of carbon and explains how the radioactive isotope carbon-14 is used in dating artefacts of historical importance.
All radiometric dating methods measure isotopes in some way. Most directly measure the amount of isotopes in rocks, using a mass spectrometer. Others measure the subatomic particles that are emitted as an isotope decays. Some measure the decay of isotopes more indirectly. For example, fission track dating measures the microscopic marks left in crystals by subatomic particles from decaying isotopes. Another example is luminescence dating, which measures the energy from radioactive decay that is trapped inside nearby crystals.
Dating rocks near Whanganui
Some of the relative and absolute methods of dating rocks near Whanganui are outlined by Dr Alan Beu of GNS Science. Fossil correlation is important as is the counting of climate cycles represented in the rocks. Fission track dating and paleomagnetism both provide absolute dates to tie the relative dating to.
Activity ideas
Help your students understand more about dating methods with one of these activities below:
Using absolute dating methods uses the interactive Absolute dating methods and Absolute dating rock layers – quiz. Students learn about and then choose the best absolute dating method for each layer of rock in a cliff, based on material present in each rock.
Which dating method? – learn to recognise some of the different relative and absolute dating methods.
Build a timescale – develop a timescale for a person’s life. The techniques of relative and absolute dating are similar to those used in the construction of a geological timescale.
Useful link
Radiocarbon dating is not a static science – this 2020 article from Nature, Carbon dating, the archaeological workhorse, is getting a major reboot features New Zealand scientists.
