Dating the past – introduction
“How old is it?” is one of the first questions you’ll probably ask when you see an interesting rock or fossil. It’s certainly one of the first things that a geologist wants to know. As you’ll discover, finding the answer could involve you in topics as varied as investigating rock layers above or below the sea, studying fossils and their evolution or using radioactive elements as geological clocks.
Ammonite fossil
Fossil ammonite from the Mesozoic era. Ammonites, which became extinct at the end of the Cretaceous period, are one of the fossil groups geologists use to help date rocks.
Dating methods
Much of the time it is enough to know what is called a ‘relative’ date. Relative dating places a rock somewhere in a time sequence – one rock is older than some rocks, younger than others – but this does not tell you the actual number of years ago that the rock was formed. However, scientists often need to know actual dates of geological events so they can study landscape and environmental change.
Methods of ‘absolute’ dating provide dates, in numbers of years, for some types of rock. Geologists know, for example, that the Jurassic period lasted from about 251 to 145.5 million years ago. However, these dates sometimes change, as geologists continually work to refine them. Many geologists find it more convenient to refer to period names rather than actual dates. After all, it’s easier to call a movie Jurassic Park than Between About 251 and 145.5 Million Years Ago Park.
Two approaches to dating
A geological timescale is one example of how geologists combine dating methods. Relative dating puts rocks and fossils into a sequence. Absolute dating provides actual dates.
The dating method a geologist chooses depends on several things. Sometimes only relative dating is possible, as the rocks being studied do not contain material suitable for absolute dating. Most absolute dating methods measure radioactive elements in some way, so can only be used on rocks that contain these elements. For example, radiometric methods are used to date fossil compasses, which record reversals of the Earth's magnetic field. Two exceptions are fission track and luminescence dating, which make use of changes that happen to materials surrounding some isotopes in rocks.
Our collection of resources on dating the past includes New Zealand research that uses a variety of dating approaches, covering different time spans:
The construction of geological timescales involves relative and absolute dating. Date range involved: up to 4.6 billion years ago.
Professor James Crampton dates dinosarus and other fossils from Hawke’s Bay uses relative dating and matching the fossils with those from other places that have been accurately dated. Date range involved: 90 to 65 million years ago.
Dr Alan Beu is interested in layers of rock near Whanganui, which record cycles of climate change over millions of years. The relative order of events has been worked out, and absolute dates have been determined for points through the sequence. Date range involved: the last 5 million years.
Dr Marcus Vandergoes does his research in a South Island peat bog, where detailed dates have been obtained for vegetation changes during the last two ice ages. Absolute dating methods were chosen to suit the material available. Date range involved: the last 150,000 years.
The Earth is 4.6 billion years old, but you won’t find rocks that old because they have been recycled into younger rocks. The Earth is an active planet and many of its processes contribute to the rock cycle, which makes and changes rocks on or below the Earth's surface.
Read how scientists are using cosmogenic surface exposure dating, an absolute dating method in A clock in the rocks – cosmic rays and Earth science.
Explore the nature of science
Our resources provide numerous opportunities for exploring the nature of science. Geologists’ ideas of how New Zealand has developed over time can change as new data is collected. For example, detailed studies of fossils and developments in dating techniques mean that dates on the New Zealand geological timescale are continually being refined. In another example, investigations of rocks near Whanganui have helped to change global models of past cycles of climate change.
Rock layers and relative dating
The image on the left shows cliffs near Whanganui. The diagram on the right shows how the original, horizontal rock layers have changed due to tectonic activity. Relative dating puts the sequences of rocks layers into chronological order. Although the layers are no longer horizontal, geologists are able to determine their order.
Other aspects of the nature of science include the role of discovery in science, choosing the most suitable method for an investigation and the use and understanding of ‘big numbers’. Students also encounter units of geological time that do not have standard lengths, in contrast to more familiar scientific units. Students also have the opportunity to communicate their ideas about science in classroom activities and to use the Science Learning Hub as a research tool.
Nature of science
The word ‘absolute’ is a bit confusing when used for dating methods. Absolute dating does not give an exact, single year for a geological event. Any date has an error associated with it. For example, the end of the Cretaceous period may be written 65.0 ± 0.3 million years ago.
In World’s largest known dinosaurs once roamed New Zealand and Ancient dinosaur footprints discovered near Nelson read about some dinosaur fossil discoveries in New Zealand.
Researchers have long tried to discover more about moa species from their fossil remains. Read about the studies on their feathers, coprolites, impact of hunting and what we can learn from oral traditions. DNA from moa bones is not only unlocking the secrets of their survival during the last ice age but it is also providing lessons for species currently threatened by climate change.
Foulden Maars
Foulden Maar and Hindon Maar, near Dunedin, are arguably the most important terrestrial fossil sites in New Zealand. Foulden Maar formed 23 million years ago after an explosive eruption. Hindon Maar formed 15 million years ago. The maars contain tens of thousands of exquisitely preserved fossils of plants and animals, all of which represent extinct biodiversity. This article details the actions taken to preserve the sites. This article features some of the rare finds made at the sites. Both articles contain stunning images of fossils. The Connected article Foulden Maar: fossils or food? looks more closely at the fossilisation process and prompts critical thinking about preserving these sites.
Well preserved fossil
Folden Maar and Hindon Maar fossils include an unusual depth of detail – as shown by the gaping mouth and large eye of this whitebait specimen.
Take up the challenge
Help your students understand more about fossils, timescales, big numbers and/or dating methods with one of these activities below:
Fossil correlation – students date fossils from one site by matching them to fossils already dated somewhere else, using real data from Mangahouanga, made famous by paleontologist Joan Wiffen.
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.
Big numbers in science – investigate the use of big numbers, such as millions and billions, and they encounter ways to understand what these big numbers mean.
Which dating method? – learn to recognise some of the different relative and absolute dating methods.
Rock layers and relative dating – observe rocks layers located near Whanganui, watch an animation about how they were formed and use relative dating to work out the order in which the rocks were created in the interactive Relative rock layers.
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.
Use this Connected article to investigate why it took 10 years to correctly identify the remains of an ancient giant parrot – Squawkzilla.
Question bank
The Dating the past – question bank provides an initial list of questions about measuring the age of rocks and fossils and places where their answers can be found. The questions support an inquiry approach.
Key terms
For explanations of key concepts, see Dating the past – key terms.
Timeline
Explore the timeline to find out about developments in how geologists discover the ages of rocks and fossils.
Useful link
See the Otago Museum Fossils learning bundle – linked to levels 1–5 of the New Zealand curriculum, it includes worksheets, video activities and crafts to make cross-curricular links.
