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Dating the Past

“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.

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.

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.

The Dating the Past context 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.
  • The dating of dinosaur 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.
  • Near Whanganui, layers of rock 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.
  • In a South Island peat bog, 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.

This context provides 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.

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.

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