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  • “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. 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.

    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.

    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.

    Take up the challenge

    Help your students understand more about fossils, timescales, big numbers and/or dating methods with one of these activities below:

    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.

      Published 11 May 2011 Referencing Hub articles
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