Relative dating
Relative dating is used to arrange geological events, and the rocks they leave behind, in a sequence. The method of reading the order is called stratigraphy (layers of rock are called strata). Relative dating does not provide actual numerical dates for the rocks.
Oldest at the bottom
The geologist looking at this cliff near Whanganui uses the principle of superposition to work out that the bottom layer is the oldest, the top layer is the youngest.
Next time you find a cliff or road cutting with lots of rock strata, try working out the age order using some simple principles:
Sedimentary rocks are normally laid down in order, one on top of another. In a sequence, the oldest is at the bottom, the youngest is at the top. This is the principle of ‘superposition’.
Sandwich stratigraphy
Next time you have a sandwich, use the principles of relative dating to work out the order it was made in.
Most sedimentary rocks are laid down in flat (horizontal) layers, although these can later tilt and fold. This is the principle of ‘horizontality’.
Layers of sedimentary rock extend sideways in the same order. A later event, such as a river cutting, may form a gap, but you can still connect the strata. This is the principle of ‘lateral continuity’.
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.
Fossils and relative dating
Fossils are important for working out the relative ages of sedimentary rocks. Throughout the history of life, different organisms have appeared, flourished and become extinct. Many of these organisms have left their remains as fossils in sedimentary rocks. Geologists have studied the order in which fossils appeared and disappeared through time and rocks. This study is called biostratigraphy.
Fossils can help to match rocks of the same age, even when you find those rocks a long way apart. This matching process is called correlation, which has been an important process in constructing geological timescales.
Matching fossils in rocks
Rocks in different places can be put into separate time sequences. Fossils in some of the rocks can be correlated to help combine these sequences into longer ones.
Some fossils, called index fossils, are particularly useful in correlating rocks. For a fossil to be a good index fossil, it needs to have lived during one specific time period, be easy to identify and have been abundant and found in many places. For example, ammonites lived in the Mesozoic era. If you find ammonites in a rock in the South Island and also in a rock in the North Island, you can say that both rocks are Mesozoic. Different species of ammonites lived at different times within the Mesozoic, so identifying a fossil species can help narrow down when a rock was formed.
Using index fossils
Trilobites lived at a different time to ammonites and belemnites. Finding a trilobite fossil in a rock tells you the rock was formed in the Paleozoic era.
Correlation can involve matching an undated rock with a dated one at another location. Suppose you find a fossil at one place that cannot be dated using absolute methods. That fossil species may have been dated somewhere else, so you can match them and say that your fossil has a similar age. Some of the most useful fossils for dating purposes are very small ones. For example, microscopic dinoflagellates have been studied and dated in great detail around the world. Correlation with them has helped geologists, such as Professor James Crampton, date many New Zealand rocks, including those containing dinosaurs.
Activity idea
Bring relative dating principles to life with the activity Rock layers and relative dating. Students begin by observing a photograph and a diagram of rock layers near Whanganui, watch an animation about how the layers were formed, then use an interactive labelling diagram to work out the order in which the rocks were created. The activity offers literacy opportunities as well as practice using the science capability 'Interpret representations'.
