Many of us are familiar with some names for parts of Earth’s distant past. There’s the Jurassic, made famous in the movie Jurassic Park, and the Cretaceous, when dinosaurs roamed New Zealand, but these are just two examples of Earth’s 4.6 billion year history.
Geologists have built a timescale for Earth’s vast history, dividing it up into eons, periods, epochs and ages – each eon is divided up into a few periods, each period is divided up into shorter epochs and each epoch is divided into shorter ages.
These divisions do not have set lengths, so they are not like precisely defined scientific units such as length or mass. Instead, the beginning and end points of geological divisions are marked mainly by events such as the appearance or disappearance of certain fossils. For example, the boundary between the end of the Cretaceous period and the start of the Paleogene is marked by a mass extinction, including that of the dinosaurs, and evidence for a meteorite impact.
Early geological timescales were based on relative dating. Geologists studied fossils and worked out the order in which they and the rocks they were found in appeared. Since the early 1900s, absolute methods methods have been used to give actual dates for the boundaries between timescale divisions. This has confirmed that the larger divisions (eons, periods, epochs and ages) occurred at similar times around the world.
Using this information, a timescale (called the global geochronological scale) was developed over many years, finally being agreed to internationally in 2004. The timescale uses standard names that can be understood by scientists everywhere.
New Zealand timescale
The larger units of the global geochronological scale can be recognised in the rocks of New Zealand. However, the country has been geologically isolated for about 80 million years, so some fossils are not found anywhere else.
Therefore a New Zealand geological timescale has been developed over many years, most recently led by scientists at GNS Science. Alongside international names for divisions, it organises New Zealand rocks into series and stages. This timescale is the result of a huge amount of research, and work continues to refine the dates of boundaries between units.
There is no one place that contains a complete record of all the rock layers in New Zealand. At many places around the country, geologists can put a number of rock layers into order, from oldest at the bottom to youngest at the top, but they need a way of linking the different places. That is where fossils come in.
As life evolved, new species appeared and disappeared over time. One result of this succession has been that particular fossil species are restricted to rocks of the same age around the world. The presence of these key species helps place rocks in the relative timescale. If the fossils in a rock layer at one place correlate with those in a layer at another place, even if it is in another part of the country, both layers can be said to be the same age.
Gradually, over many years, fossil correlation has enabled a timescale to be developed that can be applied to the whole of New Zealand. Part of the work of GNS geologists is to find out more and more details about fossils, especially when individual species appeared and disappeared through time. Dr James Crampton is just one example of this team of scientists. One of his research interests is the use of microscopic fossils, called dinoflagellates, to help refine details of the New Zealand geological timescale.
Accurate dates are needed to tie down the divisions between relative time segments and to link the New Zealand timescale to the global one. Scientists at GNS use a wide range of absolute methods to do this. These are mostly radiometric methods, such as measuring radioactive isotopes and fission track dating. They also make use of paleomagnetic reversals recorded in sea-floor sediments. These have been accurately correlated with sequences of sedimentary rock on land.
Nature of Science
The global geochronological scale uses names that are recognised by scientists around the world, so scientists can easily compare rocks around the world and communicate their ideas internationally. This is similar to the practice of giving animals and plants unique scientific names that are recognised internationally.
Copies of a poster of the New Zealand geological timescale can be obtained from GNS Science.