Home / Learning / Science Topics / NZ Geology / Measuring Geological Time

Measuring Geological Time

The Geological Timescale divides the Earth’s history into several periods of differing lengths of time. There are different ways that scientists can measure geological time. These techniques are often combined together to get the most detailed dating information from a rock sequence.

Methods of dating rocks

Absolute dating involves determining a rock’s actual age as a number of years, whereas relative dating methods provide an estimate of the age of a rock by comparing it to rocks of a known age The Geological Time Scale has been created by combining both absolute and relative dating methods.

Radioactive or radiometric dating is a very important method of determining an absolute age for a rock using radioactive isotopes. As minerals crystallise in igneous and metamorphic rocks they trap certain isotopes in their crystal structure that begin to decay radioactively as soon as the mineral forms. These radioactive isotopes are parent isotopes, which decay slowly to daughter isotopes, changing the rock’s isotopic character. The rate at which the isotopes decay is in effect our "geological clock". By measuring the amount of the parent and daughter isotopes in a crystal, and then applying the decay rate, the actual age in years since the rock crystallized can be calculated. Check out this video on the Uranium – Lead dating method:

Biostratigraphy is a relative dating method that correlates rock ages using the fossils contained within rock units. Once the age range of a number of fossil species has been established using the radiometric method, fossils can provide very useful dating information. Fossils can also tell us much about the ancient environments in which sedimentary rocks were deposited.

Paleomagnetic dating involves the analysis of the magnetic alignment of iron containing minerals within a rock, and is yet another useful tool for dating rocks younger than 100 million years. The Earth’s Magnetic Poles have alternated their polarities many times in the past (changing from North – South to South – North and back). Sedimentary and igneous rocks are often imprinted with the magnetic alignment of the Earth at the time they were formed. The record of these magnetic reversals has itself been studied to create the paleomagnetic time scale, which looks like an irregular bar code. By analysing the magnetic orientation of rocks their ages can be allocated to one of the two possible sets of time ranges.

The Paleomagnetic Time Scale

The Paleomagnetic Time Scale

Sedimentation rates: Another way of estimating geological time is to apply known rates of present-day sedimentation to the thickness of rock sequences. This can be useful especially when combined with other methods that can pin specific ages to particular layers within the sequence.