Blackwell Publishing

Fossils and the history of life - How can we estimate the age of a fossil?


Relative time

We can also estimate the age of a rock relative to that of another rock, or fossil: we have a statement of the form "rock A was laid down before/at the same time as/after rock B". Some of the procedures for finding relative times are as follows.

• At any one site, more recent sediments are deposited on top of older sediments. Fossils lower down a sedimentary column are therefore likely to be older (sometimes a large geologic convulsion, such as a volcanic explosion, may turn a sedimentary column upside down, but it is obvious when it has happened).

• The date of any one fossil deposit relative to those at different sites can also usually be estimated. It is done by comparing the fossil composition of the site, for some common fossils such as ammonites or foraminifers, with a standard reference collection. For these reference fossils, the fossils deposited at one place and time will be much the same as those being deposited at another place. They therefore show that two sites had the same relative date. These kind of study are called correlation; the paleontologist is said to be 'correlating' the two sites.

• Magnetic time zones supply a similar principle. The magnetic field of the Earth has reversed its polarity at intervals through geological history. When the magnetic field is as it is now (compasses point north) it is called normal; when it has the opposite polarity it is called reversed. The history of the Earth is an alternating sequence of normal and reversed time zones. (The reason for the reversals is not known for sure - though there are plenty of hypotheses.) Polarity switches have been commoner at some times than others; All the rocks in the globe at any one time have the same polarity, and the polarity at the time rocks were formed can be detected. The polarities can the be used in fine-scale time resolution. If two rocks are known to have been formed at similar times, but we are not sure whether they are exact or only near contemporaries, magnetic polarities can provide the answer. If the rocks have different polarities they cannot have been exact contemporaries.

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