Fossils and Fossil Dating

How do scientists date fossils?

Many people talk about carbon dating but carbon dating is only accurate up to 60,000 years.

What kind of rocks are most useful in dating fossils?

Volcanic rocks – such as tuff and basalt – can be used in dating because they are formed at a particular moment in time, during an eruption. Sedimentary rocks are rarely useful for dating because they are made up of bits of older rocks.

Absolute dating methods

Fission track

Uranium is present in many different rocks and minerals, usually in the form of uranium-238. This form of uranium usually decays into a stable lead isotope but the uranium atoms can also split – a process known as fission. During this process the pieces of the atom move apart at high speed, causing damage to the rock or mineral. This damage is in the form of tiny marks called fission tracks. When volcanic rocks and minerals are formed, they do not contain fission tracks. The number of tracks increases over time at a rate that depends on the uranium content. It is possible to calculate the age of a sample by measuring the uranium content and the density of the fission tracks.

Potassium-argon dating

The age of volcanic rocks and ash can be determined by measuring the proportions of argon (in the form of argon-40) and radioactive potassium within them. Each volcanic eruption produces a new deposit of ash and rock. Fossils and other objects that accumulate between these eruptions lie between two different layers of volcanic ash and rock. An object can be given an approximate date by dating the volcanic layers occurring above and below the object.

Argon is gas that gradually builds up within rocks from the decay of radioactive potassium. It is initially formed in the molten rock that lies beneath the Earth’s crust. The heat from a volcanic eruption releases all the argon from the molten rock and disperses it into the atmosphere. Argon then starts to re-accumulate at a constant rate in the newly formed rock that is created after the eruption.

Argon-argon dating

This relatively new technique was developed in order to achieve more accurate dates than those obtained from the potassium-argon method. The older method required two samples for dating and could produce imprecise dates if the argon was not fully extracted. This newer method converts a stable form of potassium (potassium-39) into argon-39. Measuring the proportions of argon-39 and argon-40 within a sample allows the age of the sample to be determined. Only one sample is required for this method as both the argon-39 and argon-40 can be extracted from the same sample.

Relative dating methods

Chemical analysis

In special cases, bones can be compared by measuring chemicals within them. Buried bones absorb chemicals, such as uranium and fluorine, from the surrounding ground and absorb more of these chemicals the longer they remain buried. The rates of absorption depend on a number of factors which are too variable to provide absolute dates. This technique is, however, useful for providing relative dates for objects found at the same site.

Another useful chemical analysis technique involves calculating the amount of nitrogen within a bone. The level of nitrogen gradually reduces as the bone decays. Absolute dating is not possible with this method because the rate at which the nitrogen content declines depends on the surrounding temperature, moisture, soil chemicals and bacteria. The technique can, however, provide the relative ages of bones from the same site.

Stratigraphy

Most fossils are found in sedimentary rocks deposited in layers. Where the rocks are not strongly folded or tilted it is possible to work out the order in which the layers were formed. The oldest rocks and fossils are at the bottom and the youngest are on top.

Biostratigraphy

Scientists are able to recognize fossils that are characteristic of various rock layers. With this knowledge, they can place the fossils into detailed chronological sequences. These known sequences can be compared with the layers of rock and fossils uncovered at other sites to provide relative dating. Some fossils are particularly useful for these comparisons as they show distinct changes over time.

Palaeomagnetic stratigraphy

This method of dating is based on the changes in the direction of the Earth’s magnetic field. Today this field is centered on magnetic north. Prior to 780,000 years ago it was centered near the South Pole and before that it was centered north and so on. These changes in direction are known as reversals. Scientists work out the direction of the Earth’s magnetic field in the past by looking for traces of iron-oxide minerals that are found in many rocks. Because iron oxide is magnetic, the minerals tend to be oriented in the direction of the Earth’s magnetic field at the time the rock was formed. This technique has established a known sequence of reversals from dated layers found all around the world. If a sequence of reversals is found at a particular site then it can be compared with this known sequence in order to establish an approximate date.