Historical Geology/Cosmogenic surface dating

A surface exposed by the action of glaciers, Kosterhavet national park, Sweden.

In the article we shall discuss the techniques and applications of cosmogenic surface dating.

Applications

Unlike other dating methods, which tell us how long it is since a rock was formed, cosmogenic surface dating tells us how long a rock has been exposed on the surface.

In some cases, as when the rock is a lava flow, this amounts to the same thing. But there are other ways in which a rock can become exposed, as for example when a glacier erodes the sediment covering bedrock: when the glacier melts, the bedrock will be exposed.

Cosmogenic isotopes

In the article on radiocarbon dating we have already introduced one cosmogenic isotope, ¹⁴C, which is produced by cosmic rays from ¹⁴N.

For cosmogenic surface dating, the two most commonly used isotopes are the cosmogenic isotopes ¹⁰Be, which is produced from ¹⁶O and which has a half-life of 1.39 million years; and ²⁶Al, which is produced from ²⁶Si and which has a half-life of 717,000 years.

The method

Because the isotopes we're using have a short half-life, it follows that if a rock has been buried for a few million years the quantities of these isotopes will be negligible. But when the rock becomes exposed on the surface, and so exposed to cosmic rays, these cosmogenic isotopes will begin to accumulate in the rock.

The rate at which they do so will depend on a number of factors, including:

• The exposure of the rock. A nearby obstacle such as a mountain will shield the rock from cosmic rays coming from that direction, reducing the creation of cosmogenic isotopes.

• The elevation of the rock. If the rock is on top of a mountain, then the cosmic rays have less atmosphere to travel through to get to the rock, and so more of them will make the journey all the way to the rock without being absorbed in the atmosphere on the way.

• The depth from which we take the sample. Cosmic rays can penetrate a few meters through rock or soil, but the further they travel the more likely they are to be absorbed, so a rock sample will get more exposure to cosmic rays if it is taken from the surface than if it is taken from a meter down.

If we take all the relevant factors into account, and calculate, estimate, or simply measure the amount of cosmic rays a given rock is exposed to per year, and if we measure the quantities of the cosmogenic isotopes in a sample of the rock, then we can figure out how long the rock has been exposed.

Limitations of the method

The quantity of the relevant isotopes in the rock will not simply grow without limit with longer and longer exposure to cosmic rays; rather they will tend towards a maximum (a secular equilibrium): the point at which the cosmogenic cosmogenic production of unstable isotopes is equaled by their destruction by decay. In practice, we are not going to be able to tell the difference between a rock which has reached 99.9% of this maximum and one which has reached 99.99%. Consequently, the practical limit for the use of cosmogenic surface dating seems to be about 10 million years; after that, one old rock looks much like another. The lower limit for application of the method seems to be about ten years, because of practical limits on the accuracy with which we can measure the quantities of the relevant isotopes.