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Cartoon illustrating cosmogenic nuclide exposure ages. A glacier transports an erratic boulder, and then recedes, exposing it to cosmic rays.

Spallation reactions occur in minerals in the rocks upon bombardment by cosmic rays.

Spallation reactions are those where cosmic-ray neutrons collide with particular elements in surface rocks, resulting in a reaction that is sufficiently energetic to fragment the target nucleus[3]. Counting the numbers of these isotopes, normally as a ratio to other isotopes, means that scientists can calculate how long rocks have been exposed at the Earth’s surface.

Beryllium-10 (Be) does not occur naturally in quartz, and once it forms following spallation it becomes trapped by quartz’s regular crystal lattice.

For a rock to be suitable for cosmogenic nuclide dating, quartz must occur in the rock in sufficient quantities and in the sufficient size fraction.

These cosmic rays do not penetrate deep into the earth’s surface.

This is important for glacial geologists, as it means that surfaces that have had repeated glaciations with repeated periods of exposure to cosmic rays can still be dated, as long as they have had sufficient glacial erosion to remove any inherited signal.

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