Light of the rocks

Radiocarbon dating only works on organic material, so how do you accurately measure the last time rocks and sediment saw sunlight? Luminescence dating.

Radiocarbon dating relies on the fact that all living things contain some amount of radioactive carbon (carbon-14), and when that living thing dies it stops getting more. The carbon-14 decays over time, so if you measure the amount of decay you can work out pretty accurately when that living thing died. It’s a very clever technique, but it can only measure the age of something that used to be alive. If you want to extract dates out of pottery, sediments, and rocks, you need to try something different.

Feldspar is the most common mineral in the Earth’s crust. Quartz is the second most common. Both sometimes exhibit an interesting and useful property: they absorb background radiation. Specifically, stray electrons can become stuck within grains of potassium feldspar or quartz. Lacking enough energy to escape the grains, those electrons become caught in an “electron trap.”

If the mineral is on the surface of the planet, the sun’s heat and light will soon give those electrons enough energy to escape the trap – in the case of quartz, this bleaching takes less than two minutes. Effectively, sunlight hits the reset button and clears out all the accumulated electrons. However, as soon as that material is buried away from sunlight, the electrons cannot escape – without a reset button, they keep building up as a result of the natural radiation in its burial site.

Once we dig up that mineral, all we need to do is shine some light or heat on it and see how many electrons escape. Count the emitted energy and you can get a decent estimate of just how long that mineral has been buried. Luminescence dating is an ingenious method of dating things like stone monuments, pottery, sand dunes, and sedimentary deposits.

This technique has some flaws – you need to know the background radiation of the area, and you have to account for incomplete bleaching (when some but not all of the trapped electrons have been removed by sunlight) – but it can measure dates much further back than radiocarbon dating: up to a million years, compared to radiocarbon dating’s 50,000 year maximum.

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