This is going to sounds very sci-fi:
Geologists can date how long a rock has been sitting on the surface of a landscape based on the amount beryllium (an isotope) that has built up in the rock over time. Some people compare this build up in the rock to a sun tan in the skin, but I don’t really like that analogy, it confuses people to thinking that cosmic rays come from the Sun, but they don’t, they come from beyond the Solar System.
Cosmic rays pass through Earth’s atmosphere and hit the surface of the Earth. When the energy from these rays hits a rock surface, it causes some oxygen atoms (like those in quartz -> SiO2) to break apart, one of the results is an isotope called beryllium. The longer the rock is hit by these rays, the more breaking occurs, and the more beryllium builds up over times. We sample the top of a rock, dissolve it in the lab, measure the amount of beryllium and calculate the age of ‘exposure to the rays’.
Why is this useful?
I study ice ages, and long-term climate change. If we can date the timing of past glacier advances, based on the boulders glaciers left behind, we can figure out the timing and magnitude of past climate changes and link that to a specific cause. The climate system is complicated, and what causes glaciers to grow in New Zealand may be different from what causes glaciers to grow in Europe. Cosmogenic dating has allowed us to think about glacier changes on every continent in detail that was never possible before now, it’s very exciting! The cause of ice ages and how the cooling signal moves around the world is essential to improving predictions of what will happen to our planet with increased greenhouse gases.