Cosmic Rays as Chronometers
Galactic cosmic rays — mostly high-energy protons from supernovae — bombard Earth's surface continuously. When these particles strike atoms in exposed rock (mainly oxygen and silicon in quartz), they shatter nuclei in a process called spallation, producing rare isotopes like ¹⁰Be and ²⁶Al. The longer a rock surface has been exposed, the more cosmogenic nuclides it accumulates, creating a clock that measures landscape exposure time.
Production and Scaling
The production rate depends on latitude (geomagnetic shielding), altitude (atmospheric shielding), and depth below the surface (exponential attenuation). At sea level and high latitude, ¹⁰Be production in quartz is about 4 atoms per gram per year. At 3000 m altitude, this rises to roughly 15 at/g/yr. This simulation lets you explore how altitude and the exponential scaling factor control the effective production rate at any location on Earth.
Erosion Complicates Everything
A surface that erodes continuously loses its irradiated layer, reducing the measured nuclide concentration. The apparent exposure age becomes a minimum. However, by measuring two nuclides with different decay rates (the ²⁶Al/¹⁰Be pair), geochemists can simultaneously determine both exposure time and erosion rate — a powerful technique for quantifying landscape evolution over millennia.
Dating Glacial Landscapes
The most widespread application of cosmogenic nuclide dating is determining when glacial moraines were deposited. Boulders perched on moraines have been exposed to cosmic rays since the glacier retreated. Systematic dating of moraines across continents has revealed the precise timing of deglaciation after the Last Glacial Maximum (~20 kyr ago) and shown that glacier retreat was nearly synchronous worldwide.