Radioactive clocks in granite
The Abukuma Mountains are built on a foundation of Cretaceous granitic rocks, formed between 96 and 121 million years ago. This granite contains small amounts of accessory minerals like zircon and apatite. When these crystals formed deep underground, they incorporated atoms of uranium and thorium into their structure. One isotope in particular, Uranium-238, decays spontaneously through a process called fission.
This nuclear decay is energetic. It sends fragments flying through the crystal lattice, leaving behind tiny linear trails of damage called fission tracks. Each track is a physical scar, typically only 1 to 15 micrometers long, visible only after chemical etching and examination with a powerful microscope. The number of these tracks accumulates at a predictable rate based on the half-life of Uranium-238. By counting the density of these tracks and measuring the uranium concentration, scientists can determine how long the tracks have been accumulating. This turns each microscopic crystal into a tiny geological clock.
Measuring a mountain's pace
The fission track clock comes with a thermal switch. Heat heals the crystal lattice, causing the fission tracks to fade and eventually disappear in a process called annealing. Each mineral has a specific temperature range, or closure temperature, below which the tracks are permanently retained. For apatite, this temperature is relatively low, around 60 to 110°C. For the more durable zircon, it is significantly higher, between 230 and 250°C.
This temperature-sensitivity is important for measuring erosion. Rocks deep within the Earth's crust are hot, well above the closure temperatures of these minerals. As the mountains above erode away over millions of years, this deep rock is slowly lifted towards the surface in a process called exhumation. As the rock rises, it cools. When a zircon or apatite crystal passes through its specific closure temperature, its fission track clock starts ticking, permanently recording the time since it cooled.
By calculating this "cooling age," geologists can determine how long it took for the rock to travel from the hotter depths to its cooler, near-surface position. Combining this age with the local geothermal gradient, the rate at which temperature increases with depth, allows them to calculate the long-term average rate of erosion. This technique, called fission-track thermochronology, provides a direct measurement of a mountain's erosion over geological timescales. Studies in the Abukuma mountains using similar methods have found modern, basin-scale denudation rates between 114 and 180 millimeters per thousand years.