A radiometric dating method based on the fact that when trace amounts of uranium-238 fission there is a release of highly energy-charged alpha particles which burn narrow fission tracks, or damage trails, through glassy materials such as obsidian (i. E. , volcanic glass), mica, and zircon crystals. The number of fission tracks is directly proportional to the time since the material cooled from a molten state. The rate at which fission tracks occur is related to the half-life of uranium-238, which is approximately 4. 5 billion years.