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.