The study of the rate at which a rock cooled from a molten state, based on measurements of the amount of daughter products present in a mineral. Cooling rocks contain trace amounts of radioactive isotopes, which decay, and as mineral crystal lattices form they retain the daughter products. The amount of daughter products retained in the lattice depends on temperature. Consequently, the amount of a daughter product that has accumulated in a mineral can be used to calculate the time that has elapsed since the mineral was at its blocking temperature.