Technique to determine the age of a rock by measuring the last time the rock being dated was either melted or disturbed sufficiently to rehomogenize its naturally-occurring radioactive elements. This is accomplished because of the decay of long-lived radioactive isotopes. Some of the parent-daughter isotopes used as chronometers to determine the age of extraterrestrial rocks are: aluminum-magnesium, uranium-lead, thorium-lead, rubidium-strontium, potassium-argon, samarium-neodymium, and lutetium-hafnium.