Certain rocks give off high energy radiation due to the presence of radioactive substances in them. Radioactive substances have got the property of decay (break down of active material with the passage of time). For example, uranium decays giving rise to lead. By measuring the amount of lead-present in the uranium sample scientists can determine the ages of rocks by taking the half-life period of that radio active element. This is called the uranium-lead method which is used for very old rocks.
- Simple! Just read the date on the rock edict.
- By seeing the writing on its walls.