When that mineral forms and the rock cools enough that argon can no longer escape, the "radiometric clock" starts. Brain Metrics. Numerical ages estimate the date of a geological event and can sometimes reveal quite precisely when a fossil species existed in time. The simplest and most intuitive way of dating geological features is to look at the relationships between them. For example, based on the primate fossil record, scientists know that living primates evolved from fossil primates and that this evolutionary history took tens of millions of years. Over time, the radioactive isotope of potassium decays slowly into stable argon, which accumulates in the mineral. Just as the magnetic needle in a compass will point toward magnetic north, small magnetic minerals that occur naturally in rocks point toward magnetic north, approximately parallel to the Earth's magnetic field.
Each time a new layer of sediment is deposited it is laid down horizontally on top of an older layer.
If the amount of radiation to which an object is exposed remains constant, the amount of electrons trapped in the imperfections in the crystal structure of the material will be proportional to the age of the material. Combined observations of this type have led to the development of the geomagnetic polarity time scale GPTS Figure 6b.