Elements used in absolute dating
Numerical ages estimate the date of a geological event and can sometimes reveal quite precisely when a fossil species existed in time.
Third, magnetism in rocks can be used to estimate the age of a fossil site.
As these changes have occurred, organisms have evolved, and remnants of some have been preserved as fossils.
A fossil can be studied to determine what kind of organism it represents, how the organism lived, and how it was preserved.
Radioactive atoms are inherently unstable; over time, radioactive "parent atoms" decay into stable "daughter atoms." When molten rock cools, forming what are called igneous rocks, radioactive atoms are trapped inside. By measuring the quantity of unstable atoms left in a rock and comparing it to the quantity of stable daughter atoms in the rock, scientists can estimate the amount of time that has passed since that rock formed.
Mountains have been built and eroded, continents and oceans have moved great distances, and the Earth has fluctuated from being extremely cold and almost completely covered with ice to being very warm and ice-free.
These changes typically occur so slowly that they are barely detectable over the span of a human life, yet even at this instant, the Earth's surface is moving and changing.
Using the decays of uranium and thorium, our galaxy has been found to be between 10 and 20 billion years old and the earth has been found to be 4.6 billion years old. Within experimental error, this estimate agrees with the 15 billion year estimate of the age of the Universe.
Despite seeming like a relatively stable place, the Earth's surface has changed dramatically over the past 4.6 billion years.
Relative dating puts geologic events in chronological order without requiring that a specific numerical age be assigned to each event.