The last radioactive element in a series of these transformations will decay into a stable element, such as lead.
Half-life: The time it takes for one-half of the population of a radioactive element to decay.
Stratigraphy: The branch of geology that catalogues Earth's successions of rock layers.
Half-life is a measurement of the time it takes for one-half of a radioactive element to decay.
Geologists easily identify rocks containing fossils of primitive life-forms as being older than rocks containing fossils that are more evolved or advanced.
However, the method of relative dating is exactly that, relative. In addition, complex lifeforms have existed on Earth for only the last 600 million years.
Over time, the nucleus of every radioactive element (such as radium and uranium) spontaneously disintegrates, transforming itself into the nucleus of an atom of a different element.
The half-life of a particular element, therefore, is constant and is not affected by any physical conditions (temperature, pressure, etc.) that occur around it.
The two most recent periods are further subdivided into seven epochs.
Before the eighteenth century, ideas about time and the history of Earth came mostly from religious theories.
Eras, the four largest time blocks in the scale, are named to indicate the fossils they contain: Precambrian (before ancient life), Paleozoic (ancient life), Mesozoic (middle life), and Cenozoic (recent life).
The last three eras are then subdivided into 11 periods.
Therefore, their fossils represent less than 15 percent of Earth's history.