Geologists and geomorphologists describe the Earth's geologic history through a temporal system known as the geologic time scale (Table 10b-1). On this scale, time is measured using the following four units of time: eons, eras, periods and epochs. All of these temporal subdivisions are established on the occurrence of some important geologic event. For example, Hadean Eon represents the time on Earth when life did not exist. During the Archean Eon life started and was dominated by one-celled prokaryotic life forms. Eukaryotic one-celled organisms became dominant in the Proterozoic Eon. Multicellular organisms ruled the planet during the eon known as the Phanerozoic.
Table 10b-1 describes some of the important geologic events that have occurred since the Earth's formation some 4.6 billion years ago.