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Geologic Time

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Geologic Time Geologists and geomorphologists describe the Earth's geologic history through a temporal system known as the  geologic time scale  . 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 describes some of the important geologic events that have occurred since the Earth's formation some 4.6 billion years ago.   Table:  Geologic time scale.   Eon   Era   Period