The term 'Eon Divisions, in Geology' refers to the four major divisions of geologic time: the Hadean, Archean, Proterozoic, and Phanerozoic Eons. The Hadean Eon is the earliest division of geologic time, lasting from the formation of the Earth about 4.6 billion years ago until the start of the Archean Eon about 4 billion years ago. The Archean Eon is the second division of geologic time, lasting from about 4 billion years ago until the start of the Proterozoic Eon about 2.5 billion years ago. The Proterozoic Eon is the third division of geologic time, lasting from about 2.5 billion years ago until the start of the Phanerozoic Eon about 541 million years ago. The Phanerozoic Eon is the fourth and current division of geologic time, lasting from about 541 million years ago to the present day.