when did the geologic time begin
The Geologic Time Scale. The 4.55 billion-year geologic time scale is subdivided into different time periods of varying lengths. All of Earth history is divided into two great expanses of time. The Precambrian began when Earth first formed 4.55 billion years ago and ended about 570 million years ago.The Phanerozoic Eon began 570 million years ago and continues today. This time scale, from the Decade of North American Geology, is widely used in North America.ll of Earth history is divided into two great expanses of time. The Precambrian began when Earth first formed 4.55 billion years ago and ended about 570 million years ago. The Phanerozoic Eon began 570 million years ago and continues today.