1. The Geologic Time Scale From Eons to Stages

2. Organizing Geologic Time Geologic time dates from 4.5 billion years ago (the formation of the Earth) All events prior to that are considered “astronomical time” Geologic time is organized based on events, rather than on a calendar These events are usually related to life forms, but can be other events as well (like climate changes)

4. Organizing Geologic Time The major unit of geologic time is the eon There are three eons in geologic time The Archean (4.6 – 2.5 bya), Protorozoic (2.5 bya – 542 mya), and the Phanerozoic (542 mya – present) Some geologists also recognize a fourth eon – the Hadean (4.6 – 3.8 bya) – but it is officially part of the Archean

5. Organizing Geologic Time Eons are divided into eras. For example, the current eon (the Phanerozoic) is divided into three eras – the Paleozoic (542 mya – 250 mya), the Mesozoic (250 – 65 mya) and the Cenozoic (65 mya – present) Note that as we get closer to the present, the time periods get shorter. Better fossil evidence to make more distinctions!

6. Organizing Geologic Time Eras are further divided into periods. For example, our current era (Cenozoic) is divided into three periods – the Paleogene (65 – 23 mya), the Neogene (23 mya – 2.6 mya), and the Quaternary (2.6 mya – present) Note that your textbook may list the periods as the “Tertiary” (65 – 1.8 mya) and “Quaternary” (1.8 mya – present) – this is outdated!! The names (and dates) have been changed (in 2009) to better reflect life trends

7. Organizing Geologic Time Periods are divided into epochs. For example, our current period (Quaternary) is divided into two epochs – the Pleistocene (2.6 mya – 12,000 ya), and the Holocene (12,000 ya – present) Some geologists have proposed a fifth epoch – the Anthropocene (1700 AD – present) – still being debated!

8. Organizing Geologic Time Epochs can be divided into ages. For example, the Pleistocene epoch of the Quaternary period is divided into four ages – the Gelasian (2.6 – 1.8 mya), the Calabrian (1.8 mya – 780,000 ya), the Ionian (780,000 – 126,000 ya) and the Tarantian (126,000 – 12,000 ya)

9. Does Anybody Really Know What Time It Is? So, what is our present time officially called? We are living in the Subatlantic Age of the Holocene Epoch of the Quaternary Period of the Cenozoic Era of the Phanerozoic Eon. Sound confusing? Think of it this way – it is currently period ____ on the 4 th day of the month of January, in the 16 th year of the 3 rd millennia

10. Ages Epochs Periods Eras Eons (largest amount of time)