Word Wednesday: Geologic Time Part 1

As a paleontologist, I frequently refer to the geologic time scale. It might seem overwhelming, but it actually does a pretty thorough job of breaking down the geologic history of the planet. To keep things simple, we’re not going to worry about anything before the beginning of life on Earth. Geologic time is like a set of nesting dolls, with many smaller divisions fitting into larger divisions. The largest commonly used unit of geologic time is the eon. Most life on Earth has lived during the Phanerozoic.

Phanerozoic

Phanerozoic breaks down into phaner and –zoicPhanero comes from the Greek word phaneros‘ which means visible and zoic comes from the Greek word ‘zoion‘ which means animal or life. In other words, the Phanerozoic eon has visible life. It started about 541 million years ago and continues to the present day. The suffix ‘zoic‘ is going to show up a lot in the geologic time scale.

The next largest division is the era. Since the start of the Phanerozoic, there have been 3 eras: the Paleozoic, the Mesozoic, and the Cenozoic.

The geologic time scale

Paleozoic

We have already come across both parts of Paleozoicpaleo and –zoic. It should therefore come as no surprise that Paleozoic means ‘ancient life’. It started 541 million years ago and ended 252.2 million years ago.

Mesozoic

Mesozoic breaks down into meso and –zoicMeso comes from the Greek word ‘mesos‘ which means middle and we already know that zoic means life. Therefore, the Mesozoic means ‘middle life’. It started 252.2 million years ago and ended 66 million years ago with the extinction of the dinosaurs.

Cenozoic

Cenozoic breaks down into ceno and -zoicCeno comes from the Greek word ‘kainos‘ which means recent. It started 66 million years ago and continues to the present day. The Cenozoic includes everything since the extinction of dinosaurs, most significantly the rise of the mammals. It is the smallest era, but scientists commonly break it down into the most parts.

Next week, I’ll talk about the two smaller divisions of geologic time: the period and the epoch.

University of California Museum of Paleontology Geologic Time Scale

Geological Society of America Geologic Time Scale

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