FIELD NOTES
Date: 4.5 billion years ago, to present
Site: Earth
Subject: Geologic Time
Participants: C. Monroe
Earth is a little over 4.5 billion years old. Over all those years, Earth has changed a lot. The weather, atmosphere, and land have all changed many times. Many creatures that used to roam the Earth no longer exist today. For a long time, there was no life on Earth at all!
When scientists study Earth’s past, they use something called “geologic time.” Geologic time splits Earth’s history into sections. Think about how we split the lifespan of a human into sections: baby, child, teenager, adult. Scientists do the same thing for the lifespan of the Earth.
The biggest sections are called “eons.” There have only been four eons so far. The first eon began when the Earth formed. Our current eon – the Phanerozoic Eon – began more than 500 million years ago.
Because eons are so long, scientists break them down into smaller sections called “eras.” Then they break the eras into even smaller parts called “periods.” When a scientist says that something happened during a certain period, everybody knows exactly what part of Earth’s history they are talking about. They know what kinds of plants, animals, rocks, and atmosphere existed on Earth at that time, and how long ago it was.
Today, we live in the Quaternary Period, which is part of the Cenozoic Era. The era right before ours was the Mesozoic Era. That was the time of the dinosaurs. Dinosaurs went extinct about 65 million years ago, which feels like a long time to us. In terms of geologic time, however, it’s like it happened only yesterday.
The Mesozoic Era is split into three periods: Triassic, Jurassic, and Cretaceous. Different dinosaurs existed during each period. At the end of the Cretaceous, a huge natural disaster caused the dinosaurs to go extinct and ended the Mesozoic Era. The time of the dinosaurs was over; the time of mammals was just beginning.
Marginal comment: The Triassic, Jurassic, and Cretaceous Periods all have interesting dinosaurs. However, I find the population at the end of the Cretaceous to be the most fascinating.
Want to learn more? Check out these sources!
https://geology.com/time.htm
https://sciencing.com/three-time-periods-dinosaurs-lived-8737410.html