MODULE 6: ROCK DATING AND GEOLOGIC TIME SCALE

How many years is a “long time”? We often express time in hours or days, and 10 or 20 years certainly feels like a long time. Imagine if you needed to think about one million, 100 million, or even several billion years. These exceptional lengths of time seem unbelievable, but they are exactly the spans of times that scientists use to describe the Earth.

The Earth is 4 ¹⁄₂ billion years old. That’s 4,500,000,000 years! Have places like the Grand Canyon and the Mississippi River been around for all of those years, or were they formed more recently? When did the giant Rocky Mountains form and when did dinosaurs walk the Earth? To answer these questions, you have to think about times that were millions or billions of years ago.

Historical geologists are scientists who study the Earth’s past. They study clues left on the Earth to learn two main things: the order in which events happened on Earth, and how long it took for those events to happen. For example, they have learned that the Mississippi River formed many millions of years after the Grand Canyon began forming. They have also concluded that dinosaurs lived on the Earth for about 200 million years. (https://courses.lumenlearning.com/earthscience/chapter/geologic-time-scale/)