Earth Changes Over Millions of Years

Geological time is the way scientists measure Earth's incredibly long history. Geological time refers to the billions of years that Earth has existed. Understanding geological time helps us learn how mountains, canyons, and other features formed on our planet's surface.

Earth is about 4.5 billion years old. That number is so large it's hard to imagine! To understand this better, think of Earth's entire history as a 24-hour clock. If Earth formed at midnight, humans wouldn't appear until the very last second before the next midnight. Dinosaurs—the giant reptiles that once lived on Earth—existed between 230 and 66 million years ago. That seems like a long time, but it's only a small part of Earth's total history.

Many of Earth's features took millions of years to form. The Rocky Mountains, a major mountain range in North America, began forming about 80 million years ago and are still being shaped today by wind, water, and weather. The Grand Canyon, a massive valley in Arizona, took about 5 to 6 million years for the Colorado River to carve through layers of rock.

Humans have been on Earth for only about 300,000 years. This is a tiny fraction of Earth's history. Most of the dramatic changes to Earth's surface—like the formation of mountains and the carving of canyons—happened long before people existed.

Interesting Fact: If you could stack all the years humans have existed into a pile of pennies, and stack all of Earth's history the same way, the human pile would be about as tall as a pencil, while Earth's pile would be taller than a 30-story building!