The Oldest Rocks Are at Bottom

The Earth is made of many layers of rock. When you look at a cliff or a canyon, you can often see these layers stacked on top of each other. Each layer is called a stratum (plural: strata). These layers form over millions of years as mud, sand, and small pieces of rocks called sediment pile up on the ground or under water.

The oldest rocks are usually found at the bottom of these layers. This idea is called the law of superposition. According to this law, in undisturbed layers, the bottom layer was laid down first, so it is the oldest. Newer layers form on top of the older ones as more sediment settles over time.

Scientists called geologists study these rock layers to learn about Earth’s past. For example, if a geologist finds a fossil deep in the bottom layer, they know it is very old. Fossils are the remains or traces of plants or animals that lived a long time ago. By studying which fossils are found in each layer, geologists can discover when certain animals or plants lived and how the Earth has changed.

Sometimes, forces like erosion, earthquakes, or volcanoes can disturb the layers. This can make it harder to tell which rocks are oldest. However, in places where the layers have not been disturbed, the law of superposition helps scientists read the Earth’s history, just like pages in a book.

For example, at the Grand Canyon, you can see many rock layers. The rocks at the very bottom are over a billion years old, while the rocks at the top are much younger. This helps scientists learn about the different times in Earth’s history.

Interesting Fact: Some of the oldest rocks on Earth are more than 4 billion years old! That means they formed soon after the planet was created.