Beneath your feet is a record of everything Earth has ever been — billions of years of history written in stone.
Before we explain anything — look at these three formations and trust your instincts. What do you notice?
Those patterns you just noticed — flat layers, tilted layers, things that cut through — all come from how rocks form in the first place.
The Grand Canyon is over a mile deep — and each layer of rock is a page in Earth's history. The very bottom layers are nearly 2 billion years old. The Colorado River didn't build the canyon; it just carved through rock that was already there, revealing what had been buried for hundreds of millions of years. Standing at the rim, you're looking back in time.
All rocks on Earth fit into one of three categories based on how they formed. Understanding this is the key to reading Earth's history.
Formed when small pieces of rock, minerals, and dead organisms pile up in layers over time. Pressure from above slowly compacts them into solid rock.
★ Most fossils are found in sedimentary rock — it's the best natural recorder of life.
Formed when melted rock (magma underground, lava above ground) cools and hardens. Can form inside Earth or on the surface after a volcanic eruption.
★ Igneous rock can be used to determine the age of surrounding layers — a geologic clock.
Weathering is the breakdown of rock into smaller pieces. Wind, rain, freezing and thawing temperatures, and even plant roots can crack and crumble rock over time. Weathering happens in place — the rock breaks apart but doesn't go anywhere yet.
Erosion is when those broken pieces get picked up and carried somewhere else — by rivers, wind, glaciers, or ocean waves. When the moving water or wind slows down, the sediment drops and begins to pile up.
You made predictions about all three of these in the Investigation. Here's what each one is actually called — and why it works.
Rock strata are layers of sedimentary rock stacked on top of each other. Each layer records the environment that existed when it formed — the organisms that lived, the climate, even the events like volcanic eruptions.
Index fossils are like timestamps in the rock record — from species that lived for a short, well-defined window of time and were found all over Earth. Find one, and you know exactly which period that layer is from, anywhere on the planet.
Scientists use two types of dating to figure out how old rock layers are. They answer different questions — and geologists often use both together.
| Relative Dating | Absolute Dating | |
|---|---|---|
| What it tells you | Which layer is older or younger — the sequence, not the age | The actual age in years — a number |
| Tools used | Superposition, cross-cutting, index fossils | Radioactive decay (carbon-14, uranium-lead) |
| Everyday equivalent | "You are younger than me" | "You are 12 years old" |
| Limitation | Can't say how many years ago something happened | Requires the right minerals to be present in the rock |
10 questions on rock types, geological processes, and reading the rock record. Fill in your info below before you begin — your score will be sent to your teacher when you submit.
The lesson is just the beginning — go deeper, test your skills, or see how it all connects.