Beneath your feet is a record of everything Earth has ever been — billions of years of history written in stone.
Rock isn't just rock — it's a record. Every layer tells a story about what Earth was like when it formed. Here's how to read it.
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.
Before sediment can pile up and become rock, something has to break existing rock apart — and then move it. Two processes handle this.
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.
Scientists use four key principles to figure out the age and order of rock layers — even when they can't see the whole picture.
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. They come from species that:
Scientists use two types of dating to figure out how old rock layers are:
Relative dating tells you the order — which layer came first — but not the exact age. "You are younger than me" is relative dating. Superposition and index fossils are tools for relative dating.
Absolute dating gives an actual number — years. "You are 12 years old" is absolute dating. Scientists use radioactive decay (like carbon-14 or uranium-lead dating) to calculate precise ages in millions of years.
Watch how geologists read rock layers to uncover Earth's deep history — from sediment formation to fossil discovery.
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.