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Newer or younger rock layers are found on top. Older rock layers are found at the bottom.
I am learning how Earth's history is organized using evidence from rocks and fossils.
I can analyze rock formations and fossils to establish the relative ages of major events in Earth's history.
I can interpret evidence of major historical events.
I can use models to study the geologic time scale and explain how it organizes Earth's history.
I can construct scientific explanations about how geological processes have shaped Earth's surface features and watersheds.
Strata - rock layers
and a stratigraphic column
Reading Earth's Ancient Story
Aligned to SC Standard 6-ESS1-4
Scientists use rock strata like the pages of a history book to understand Earth's past. These layers of sedimentary rock form when sediment settles and hardens. By studying these layers, geologists can piece together the geologic time scale, which organizes our planet's history.
One important tool scientists use is relative dating. This method helps determine which rock layers are older or younger without knowing their exact age. The principle of superposition states that in undisturbed layers, the oldest rocks are at the bottom and the youngest are at the top. A stratigraphic column shows these layers stacked in order, creating a timeline of Earth's history.
Sometimes, this timeline gets interrupted. A fault—a crack in Earth's crust—can shift rock layers. Lava flows from volcanic eruptions can cut across existing layers. These are examples of cross-cutting relationships, where newer geologic features cut through older ones. Even weathering can break down and reshape ancient rock formations over time.
The fossil record preserved in these rocks tells amazing stories. Fossils show us which organisms lived during different time periods. Some fossils reveal massive extinction events when many species disappeared suddenly. Evidence suggests that an asteroid/meteorite impact caused one such extinction, leaving behind a huge impact crater and changing Earth forever.
Earth's surface has transformed dramatically over the years. Powerful forces created mountain ranges and carved out ocean basins. Volcanoes erupted, building new landforms. By studying all these clues in the rock layers, scientists can reconstruct Earth's incredible journey through time.
More Complex Rock Layering
Complex layering due to intrusions and extrusions, or faults can make dating rocks and fossils challenging.
Beneath the surface, magma may push into bodies of rock. There, the magma cools and hardens into a mass of igneous rock called an intrusion. An intrusion is always younger than the rock layers around and beneath it.
A fault is a break in Earth’s crust. A fault is always younger than the rock it cuts through.
Can you determine which occurred more recently E or A?
Can you determine which is layer is older D or B?
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