Rock layers as evidence of change
Identify evidence from patterns in rock formations and fossils in rock layers to support an explanation for changes in a landscape over time
What a learner can do afterwards
- Explain that rock layers (strata) form over time, with the oldest at the bottom
- Describe how fossils in different layers provide evidence of organisms that lived at different times
- Use patterns in rock formations to explain how a landscape has changed over millions of years
1 · Read
Rock layers build up over long stretches of time. Sand, mud, and ash settle down and harden into rock, one layer at a time. Scientists call these layers strata. Since new layers always settle on top of old ones, the layer at the bottom of a cliff or canyon wall is normally the oldest, and the layer at the top is the youngest.
In a real canyon wall, scientists once found trilobite fossils only in the bottom layers. Trilobites died out hundreds of millions of years ago, so finding them low down told scientists that layer formed a very long time ago. Higher up, they found shell fossils from animals still alive today, showing those layers are much younger.
Layers settle flat. So a layer that is tilted, folded, or cracked is a clue on its own: forces pushed, bent, or broke that land sometime after the layer had already hardened. Put the clues together and you can explain how a whole landscape changed over millions of years.
Oldest rock layers sit at the bottom, and both the fossils inside them and any tilting or folding of the layers reveal how a landscape changed over millions of years.
More to read
Fossils: rocking the Earth
Fossils reveal connections between ancient and modern organisms, providing strong evidence for evolution. The fossil record shows gradual evolutionary change, physical proof of extinction, species splitting, and transitional forms. By studying fossils, we can trace the development of features in living organisms and understand how life on Earth has evolved over time. Rock layers (strata) form over time, with the oldest layers at the bottom. Fossils in different layers show organisms that lived at different times. This layering is like a history book of life on Earth.
Activity: Comparing the Most Populous Cities by Century, 1500 to Present
Rock layers form over time, with the oldest at the bottom. This is the principle of superposition. Each layer records the environment when it formed: sand becomes sandstone, mud becomes shale, coral reefs become limestone. Fossils trapped in layers provide evidence of past life. When you see a cliff with distinct layers, you are looking at millions of years of history. The bottom layers are oldest. Fossils in lower layers are older than fossils in upper layers. This helps scientists reconstruct how landscapes and life have changed.
Activity: biodiversity patterns of speciation and extinction
The geological timeline shows the relative lengths of different time intervals and major events like mass extinctions. The oldest rocks on Earth are over 4 billion years old. The fossil record shows that life began simply and became more complex over time. Major extinction events punctuate the timeline, wiping out many species and clearing the way for new ones. By building a scale model of Earth's timeline, we can see how recent human history is compared to the vastness of geological time. Rock layers and fossils are the primary evidence for this timeline.
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