Fossils and geological time — Earth science, 14–17 years
Fossils are evidence of past life or past biological activity preserved in rocks. By comparing layers, fossil types and numerical ages, geologists reconstruct changing environments across Earth’s immense history.
Rocks as records
A fossil is not always a complete skeleton. It may be a shell, footprint, burrow or chemical trace left by an organism. Sediment can cover the evidence, minerals can replace the original material, and later rock layers can preserve a record of life and its surroundings.
Why a time scale was needed
Human lifetimes are too short to witness most geological change. Geologists therefore needed a way to order events and estimate their ages. Layer relationships give relative order, while radioactive decay provides numerical ages, allowing Earth history to be read from thousands to billions of years.
Dating a rock layer
A volcanic ash layer contains a mineral with 25% of its original radioactive parent isotope and a half-life of 10 million years. Step 1: 25% means two half-lives have passed, from 100% to 50% to 25%. Step 2: 2 × 10 million gives an age of 20 million years.
The single-fossil trap
It is reasonable to assume that finding a sea fossil means the whole region was always under the sea. But a fossil records one organism in one past setting, not every later change. Uplift can raise seabed, erosion can remove layers, and similar fossils may live in different environments.
Where it is used
Fossils help scientists reconstruct ancient climates, locate rocks of similar age and identify past extinctions. They also guide searches for resources, because particular layers can contain useful fuels or groundwater. Fossils do not predict the future by themselves; they provide evidence about earlier conditions.
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