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Grade 11-12 (age 16-18)

high school Geological Time & Environment

Earth's history, from its formation about 4.6 billion years ago to today, is divided by major changes in life into the Precambrian, Paleozoic, Mesozoic, and Cenozoic. The Precambrian takes up about 88% of that span; everything after it, when fossils start to turn up in abundance, adds up to just over 12%. The Paleozoic–Mesozoic and Mesozoic–Cenozoic boundaries coincide with mass extinctions, and trilobites flourished in the Paleozoic, dinosaurs and ammonites in the Mesozoic, and mammals in the Cenozoic. In the diagram, the upper bar shows all 4.6 billion years to scale and the lower bar enlarges the last 541 million, so switching eras lets you compare their lengths and the mass extinctions within each.

How do we divide 4.6 billion years?

⏳ How geologic time is divided
①Geologic time is divided by major changes in life recorded in rock layers and fossils
②The order is Precambrian → Paleozoic → Mesozoic → Cenozoic, and the last three together form the Phanerozoic Eon
③The Paleozoic–Mesozoic boundary (about 252 million years ago) and the Mesozoic–Cenozoic boundary (66 million years ago) coincide with mass extinctions
④The Precambrian–Paleozoic boundary (about 541 million years ago) is not a mass extinction but the start of the Cambrian, when fossils of animals with shells and skeletons suddenly become abundant

The geologic time scale, drawn to scale

Paleozoic
💡 What to read in the diagram
①In the upper bar the Precambrian takes up about 88%, and the Paleozoic, Mesozoic, and Cenozoic are squeezed into the narrow strip at the right end
②The lower bar enlarges that strip: the Paleozoic lasted about 289 million years, the Mesozoic about 186 million, and the Cenozoic 66 million
③Of the Big Five, the end-Ordovician, Late Devonian, and end-Permian extinctions happened in the Paleozoic and the end-Triassic and end-Cretaceous in the Mesozoic, with the end-Permian and end-Cretaceous each closing their era
④The Cenozoic is only 1.4% of Earth's history, and modern humans appeared about 300,000 years ago; if Earth's history were one day, that would be about 6 seconds before midnight

Fossils and radiometric dating

Types of fossils

Index fossils vs. facies fossils
ItemIndex fossilsFacies fossils
Purposedetermine the age of stratainfer depositional environment
Lifespanshort (specific era)long (extended period)
Distributionwide (worldwide)narrow (specific environment)
Examplestrilobites (Paleozoic), ammonites and dinosaurs (Mesozoic), nummulites (Cenozoic)corals (warm, shallow seas), ferns (warm, humid land)
Radiometric dating
t = T½ × log₂(N₀/N)
From the half-life (T½), the starting amount (N₀), and the amount left (N), find how long ago the rock formed.
Half-life concept
1 half-life: 50% remains → 2: 25% → 3: 12.5%
C-14 half-life 5730 years (organic remains up to about 50,000 years old) / U-238 half-life about 4.5 billion years (old rocks)

The Big Five mass extinctions

Big Five extinctions

The five largest extinctions of the Phanerozoic
End-Ordovician (about 443 million years ago)
spreading glaciers lowered sea level and ocean temperature
about 85% of marine species lost
Late Devonian (about 372 million years ago)
oxygen-poor oceans (anoxia)
about 75% of marine species lost
End-Permian (about 252 million years ago)
massive volcanic eruptions in Siberia → rapid warming and ocean acidification
up to about 96% of marine species lost (the largest ever)
End-Triassic (about 201 million years ago)
massive volcanism as Pangaea began to break apart
about 80% of species lost
End-Cretaceous (66 million years ago)
asteroid impact + massive Deccan volcanism
about 75% of species lost (including the dinosaurs)

Worked Examples and Exam Practice

Example 1
A rock's radioactive element has only 1/4 of its original amount left. If the half-life is 70 million years, how old is the rock?
1
Each half-life halves the amount. 1/4 is (1/2)², so two half-lives have passed.
2
Two half-lives = 70 million years × 2 = 140 million years.
▸ 140 million years (2 half-lives)
If 1/2, 1/4, or 1/8 is left, 1, 2, or 3 half-lives have passed; first find the number of half-lives from the fraction left, then multiply by the half-life.
Example 2
What do we call a fossil used to date a stratum, and one used to infer the environment in which the stratum was deposited?
1
A fossil that lived only briefly but spread widely marks a specific period, so it is used to date a stratum (index fossil).
2
If a fossil of a long-lived organism that lived only in a specific environment turns up, it points to the environment at the time of deposition (facies fossil).
▸ Age → index fossil / Environment → facies fossil
An index fossil is more useful the shorter its range and the wider its spread; a facies fossil, the narrower the environment it lived in.
Mock-test style
Which statement about fossils and dating is correct?
①A facies fossil is best for determining the age of a stratum
②An index fossil is better when it is short-lived and widespread
③A radioactive element disappears completely after one half-life
④A shorter half-life is better for dating older rocks
⑤After one half-life, the amount drops to 1/4
▸ ② An index fossil is better when it is short-lived and widespread
1
An index fossil must point precisely to one period, so it is better when short-lived and spread across many regions.
2
Index fossils determine the age (①), a radioactive element only halves each half-life and never vanishes (③), long half-lives suit old rocks (④), and one half-life drops the amount to 1/2 (⑤).

Summary

Geological time core
Precambrian (about 88%) → Paleozoic (trilobites) → Mesozoic (dinosaurs, ammonites) → Cenozoic (mammals)
Eras are divided by major changes in life, and index fossils and radiometric dating give the ages of rock layers.
🎯 Exam Points
①Index fossils are short-lived and widespread, so they are used to date strata
②Facies fossils are long-lived and live only in particular environments, so they are used to infer the environment at the time of deposition
③Radiometric dating uses half-lives: C-14 is 5730 years, U-238 about 4.5 billion years
④The end-Permian extinction is the largest in history
⑤After the end-Cretaceous extinction wiped out the dinosaurs, mammals flourished in the Cenozoic
⑥The Precambrian takes up about 88% of Earth's history
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