Geological Time & Environment

Earth's 4.6-billion-year history is divided by biotic change into Precambrian, Paleozoic, Mesozoic, and Cenozoic. Era boundaries are mass extinctions; the Precambrian is about 88% of that history, with trilobites in the Paleozoic, dinosaurs in the Mesozoic, and mammals in the Cenozoic. Fossils and radiometric dating fix the ages of rock layers. Here you can use the era slider to walk the geological time scale and its mass-extinction events era by era.

4.6-billion-year history of Earth

⏳ What is geological time?

①Earth's 4.6 Ga history divided based on biotic changes

②Precambrian → Paleozoic → Mesozoic → Cenozoic

③Era boundaries = mass extinction events

Geological time scale

Select era1

💡 Key observations

①Precambrian (0): ~88% of Earth's history — only simple life

②Paleozoic (1): Cambrian explosion; trilobites, fish, amphibians, ferns

③Mesozoic (2): Age of dinosaurs, angiosperms appear / Cenozoic (3): mammals, humans

Fossils and dating

Types of fossils

ChartIndex fossils vs Facies fossils

Item	Index fossils	Facies fossils
Purpose	determine the age of strata	infer depositional environment
Lifespan	short (specific era)	long (extended period)
Distribution	wide (worldwide)	narrow (specific environment)
Examples	trilobite (Paleozoic), dinosaur (Mesozoic)	coral (warm seas), fern (humid environments)

Radiometric dating

t = T_½ × log₂(N₀/N)

use half-life (T½) and remaining radioactive amount (N) to date rocks

Half-life concept

1 half-life: 50% remains → 2: 25% → 3: 12.5%

C-14: 5,730 years (young samples) / U-238: 4.5 Ga (ancient rocks)

Mass extinctions

Big Five extinctions

ListMajor mass extinctions in Earth's history

Late Ordovician (~440 Ma)

glaciation onset

~85% of marine species lost

Late Devonian (~360 Ma)

oceanic anoxia

~75% of marine species lost

End-Permian (~252 Ma)

Siberian Traps, Pangaea formation

~96% of species lost (largest)

End-Triassic (~200 Ma)

volcanic activity

~80% of species lost

End-Cretaceous (~66 Ma)

asteroid impact + volcanic activity

dinosaur extinction

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?

Each half-life halves the amount. 1/4 is (1/2)², so two half-lives have passed.

Two half-lives = 70 million years × 2 = 140 million years.

▸ 140 million years (2 half-lives)

Remaining 1/2 → 1/4 → 1/8 means 1 → 2 → 3 half-lives. The remaining fraction works backward to the age.

Example 2

What is the fossil that reveals the age of a stratum called, and what is the fossil that reveals the depositional environment called?

A fossil that lived only briefly but spread widely marks a specific period, so it is used to date a stratum (index fossil).

A fossil that lived for a long time but only in a specific environment reveals the depositional environment (facies fossil).

▸ Age → index fossil / Environment → facies fossil

An index fossil is better when widespread and short-lived; a facies fossil is better when restricted to a specific environment.

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

An index fossil must point precisely to one period, so it is better when short-lived and spread across many regions.

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 (88%) → Paleozoic (trilobites) → Mesozoic (dinosaurs) → Cenozoic (mammals)

eras are defined by mass extinctions; radiometric dating gives ages

🎯 Exam Points

①Index fossils: short lifespan, wide distribution → date strata

②Facies fossils: long lifespan, narrow distribution → infer depositional environment

③Radiometric dating: uses half-life; C-14 (5,730 yr), U-238 (4.5 Ga)

④End-Permian: largest in history (~96% lost)

⑤K-T extinction: asteroid impact → dinosaurs gone → mammals rise

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