Text lesson

Unit 4 — Dating Planet Earth

Science — Third Preparatory

2.2 The geologic time scale

We divide ordinary time into units (years, months, days, hours).
In the same way scientists divide Earth’s 4.6 billion year history into units, based on rock and fossil evidence.
This division is the geologic time scale.
It helps us understand the sequence of:

Eon	Time span	Notes
Hadean	4600 – 4000	Formation of Earth intense volcanic activity no life
Archean	4000 – 2500	First simple life (micro-organisms)
Proterozoic	2500 – 541	More abundant simple life
Phanerozoic	541 – present	Diversification of life complex plants and animals

The Phanerozoic Eon has three eras Paleozoic (about 541–252 million years ago), Mesozoic (about 252–66 million years ago; the age of dinosaurs) and Cenozoic (about 66 million years ago to the present; the age of mammals).
Making a timeline.
To model Earth’s history, choose a scale such as 1 cm = 500 million years.
A line for 4.6 billion years then needs 4600/500 = 9.2 cm.
Mark events: formation of Earth (4.6 billion years ago, left end), first life (about 3.5 – 4 billion years ago), extinction of dinosaurs (66 million years ago, only 0.13 cm from the right end), and the appearance of modern humans (a few hundred thousand years ago, just a tiny fraction of a millimetre).
Such a scale shows how recent the human era is.

Unit 4 — Dating Planet Earth · 11 / 28

2.2 The geologic time scale