History of the World, pt 1
The Geological Time Scale and
Major Time Periods
Major Divisions of Time
Earth’s history is divided into multiple time periods,
characterized mainly by major changes in the
geologic record.
Divisions:
Eons: the largest geologic time periods.
• Hadean: 4.6 Billion – 3.8 Billion years ago
• Archaen: 3.8 Billion – 2.5 Billion years ago
• Proterozoic: 2.5 Billion – 540 Million years ago
• Phanerozoic: 540 Million years ago - present
Major Divisions of Time
Eras: the Phanerozoic Eon is divided into smaller units
called eras:
• Paleozoic: 540 m.y.a. – 248 m.y.a.; marked by the
first complex life forms, began with the Cambrian
Period and ended with the great mass extinction
at the end of the Permian Period.
• Mesozoic: 248 m.y.a. – 65 m.y.a.; time of the
dinosaurs. Ended with an asteroid impact.
Destroyed all dinosaurs and most plant life.
• Cenozoic: 65 m.y.a. – present; rise of the mammals
and formation of the world as we know it.
Major Divisions of Time
Periods: Smaller time units within eras.
Major periods of interest in this course:
• Cambrian: marked by enormous increase in complex life forms
in the oceans. No land organisms. Much of the oil and
natural gas that we use today come from the remains of
animals of this period.
• Mississippian/Pennsylvanian: Huge tracts of tropical
swamps on the land. Large plants, few small animals. A.K.A.
the Carboniferous period because remains of plants
produced the coal we use today
• Permian: Rise of the large land dwelling lizards. Predinosaurs. Ended in the great mass extinction caused by the
eruption of the Siberian Traps.
Major Divisions of Time
Periods: Smaller time units within eras.
Major periods of interest in this course, cont.:
• Triassic, Jurassic, and Cretaceous: Mesozoic
periods marked by the rise of the dinosaurs, first
birds and small mammals. Ended by impact of
huge asteroid off coast of present-day Mexico.
• Holocene: Current Phanerozoic period; 10,000
years ago – present. Modern world as we know it.
Marked by the rise of human civilization,
cultivation and human-caused changes to the
surface of the earth and the environment
Major Divisions of Time
Controversy in Geologic Time Scale:
Have we entered a geological era in which the humaninduced changes to the earth will be evident in the
geologic record?
• Anthropocene: where does it begin?
• Rise of agriculture (8,000 -10,000 years ago)
• Industrial revolution (late 1700s)
• Earlier (14,000 – 15,000 years ago)
END