12.2 The Geologic Time Scale
KEY CONCEPT
The geologic time scale divides Earth’s history based
on major past events.
12.2 The Geologic Time Scale
Index fossils are another tool to determine the age of rock
layers.
• Index fossils can provide the relative age of a rock layer.
– existed only during specific spans of time
– occurred in large geographic areas
• Index fossils include fusulinids and trilobites.
12.2 The Geologic Time Scale
The geologic time scale organizes Earth’s history.
• The history of Earth is
represented in the geologic time
scale.
100
250
550
1000
2000
PRECAMBRIAN TIME
This time span makes up the
vast majority of Earth’s history.
It includes the oldest known
rocks and fossils, the origin of
eukaryotes, and the oldest
animal fossils.
Cyanobacteria
12.2 The Geologic Time Scale
• Eras last tens to hundreds of millions of years.
– consist of two or more periods
– three eras: Cenozoic, Mesozoic, Paleozoic
12.2 The Geologic Time Scale
• Periods last tens of millions of years.
– most commonly used units of time on time scale
– associated with rock systems.
• Epochs last several million years.