1. KEY CONCEPT The geologic time scale divides Earth’s history based on major past events. Earth is approx. 4.5 billion years old.

2. Index fossils are another tool to determine the age of rock layers
Index fossils are another tool to determine the age of rock layers. Index – point of reference
Index fossils can provide the relative age of a rock layer.
existed only during specific spans of time (few hundred thousand), common or plentiful
occurred in large geographic areas, easy to recognize
The shorter the timespan, the more precise the date.
Index fossils include fusulinids ( million), Trilobites

3. The geologic time scale organizes Earth’s history.
The history of Earth is
represented in the geologic
time scale. Geologic time is
not divided uniformly into
standard units of time.
Geologic time is divided
based on rock layers as well
as Animal and plant life.
Precambrian time makes up
about 87% of the Earth’s History.
The 3 Basic divisions are Eras,
Periods, and Epochs

4. Eras last tens to hundreds of millions of years.
consist of two or more periods
Three eras: Cenozoic “recent life”, Mesozoic “middle life”, Paleozoic “ancient life”

5. Periods last tens of millions of years.
most commonly used units of time on time scale
associated with rock systems and divided by mass extinctions

6. Epochs last several million years.
Smallest unit of time