Dating Fossils

Index Fossils and Geologic Time
Dating Fossils
1. Scientists use two methods to date
fossils: absolute dating & relative dating
2. Scientists use radiometric dating to
determine the absolute age of a rock or
3. Relative dating uses layers of rock to
determine a relative age. Ex: Deeper rocks
are older and therefore have older fossils
Using Fossils to Date Rocks
4. Index fossils are fossils of organisms
that scientists use to determine how old
rock layers are. To be an index fossil the
organism must:
A. Be common in rocks from most of
the world.
B. Have lived for only a geologically
short period of time(a few million years)
C. Be easy to identify Ex: trilobites &
Using Fossils to Date Rocks
5. Index fossils are useful to scientists
because they can be used to help tell the
relative ages of the rock layers in which
they occur.
Index fossil
North America
South America
Geologic Time: Why Do We Use It?
Geologic Time Scale
The geologic time scale is a record
of the life forms and geologic events in
Earth’s history.
7. Scientists use the geologic time
scale instead of years because the
span of Earth’s history is so great.
Geologic Tim 8. The
Earth is
about 4.6
Years old
and is
older than
the oldest
fossils on