1. II. Why Do We Study Fossils Found in Rocks?
Studying The Past
What is a Fossil?
A. Definition: The evidence or remains of once-living plants or animals
II. Why Do We Study Fossils Found in Rocks?
To provide evidence of the past existence of life forms
To provide information about past environmental conditions
To provide evidence that populations have undergone change over time due to environmental changes (evolution)

2. III. Types of Fossils Unaltered Remains
Description: plant or animal remains that have not undergone change since death.
Uncommon because frozen, extremely dry, or oxygen-free environments are required to form these fossils
Examples:
Mummified humans
Frozen organisms (Ice Man)
Mammoths & cats in La Brea Tar Pits
Fossilized insects in tree sap (amber)

3. B. Altered Remains
Description: all organic material has been removed and the hard parts of the organism have been changed
Minerals seep in slowly and replace the original organic tissue, forming a rock-like fossil
The fossil has the same shape as the original object, but is chemically more like a rock!
Examples:
Petrified wood
Recrystallized shells

4. B. Altered Remains - Continued
Types of Altered Remains - Molds and Casts
An organism dies and gets trapped/buried in sediment (sand, ash, etc)
The original organism decomposes or dissolves away leaving a hollow impression of the organism (mold)
This cavity might later become filled with minerals or sediment to create a cast.
Examples:
Plant fossils
Trilobites

5. C. Indirect Evidence
Description: Trace fossils of plant and animal life
Provide information about how an organism lived, moved or obtained food
Examples:
Coprolites (animal waste)
Burrows/trails
footprints

6. IV. Dating Fossils Relative-Age Dating:
Definition: Dating rocks and fossils by placing them in chronological order without exact dates.
Geologic Principles (used in this dating process, also called Steno’s laws):
Original Horizontality
Sedimentary rocks are deposited in horizontal layers
The Law of Superposition
in an undisturbed sequence the oldest rocks are at the bottom and each successive layer is younger
Principle of Cross-Cutting Relationships:
an intrusion or a fault is younger than the rock it cuts across

7. 3. Other Means of Determining Relative Age
Correlation
Used to date rock layers that are far apart from each other
Geologists examine rocks for distinctive fossils and features to help identify and date them
Inclusions
If a rock contains fragments of another rock, then the rock that is containing the fragments must be younger than the fragments themselves
Unconformities
Gaps in the geologic record
May indicate erosion or
deformation of the rock layers

8. Relative Dating - Applying the Concepts
Which is the oldest rock unit in the outcrop?
Which is the youngest rock unit in the outcrop?
What does “H” represent?
Why don’t the layers on the left side of the diagram match with the layers on the right side of the diagram?
What do we call what is happening between layer “F” and “M”? I R
An intrusion
Because of the fault
Faulting, unconformity

9. B. Absolute-Age Dating:
Definition: Dating rocks and fossils by using techniques to determine their actual age.
Radioactive Dating: Dating fossils based on the amount of radioactive material remaining in a substance over time
When nuclei are unstable they spontaneously break apart (decay) in a process called radioactivity
The original unstable element (parent) is converted to a different stable element (daughter)
Since the rate of decay is constant, you can measure the parent to daughter ratio to determine
the age of the rock
The length of time it takes for
one-half of the original
radioactive amount to decay is
called the elements half-life

10. Absolute Age Dating – Continued
3. Uranium 238
When rock forms from magma, it contains U-238 and
there is no lead present (U-238 decays into Lead-206)
U-238 can be used to date rocks formed in the early Earth
4. Carbon 14
When an organism is alive, its C-14 is continuously replaced
When that organism dies, the C-14 decays to Nitrogen-14 and it
does not get replaced
C-14 can be used to date geologic events involving organisms
within the past 30,000 years

11. 1 half-life 2 half-lives 9 billion years 4.5 billion years 0 years
Example: Uranium-238Lead-206
1 half-life
2 half-lives
9 billion years
25 % U
75 % lead
4.5 billion years
50 % U
50 % lead
0 years
100 % U
Graph
%U left
time

12. C. A Special Case of Relative/Absolute Dating - Index Fossils
Description: Remains of unique species that can be used to correlate rock layers or to date a particular rock layer
Must be easily recognized, abundant, and widely distributed geographically
Must have lived during a relatively short time period
If we can date a rock layer absolutely (exact age), then we can use the index fossil to date similar rock layers absolutely
If we can only date the rock layer relatively (approximate age), then we can use the index fossil date similar rock layers relatively.
Examples:
Ammonites were common 245
to 65 million years ago, when
they went extinct