II. Why Do We Study Fossils Found in Rocks?

I. What is a Fossil?A. Definition: The evidence or remains of once-living plants or

animals

A. To provide evidence of the past existence of life formsB. To provide information about past environmental conditionsC. To provide evidence that populations have undergone change

over time due to environmental changes (evolution)

Studying The Past

III. Types of FossilsA. Unaltered Remains

1. Description: plant or animal remains that have not undergone change since death.a. Uncommon because frozen, extremely dry, or oxygen-

free environments are required to form these fossilsb. Examples:

• Mummified humans• Frozen organisms (Ice Man)• Mammoths & cats in La Brea Tar Pits• Fossilized insects in tree sap (amber)

B. Altered Remains1. Description: all organic material has been removed and the

hard parts of the organism have been changeda. Minerals seep in slowly and replace the original

organic tissue, forming a rock-like fossilb. The fossil has the same shape as the original object,

but is chemically more like a rock!c. Examples:

• Petrified wood• Recrystallized shells

B. Altered Remains - Continued2. Types of Altered Remains - Molds and Casts

a. An organism dies and gets trapped/buried in sediment (sand, ash, etc)

b. The original organism decomposes or dissolves away leaving a hollow impression of the organism (mold)

c. This cavity might later become filled with minerals or sediment to create a cast.

d. Examples: • Plant fossils• Trilobites

C. Indirect Evidence1. Description: Trace fossils of plant and animal life

a. Provide information about how an organism lived, moved or obtained food

b. Examples: • Coprolites (animal waste)• Burrows/trails• footprints

IV. Dating Fossils

A. Relative-Age Dating: 1. Definition: Dating rocks and fossils by placing them in

chronological order without exact dates.2. Geologic Principles (used in this dating process, also called

Steno’s laws):a. Original Horizontality

• Sedimentary rocks are deposited in horizontal layersb. The Law of Superposition

• in an undisturbed sequence the oldest rocks are at the bottom and each successive layer is younger

c. Principle of Cross-Cutting Relationships:• an intrusion or a fault is younger than the rock it cuts

across

3. Other Means of Determining Relative Agea. 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 themb. Inclusions

• If a rock contains fragments of another rock, then the rock that is containing the fragments must be younger than the fragments themselves

c. Unconformities• Gaps in the geologic record• May indicate erosion or

deformation of the rock layers

Relative Dating - Applying the Concepts

1. Which is the oldest rock unit in the outcrop?

2. Which is the youngest rock unit in the outcrop?

3. What does “H” represent?

4. Why don’t the layers on the left side of the diagram match with the layers on the right side of the diagram?

5. What do we call what is happening between layer “F” and “M”?

IH

An intrusion

Because of the fault

Faulting, unconformity

B. Absolute-Age Dating: 1. Definition: Dating rocks and fossils by using techniques to

determine their actual age.2. 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

B. Absolute Age Dating – Continued3. Uranium 238

• When rock forms from magma, it contains U-238 andthere is no lead present (U-238 decays into Lead-206)

• U-238 can be used to date rocks formed in the early Earth4. 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

0 years

100 % U

4.5 billion years

50 % U

50 % lead

9 billion years

25 % U

75 % lead

%U left

time

Graph

1 half-life 2 half-lives

Example: Uranium-238➜Lead-206

C. A Special Case of Relative/Absolute Dating - Index Fossils1. Description: Remains of unique species that can be used to

correlate rock layers or to date a particular rock layera. Must be easily recognized, abundant, and widely

distributed geographicallyb. Must have lived during a relatively short time periodc. If we can date a rock layer absolutely (exact age), then

we can use the index fossil to date similar rock layers absolutely

d. If we can only date the rock layer relatively (approximate age), then we can use the index fossil date similar rock layers relatively.

e. Examples: • Ammonites were common 245

to 65 million years ago, whenthey went extinct