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 forms

B. To provide information about past environmental conditions

C. To provide evidence that populations have undergone change over time due to environmental changes (evolution)

Studying The Past

III. Types of Fossils

A. Original Preservation1. Description: plant or animal remains

that have not undergone elemental change since death.a. Uncommon because frozen, extremely

dry, or oxygen-free environments are required to form these fossils

b. Examples: • Mummified humans• Frozen organisms (Ice Man)• Mammoths & cats in La Brea Tar

Pits• Fossilized insects in tree sap

(amber)

B. Altered Hard Parts1. 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 fossil

b. The fossil has the same shape as the original object, but is chemically more like a rock!

c. Examples: • Petrified wood• Recrystallized shells

C. 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 geographically

b. Must have lived during a relatively short time period

c. Examples: • The mollusk Ecphora has a

distinctive shape

D. Molds and Casts1. Description: Fossils that do not

contain any shell or bonea. A mold is formed when original shell

parts are weathered and eroded, leaving an impression of the shell.

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

c. Examples: • Common with shellfish

E. Trace Fossils1. Description: Indirect evidence of plant

and animal lifea. Provide information about how an

organism lived, moved or obtained food

b. Examples: • worm trails• burrows• 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):a. Original Horizontality

• Sedimentary rocks are deposited in horizontal layers

b. 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 (index) and features to help identify and date themRelative Dating - Applying the Concepts

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

2. Explain why the rock layers on the west side of the outcrop do not match the east side.

3. Which is the younger layer, layer A or layer C?

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

using techniques to determine their actual age.

2. Methods:a. Tree Rings and Seasonal Climatic

Changes• Each tree ring represents 1 year of

growth• Varves are bands of sediment that

show a yearly cycle from climate change

• Although accurate, neither method can be used to date very far back in time.

Methods cont.:b. Radioactive Dating: Dating fossils

based on the amount of radioactive material remaining in a substance over time• Radioactive substances (unstable atoms) emit protons

and neutrons at a constant rate• The original element (parent) is converted to a

different 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 amount to decay is called the elements half-life.

Ex. Uranium-238 will decay into an isotope of lead, Pb - 206

Absolute Dating - Applying the Concepts

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-238Lead 206

•Do you want to see a simulation?