ii. why do we study fossils found in rocks? i.what is a fossil? a. definition: the evidence or...

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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

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Page 1: 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

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

Page 2: 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

III. Types of Fossils

A. Original Preservation1. 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)

Page 3: 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

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 fossilb. The fossil has the same shape as the original object,

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

• Petrified wood• Recrystallized shells

Page 4: 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

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 geographicallyb. Must have lived during a relatively short time periodc. Examples:

• The mollusk Ecphora has a distinctive shape

Page 5: 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

D. Molds and Casts1. Description: Fossils that do not contain any shell or bone

a. 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

Page 6: 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

E. Trace Fossils1. Description: Indirect evidence of plant and animal life

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

b. Examples: • worm trails• burrows• footprints

Page 7: 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

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 youngerc. Principle of Cross-Cutting Relationships:

• an intrusion or a fault is younger than the rock it cuts across

Page 8: 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

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 them

Relative 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?

Page 9: 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

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.

Page 10: 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

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.

Page 11: 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

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

Absolute Dating - Applying the Concepts

Page 12: 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

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?