Download How Do You Study the Past? (The Rock Record: Absolute

Studying The Past
I.
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?
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)
III. Types of Fossils
A. Unaltered Remains
1. Description: plant or animal remains that have not
undergone change since death.
a. Uncommon because frozen, extremely dry, or oxygenfree 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 Remains
1. Description: all organic material has been removed and the
hard parts of the organism have been changed
a. 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
B. Altered Remains - Continued
2. 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 Evidence
1. 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 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 Age
a. 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
b. 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?
I
2. Which is the youngest rock unit in the outcrop? R
3. What does “H” represent?
An intrusion
4. Why don’t the layers on the left side of the diagram match with the
layers on the right side of the diagram? Because of the fault
5. What do we call what is happening between layer “F” and “M”?
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 – 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
Example: Uranium-238Lead-206
1 half-life
2 half-lives
0 years
4.5 billion years
9 billion years
100 % U
50 % U
25 % U
50 % lead
75 % lead
Graph
%U left
time
C. A Special Case of Relative/Absolute Dating - Index Fossils
1. Description: Remains of unique species that can be used to
correlate rock layers or to date a particular rock layer
a. Must be easily recognized, abundant, and widely
distributed geographically
b. Must have lived during a relatively short time period
c. 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, when
they went extinct