Transcript newes17-1 sol version

Chapter 17
The Rock Record
Page 322
BELLWORK: Order the following rock layers from
oldest  youngest
How old is the Earth?????
• 4.6 billion years old
• How do we know???
– By the geologic record
§17-1 Determining Relative Age
• Objectives:
1. State the principle of uniformitarianism.
2. Explain how the law of superposition can be
used to determine the relative age of rocks.
3. Compare the 3 types of unconformity.
4. Apply the law of crosscutting relationships to
determine the relative age of rocks.
Uniformitarianism
• originated with Scottish geologist
James Hutton in 1785
• history could be interpreted in terms
of processes currently observed.
Relative Dating of Rocks
• In sedimentary rocks, Age is based on position
• “Law of Superposition”
Law of Superposition
Unconformities
• Gaps in the
geologic
record.
• Periods of
erosion
• Unknown
time of
creation.
Nonconformity
• igneous intrusions into the sedimentary rock layers.
Disconformity
• Hard to recognize
because all layers are
horizontal.
Cross-Cutting
Relationships
Here are some real life examples of cross-cutting
relationships:
§17-2 “Determining Absolute Age”
• Objectives:
1) Summarize the limitations of using the
rates of erosion and deposition to
determine the absolute age of rocks.
2) Describe the formation of varves.
3) Explain how the process of radioactive
decay can be used to determine the
absolute age of rocks.
• Absolute age =>
Relative Age = little sister,
big brother, mom & granddad
–
–
–
–
Little sister is 2 Years old
Big brother is 6 years old
Mom is 30 years old
Granddad is 65 years old
Relative Age based on Rates of
Erosion
• Only works for the past 1020,000 years.
• Example:
– Niagara Falls is 9900 years
old based on current erosion
of the edge of 1.3 m per year
Relative Age based on rates of
deposition
• 30 cm every 1000
years for sedimentary
rock
• BUT if there is a flood
or other event this is
not very accurate.
Varve Count
• Varve = yearly layers of
sediment
• A lot like counting tree
rings.
• Each year has:
– a light colored, course layer
of sediment &
– a dark,fine layer of
sediment.
Dating using Radioactive Decay
• Uranium- 238 is the most widely
used element.
• It breaks down at a predictable rate
to Lead-206:
– ½ every 4.5 billion years
Activity today is:
Gizmo on Radioactive Decay
Explorelearning.com
Explorelearning.com
§17-3 The Fossil Record
• Objectives:
1) Describe 4 ways in which entire
organisms can be preserved as fossils.
2) List 4 examples of fossilized traces of
organisms.
3) Describe how index fossils can be used to
determine the age of rocks.
Paleontologists – study fossils
• Fossils can only be found
in sedimentary rock
Preservation is the Key !!
Remains Instantly Covered by
–
–
–
–
Mud
Sand
Silt/clay
Other organisms
Mummification
Occurs in deserts
Amber = pine sap
• Actually preserves
insects perfectly
including the DNA
Tar Seeps
• Like the La Brea Tar
Pits in California.
Freezing
• Like “Ice Man” & the
mammoth found in
Siberia.
Petrification
• Example:
– The Petrified Forest in
Arizona
Trace Fossils
Very Gross trace fossils
• Coprolites
– Fossilized poop
– Show scientists what the
animals ate.
• Gastroliths
– Stomach Stones used by
dinosaurs to grind their
food
– Like birds do today.
Interpreting the fossil record
• Used to date rocks
• How environments
have changed
– Remember Wegener?
• How animals were
affected & adapted to
the environment
– Remember Darwin?
Index Fossils
Example: Trilobites