Download - Chapter 17 The Rock Record Page 322. BELLWORK: Order the following rock layers from oldest youngest
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.
Relative Age = little sister, big brother, mom & granddad
• Absolute age => – 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 10-20,000 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
Gizmo on Radioactive Decay
Explorelearning.com
Explorelearning.com
Activity today is:
§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