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Ch. 23.6: Interpreting the Rock Record

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1 Ch. 23.6: Interpreting the Rock Record
OBJECTIVE: Use principles of relative and absolute dating to determine a sequence of events (climate, tectonic, & environmental) in Earth’s history. Key terms: Law of Superposition; Principle of Horizontality; Unconformities; Crosscutting Relationships; Index fossils; Radiometric dating; isotopes; half-life

2 Earth’s Age Up until the 1700s E’s age was estimated to be ~ 6,000 years old Today: E’s age is estimated to be 4.6 billion years old. Determined by absolute dating or radiometric isotopes (we’ll get back to)

3 Importance of Rock Record
Paleoenvironment & Climate Was this place a swamp? Coral reef? Desert? Tropical forest? Covered in ice? Rates of Climate Change Has Earth rapidly warmed or cooled before? What’s Earth’s normal? Document Evolution Fossil record Major Events: Meteroid impact; Mountain building (uplift); Rifting; Glaciation

4 Relative Dating of Earth’s Layers
Allows you to determine the SEQUENCE OF EVENTS Order that rock layers formed (1st, 2nd, etc.) No specific date

5 Relative Age 1. Law of Superposition
A sedimentary rock layer is older than the layer above; younger than layer below * Undeformed layers Sediments are deposited on top of existing layers and lithified.

6 Relative Age 2. Principle of Horizontality
Sedimentary rock layers started out HORIZONTAL. If layers are TILTED or CURVED, tectonics deformed them (Mt. Building or Faulting)

7 Relative Age 3. Unconformities
Breaks in geologic record = Missing Time Deposition stopped or Rock layers were removed (usually after uplift and erosion)

8 Relative Age Types of Unconformities
Look for erosional surfaces; tilted layers; or igneous intrusions Left: Nonconformity = Igneous or metamorphic rock is uplifted, exposed, and eroded. Sed layers deposited on top. Middle: Angular Unconformity = layers are folded or tilted, then eroded. New layers sed layers deposited on top. Right: Disconformity = Horizontal layers are uplifted and eroded. New sed. Layers deposited on top.

9 Relative Age 4. Crosscutting Relationships
If a fault or igneous intrusion cuts across a layer … it happened after that layer Which happened first: faulting or igneous intrusion? Write a summary of events for this region (oldest --> most recent).

10 Relative Age: Index Fossils
Fossils that narrow age of rock to a geologic period or era (millions of years) Requirements: Abundant - found in many regions Lived during “short” , specific span of time Distinguishing features

11 Relative Age: Index Fossils
Example: Ammonite fossils in layer 4 formed in rocks mya

12 Problem 1 Sequence the order of rock layers (oldest --> youngest) 2. All of the numbered layers are sedimentary except for ___ and _____. There is an unconformity present. Where is it? What does this mean?

13 Problem 1 What evidence is there that a tectonic event affected this area in the past? Describe and interpret this evidence. 5. What happened first: Faulting (B) or Intrusion (3)?

14 Problem 2 Label youngest and oldest sedimentary layers (bottom drawing). Describe the tectonic setting that would produce the folded layers. 3. Why are the tops of the folded layers cut off? How did this happen?

15 Problem 3 List sequence of events in relative order (oldest --> youngest) Events may include: Deposition of sedimentary layers Intrusion of igneous rock Tectonics: Uplift; folding; faulting Erosion

16 Problem 4 Put sedimentary layers in order.
Indicate when the intrusion happened.

17 Absolute Age: Radiometric Dating
Uses Radioactive Isotopes Compares relative % of parent:daughter Gives specific age of rock

18 Absolute Age: Radiometric Dating
Nucleus = Particles w/Mass Protons (+), determine element identity Neutrons (no charge), can vary Isotopes = Atoms of the same element with different # of neutrons. Ex: 12 C (6 protons + 6 neutrons), 14 C (6 protons + 8 neutrons) Radioactive Isotopes = Atoms that have nuclei that break apart (unstable) naturally. Release energy & particles

19 Absolute Age: Radioactive Decay
Unstable PARENT Isotope breaks down to stable DAUGHTER Isotope (& releases energy) Decay happens at a constant rate (not changed by Temp., Pressure, or environmental conditions).

20 Absolute Dating: Radiometric Decay

21 Absolute Age: Half Life
The time it takes for 1/2 the mass of PARENT --> DAUGHTER. Half life of 14C = 5,730 years 100 g 14 C -----> 50g 14 C + 50g 14N after 5,730 years

22 Half- Life of U 238 = 4.5 billion years


24 Absolute Dating: Half Life

25 Complete the chart below
Time Parent isotope (g) Daughter isotope (g) Remaining Parent Time (Years) Rock cyrstallizes (forms) 100 100% 1 half-life 50 50% (1/2) 700 2 half - lives 25 75 25% (1/4) 1400 3 half-lives 12. 5 87. 5 12. 5% (1/8) 2100 4 half-lives 6.25 93.75 6.25% (1/16) 2800

26 Complete the chart below
Time Parent isotope (g) Daughter isotope (g) Remaining Parent Time (Years) Rock cyrstallizes (forms) 100 1 half-life 50% (1/2) 700 2 half - lives 25 25% (1/4) 3 half-lives 87. 5 2100 4 half-lives 6.25 6.25% (1/16)

27 Absolute Dating: Carbon Dating
Used for dating organic matter found in younger rocks (< 70,000 years) Wood, bones, shells 14 C made by cosmic radiation & incoporated into plants via photosynthesis (plants take in CO2 from air) Alive - Organisms have constant ratio of 12C: 14C Dead - 14C decays and 14N increases


29 Answers to Quick Lab p.196 1. Parent Isotope
After 3 intervals: 12.5% After 6 intervals: 1. 5% After 9 intervals: % 2. Daughter Isotopes created by decay seconds 5. No new parent (paper) added or removed; cut at constant rate (half-life)

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