Title: Ch. 23.6: Interpreting the Rock Record
1Ch. 23.6 Interpreting the Rock Record
- OBJECTIVE
- Use principles of relative and absolute dating to
determine a sequence of events (climate,
tectonic, environmental) in Earths history. - Key terms Law of Superposition Principle of
Horizontality Unconformities Crosscutting
Relationships Index fossils Radiometric dating
isotopes half-life
2Earths Age
- Up until the 1700s Es age was estimated to be
6,000 years old - Today Es age is estimated to be 4.6 billion
years old. - Determined by absolute dating or radiometric
isotopes (well get back to)
3Importance 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?
Whats Earths normal? - Document Evolution
- Fossil record
- Major Events Meteroid impact Mountain building
(uplift) Rifting Glaciation
4Relative Dating of Earths Layers
- Allows you to determine the SEQUENCE OF EVENTS
- Order that rock layers formed (1st, 2nd, etc.)
- No specific date
5Relative Age1. 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.
6Relative Age2. Principle of Horizontality
- Sedimentary rock layers started out HORIZONTAL.
- If layers are TILTED or CURVED, tectonics
deformed them (Mt. Building or Faulting) -
7Relative Age3. Unconformities
- Breaks in geologic record Missing Time
- Deposition stopped or Rock layers were removed
(usually after uplift and erosion)
8Relative AgeTypes 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.
9Relative Age4. 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
--gt most recent).
10Relative 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
11Relative Age Index Fossils
- Example Ammonite fossils in layer 4 formed in
rocks 108 - 206 mya
12Problem 1
- Sequence the order of rock layers (oldest --gt
youngest) - 2. All of the numbered layers are sedimentary
except for ___ and _____. - There is an unconformity present. Where is it?
What does this mean?
13Problem 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)?
14Problem 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?
15Problem 3
- List sequence of events in relative order (oldest
--gt youngest)
- Events may include
- Deposition of sedimentary layers
- Intrusion of igneous rock
- Tectonics Uplift folding faulting
- Erosion
16Problem 4
- Put sedimentary layers in order.
- Indicate when the intrusion happened.
17Absolute Age Radiometric DatingHistory
Channels How the Earth was Made
http//www.history.com/shows/how-the-earth-was-mad
e/videos/the-age-of-earth?m51896f797a81d
- Uses Radioactive Isotopes
- Compares relative amounts of parentdaughter
-
- Gives specific age of rock
18Absolute 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
19Absolute 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).
20Absolute Dating Radiometric Decay
21Absolute Age Half Life
- The time it takes for 1/2 the mass of PARENT --gt
DAUGHTER. - Half life of 14C 5,730 years
- 100 g 14 C -----gt 50g 14 C 50g 14N after 5,730
years
22Half- Life of U 238 4.5 billion years
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24 Absolute Dating Half Life
25Complete the chart below
Time Parent isotope (g) Daughter isotope (g) Remaining Parent Time (Years)
Rock cyrstallizes (forms) 100 0 0
1 half-life 50 (1/2) 15 million
2 half - lives 25 25 (1/4)
3 half-lives 87. 5 45 million
4 half-lives 6.25 6.25 (1/16)
26Complete the chart below
Time Parent isotope (g) Daughter isotope (g) Remaining Parent Time (Years)
Rock cyrstallizes (forms) 100 0 100 0
1 half-life 50 50 50 (1/2) 15 million
2 half - lives 25 75 25 (1/4) 30 million
3 half-lives 12. 5 87. 5 12. 5 (1/8) 45 million
4 half-lives 6.25 93.75 6.25 (1/16) 60 million
27 Absolute Dating Carbon Dating
- Used for dating organic matter found in younger
rocks (lt 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
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29Answers to Quick Lab p.196
1. Parent Isotope After 3 intervals 12.5 After 6 intervals 1. 5 After 9 intervals 0.195
2. Daughter Isotopes created by decay After 3 intervals 12.5 After 6 intervals 1. 5 After 9 intervals 0.195
3. 20 seconds 5. No new parent (paper) added or removed cut at constant rate (half-life)