Class 4: Stratigraphy & Age Dating
Geologic History and Extinctions
Today’s topics:
Today’s topics:
 Characterizing geologic time
 Earth History
 Relative ages (stratigraphy)
 Mass Extinctions
 Absolute ages (radioactive decay)  Geologic Time Scale
Class 4: Stratigraphy & Age Dating
Geologic History and Extinctions
Class updates:
 Reading: Section 4.0-4.6
 Homework 1 due today
 Homework 2 online, due next Mon.
 Midterm practice online (website)
 Midterm review online (website)
 Midterm 1 on Feb. 28th (next Mon.)
How old is that rock?
Determining Relative Geologic
Time
1. Principle of Original Horizontality
2. Principle of Superposition
3. Principle of Cross-Cutting
Relationship
4. Principle of Faunal Succession
Principle of Original
Horizontality
Principle of Original
Horizontality
San Juan River, Utah
Canadian Rockies, Alberta
Principle of Superposition
Principle of Superposition
Principle of Cross-Cutting
Relationships
Igneous intrusion
Faulted rock
Principle of Cross-Cutting
Relationships
Principle of Cross-Cutting
Relationships
Grand Canyon
Order of Events?
2
1
4
3
Order of Events?
Grand Canyon
Example
3
4
2
1
Principle of Faunal (fossil) Succession
Index Fossils
 Def.: accurately indicate the ages of sedimentary rocks
1) Abundantly preserved in rocks
2) Geographically widespread
3) Fast evolving - existed for a relatively short time
4) Easily identified
Relative Dating:
Unconformities
comformity
unconformity
The Great Unconformity
Types of Unconformity
Types of Unconformity
Types of Unconformity
Relative Dating:
Correlation
How old is that rock?
 Relative age: order of events
Relative dating tells us what order things
happened, but not how many years ago they
happened.
 Absolute age: age in years
Absolute Dating: Radioactive Decay
Radioactive Half-Life
Half-life Example
• You start with $1000 in your bank account
• Every week your spouse or
boyfriend/girlfriend removes half of the
money
• The half-life of your account = 1 week
1 week
Your Bank Account
•
(1 half-life) •
•
•
•
•
•
•
•
•
Start: $1000
Week 1: $500
Week 2: $250
Week 3: $125
Week 4: $62.50
Week 5: $31.25
Week 6: $15.63
Week 7: $7.81
Week 8: $3.90
Week 9: $1.95
•
•
•
•
•
•
•
Week 10: $0.98
Week 11: $0.49
Week 12: $0.24
Week 13: $0.12
Week 14: $0.06
Week 15: $0.03
Week: 16: $0.015 left in
your bank account
Isotopes & Half Lives
Young things
Old things
Radiometric Dating
Radioactive Decay Curve
Calculating Absolute Age
• Potassium-40 included in mineral (start clock)
• Begins decaying to Argon-40
• Brilliant scientists have told us the half-life of K-Ar is
1.3 billion yrs (Gyr)
• We count # parents & daughters in sample
• We find that our sample contains 50% K & 50% Ar
• So 1/2 of our initial sample (K) has decayed
• How old is it?
• We find another sample that is 25% K & 75% Ar
• How old is it?
Mass Extinctions
A Bad Day…. 65 Million Years Ago
Mass Extinctions:
Extraterrestrial Impacts
Mass Extinctions:
Volcanic Eruptions
Fig. 4-2, p.75
Example Eruptions:
Fig. 4-2, p.75
Mass Extinctions:
Atmosphere-Ocean-Continent Interactions
Evolution of Earth Atmosphere
All Others
Oxygen
Nitrogen
Mass Extinctions:
Atmosphere-Ocean Interaction (1)
Fig. 4-2, p.75
Mass Extinctions:
Atmosphere-Ocean Interaction (2)
Mass Extinctions:
Atmosphere-Ocean-Continent Interactions
Mass Extinctions:
Ocean-Continent Interaction
Geologic time (millions of years)
Geologic Time & Biology
Geologic time (millions of years)
Geologic Time & Biology
Fig. 4-1b, p.74
Geologic time (millions of years)
Geologic Time & Biology
Geologic Time
 Major divisions

Cenozoic: 66 - 2.5 m.y.a

Mesozoic: 245 - 66 m.y.a.

Paleozoic: 544 - 245 m.y.a.

Precambrian: pre-544 m.y.a.
(proterozoic)
Geologic Time
Precambrian (4.6 Ga – 570 Ma)
Paleozoic Era (570 Ma - 245 Ma)
Mesozoic Era
(245 Ma – 66 Ma)
Cenozoic Era (66 Ma - present)
Earth
Satellites
• Size (radius)
• Bulk density
• Surface gravity
• Tectonism, volcanism
• Rotation
• Orbit
• Atmosphere
• Surface pressure
• Surface temperature
• Plate tectonics
• Water
•
1 (Moon)
6378 km
5.5 g/cm3 (4.1 g/cm uncompressed)
9.8 m/s2
yes (today)
1 day
365.25 days
78% N2, 21% O2
1 atm
20°C (diurnal, seasonal changes)
yes
yes
3