Class 4: Stratigraphy & Age Class 4: Stratigraphy & Age DatingDating

Geologic History and Geologic History and ExtinctionsExtinctions

Today’s topics: Characterizing geologic time Relative ages (stratigraphy) Absolute ages (radioactive decay)

Today’s topics: Earth History

Mass Extinctions Geologic Time Scale

Class 4: Stratigraphy & Age Class 4: Stratigraphy & Age DatingDating

Geologic History and Geologic History and ExtinctionsExtinctions

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 Horizontality2. Principle of Superposition3. Principle of Cross-Cutting

Relationship4. Principle of Faunal Succession

Principle of Original Horizontality

Principle of Original Horizontality

San Juan River, UtahSan Juan River, Utah Canadian Rockies, AlbertaCanadian Rockies, Alberta

Principle of Superposition

Principle of Superposition

Principle of Cross-Cutting Relationships

Faulted rockFaulted rockIgneous intrusionIgneous intrusion

Principle of Cross-Cutting Relationships

Principle of Cross-Cutting Relationships

Grand CanyonGrand Canyon

Order of Events?

1122

3344

Grand CanyonExample

Order of Events?

11

22

33

44

Principle of Faunal (fossil) Succession

Index Fossils Def.: accurately indicate the ages of sedimentary rocks

1) Abundantly preserved in rocks2) Geographically widespread3) Fast evolving - existed for a relatively short time4) Easily identified

Relative Dating: Unconformities

comformitycomformity unconformityunconformity

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

Absolute age: age in years

Relative dating tells us what order things happened, but not how many years ago

they happened.

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

• 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

(1 half-life)

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

Fig. 4-2, p.75

Mass Extinctions:Volcanic Eruptions

Fig. 4-2, p.75

Example Eruptions:

Mass Extinctions:Atmosphere-Ocean-Continent Interactions

Evolution of Earth Atmosphere

Nitrogen

Oxygen

All Others

Fig. 4-2, p.75

Mass Extinctions:Atmosphere-Ocean Interaction (1)

Mass Extinctions:Atmosphere-Ocean Interaction (2)

Mass Extinctions:Atmosphere-Ocean-Continent Interactions

Mass Extinctions:Ocean-Continent Interaction

Geologic Time & Biology

Geolo

gic

tim

e (

million

s o

f years

)

Fig. 4-1b, p.74

Geologic Time & Biology

Geolo

gic

tim

e (

million

s o

f years

)

Geologic Time & Biology

Geolo

gic

tim

e (

million

s o

f years

)

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)

• Satellites 1 (Moon)• Size (radius) 6378 km• Bulk density 5.5 g/cm3 (4.1 g/cm3uncompressed)

• Surface gravity 9.8 m/s2

• Tectonism, volcanism yes (today)• Rotation 1 day• Orbit 365.25 days• Atmosphere 78% N2, 21% O2

• Surface pressure 1 atm• Surface temperature 20°C (diurnal, seasonal changes)

• Plate tectonics yes• Water yes

Earth