Class 4: Stratigraphy & Age Dating Geologic History and Extinctions 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 Dating Geologic History and Extinctions Class updates:  Reading: Section  Homework 1 due today  Homework 2 online, due next Mon.  Midterm practice online (website)  Midterm review online (website)  Midterm 1 on Feb. 28 th (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

San Juan River, Utah Canadian Rockies, Alberta

Principle of Superposition

Principle of Cross-Cutting Relationships Faulted rock Igneous intrusion

Principle of Cross-Cutting Relationships

Grand Canyon

Order of Events?

Grand Canyon Example Order of Events?

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

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 Geologic time (millions of years)

Fig. 4-1b, p.74 Geologic Time & Biology Geologic time (millions of years)

Geologic Time & Biology Geologic time (millions of years)

Geologic Time  Major divisions  Cenozoic: m.y.a  Mesozoic: m.y.a.  Paleozoic: m.y.a.  Precambrian: pre-544 m.y.a. (proterozoic)

Geologic Time

Precambrian (4.6 Ga – 570 Ma)

Paleozoic Era (570 Ma Ma)

Mesozoic Era (245 Ma – 66 Ma)

Cenozoic Era (66 Ma - present)

Satellites1 (Moon) Size (radius)6378 km Bulk density5.5 g/cm 3 (4.1 g/cm 3 uncompressed) Surface gravity 9.8 m/s 2 Tectonism, volcanismyes (today) Rotation 1 day Orbit days Atmosphere 78% N 2, 21% O 2 Surface pressure1 atm Surface temperature 20°C (diurnal, seasonal changes) Plate tectonicsyes Wateryes Earth