Relative and

Absolute Dating

Relative dating - the age of a rock, fossil, or other feature measuredcompared to another.

Absolute dating - (numerical

dating) the age of a rock in years.

Relative Dating Laws:

#1 Original Horizontality

Sedimentary rocks are formed in layers which

were originally horizontal

Observe flat layers - rocks undisturbed

Observe tilted layers - tectonics at work

#2 Lateral Continuity

The horizontal layers continue in

all directions at the time that they form

Lateral Continuity image

#3 SuperpositionOldest rocks are on the bottom,Youngest rocks are on the top

Superpositionimage

You

nger

#4 Cross-cutting relationships

Igneous intrusions and faults are younger than the sedimentary rocks that they cut across

Cross-cutting relationships image

Unconformities• Represent intervals of time when layers are missing from the geologic record

• Caused by Uplift and Erosion

Grand Canyon ExampleGrand CanyonExample

Principles of Fossil Succession

• Fossil organisms succeed one another in a definite order

• Any time period can be recognized by its fossil content Examples: age of humans, ages of fishes, age of

dinosaurs, etc.

• This is application of the

Principle of Superposition• Important for Correlation

Index Fossils

•Fossils useful as time indicatorsShort age rangeEasily recognizedWidespread distribution

Index Fossils image

Relative dating tells us what order things happened, but not

how many years ago they happened

For that, we need Absolute dating

Absolute dating:

Radiometric DatingProcess used to determine the absolute age of a rock or a fossil by determining

the ratio of parent nuclei to daughter nuclei within a

given sample.

Radioactive atoms are like clocks

All minerals contain some radioactive atoms

Only works for IGNEOUS rocks

Radiometric Dating

Procedures that count the products of

radioactivity to date rocks

Radioactivity• When atomic nuclei spontaneously break apart

• Types:Alpha emissionBeta emissionGamma rays

Alpha Emission

Beta Emission

Radiometric dating keys:

•Knowing the HALF LIFE

•Having the parent and the daughter together in your mineral

Half-life Example #1

• 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

• Questions to think about: When is the most money taken out? Will the amount in your bank account ever go to

zero?

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: One and

a half cents left in your bank account

Radioactive Decay Curve image

Radiometric Dating Limitations• Sedimentary Rocks

Weathered material of other rocks Radiometric clock has been altered

• Metamorphic Rocks Radiometric clock has been altered?

• Igneous Rocks Minerals form when rock forms No Problem – best source for radiometric dating

Radiometric Decay Data

Front Page of ESRT’s

Carbon 14 Only useful in finding the age of ORGANIC materials up to 75,000

years old.

Carbon 14 Decay Curve

Other types of Absolute Dating

Tree Rings

• Annual growth rings are a function of their environment, i.e. temperature, humidity, precipitation, insects, fires, etc.

• Limited to recent geologic past (furthest back 8000 years)

Ice Layers

• Ice Sheets in Greenland and Antarctica record annual changes in accumulation and snowmelt

• Records up to 65,000 years

• Ice also traps volcanic ash which can be radiometrically dated

• Record climatic conditions of the Earth

Ash Layers

• Ash Layers from major volcanic eruptions create KEY BEDS – a distinct layers of known age

GeologicTime Scale

• Eon/Era/Period names - based on fossil assemblages, superposition

• Absolute ages, from radiometric dating