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
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