Relative and Absolute Dating

Relative dating - the age of a rock, fossil, or other feature measured Relative dating - the age of a rock, fossil, or other feature measured compared 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 Superposition Oldest rocks are on the bottom, Youngest rocks are on the top

Superposition image Younger

#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 Example Grand Canyon Example

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 indicators Short age range Easily recognized Widespread distribution

Index Fossils image

For that, we need Absolute dating 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 Dating Process 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

Procedures that count the products of radioactivity to date rocks Radiometric Dating Procedures that count the products of radioactivity to date rocks

When atomic nuclei spontaneously break apart Types: Alpha emission Radioactivity When atomic nuclei spontaneously break apart Types: Alpha emission Beta emission Gamma 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: 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

Geologic Time Scale Eon/Era/Period names - based on fossil assemblages, superposition Absolute ages, from radiometric dating