William E. Ferguson Relative DatingRelative Dating Absolute DatingAbsolute Dating.

William E. Ferguson

•Relative DatingRelative Dating•Absolute DatingAbsolute Dating

Kelvin Calculation of Earth AgeKelvin Calculation of Earth Age

Unconformity TypesUnconformity Types

Figure 8.1 LabFigure 8.1 Lab

Folding RockFolding Rock

Relative Dating PrinciplesRelative Dating Principles

Geologic Puzzle Time!Geologic Puzzle Time! - superposition, horizontality, cross- - superposition, horizontality, cross-

cutting relationshipscutting relationships

• In assigned groups of 3, work out the age relationships In assigned groups of 3, work out the age relationships in Figs. 8.4 - 8.12 in your lab bookin Figs. 8.4 - 8.12 in your lab book

• Pay attention to contact metamorphism!Pay attention to contact metamorphism!• Use pencilUse pencil!!!!!!!!

• Put in squiggly lines between events for uncomformities Put in squiggly lines between events for uncomformities

• Be ready as a group to transfer your answers to an Be ready as a group to transfer your answers to an overhead & explain your relationships to the class.overhead & explain your relationships to the class.

Figure 8.4bFigure 8.4b

Figure 8.10bFigure 8.10b

Figure 8.9 LabFigure 8.9 LabFFBBKKNNAAJJDDMMHHCCLLGGEE

Figure 8.10a LabFigure 8.10a LabGGHHIIAABBCCDDLLFFEEMMJJKK

CouldCouldbebe

youngeryounger

Figure 8.11 LabFigure 8.11 LabSSJJKKDDXXEEAAHHMMRRBBGGPPFF

Figure 8.12 LabFigure 8.12 LabRRNNHHPPDDAATTFFLLGGJJZZSSBBEEKKXXMMOOCCVV

Radioactive DecayRadioactive Decay

What we need to know to apply radiometric What we need to know to apply radiometric datingdating

• The half-life of a radioactive isotope = time required for half of it to decay to a stable (non-radioactive) daughter isotope.

• 40K decays to 40Ar with half-life = 1300 million years.

• 40K and 40Ar can be measured in a mass spectrometer.

• 40K is decaying to 40Ar all the time, but 40Ar is a gas and can escape if a mineral is above the “blocking temperature”. Below the blocking temperature, 40Ar is trapped in the mineral.

• If a mineral is heated above the blocking temperature, 40Ar is lost.

• Different isotopes (of U, K, Rb, Sr, etc) used in dating have different blocking temperatures.

40K has the simplest decay scheme, and is the only one we’ll examine in detail.

# of # of ParentParent & Daughter & Daughter Atoms as a Function of TimeAtoms as a Function of Time

# o

f p

aren

t o

r d

aug

hte

r is

oto

pes

ParentParent

DaughterDaughter

ProportionProportion of Parent Atoms Remaining of Parent Atoms Remaining as a Function of Timeas a Function of Time

Fig. 10.14Fig. 10.14

Percent of Percent of Parent Atoms Parent Atoms

Remaining as a Remaining as a Function of Function of

TimeTime

100

50

25

12.56.25

3.125

% o

f p

aren

t is

oto

pe

rem

ain

ing

As more parent decays, the As more parent decays, the decay rate slows down, but decay rate slows down, but the half-life doesn’t changethe half-life doesn’t change

100

50

25

12.56.25

3.125

% o

f p

aren

t is

oto

pe

rem

ain

ing

Slope =Slope = rate of decayrate of decay

Decay rate = Decay rate = PP

P = amt. Of parentP = amt. Of parent

= decay constant= decay constant = 0.693/half-life= 0.693/half-life