william e. ferguson relative datingrelative dating absolute datingabsolute dating
Post on 22-Dec-2015
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Kelvin Calculation of Earth AgeKelvin Calculation of Earth Age
Unconformity TypesUnconformity Types
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.
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