Richard J. Lloyd
Nov. 18, 2008
Edinboro University of Pennsylvania

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a) b)
Biblical Chronologies
Chronologie de l’historie
sainte (1738) collected over 200 computations with creation dates ranging from 3483 B.C. to 6984 B.C.
Johannes Kepler combined biblical and astronomical arguments. Result:
3993 B.C.
Mostly based on determining the time elapsed between known historical events— i.e. the Flood, Abraham, etc. by summing lists of generations and the reigns of various rulers. Assumed roughly 3 generations per century and about 25 years/ruler typically

Kepler’s Chronology
Based on the belief that Earth was created at the summer solstice when the solar apogee was at the head of the constellation Aries.
Using the known rate at which the solar apogee moved, a date could be calculated.
Ussher’s Chronology
Most famous for his prediction of creation in
October 22, 4004 B.C. at midnight using astronomical cycles, historical accounts and biblical chronology.

Credibility of biblical chronologies eroded in
17 th and 18 th centuries when observations of nature were becoming more precise in concert with the theories that explained natural phenomenon.
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Methods based on scientific explanation became popular, variously depending on observations of:
Declining sea levels
Cooling of Earth and Sun
Scenarios involving the
Moon’s origin
Salt clocks and sedimentation

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De Maillet (1720’s)
Based on assuming Earth was once entirely covered with water
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This was NOT stupid!
Marine strata bearing sea shells were found in inland mountains
Tried to measure present rate of sea level decline from historical records and modern data. Result: 2.4
Gyr—half the modern result!
Why It Was Wrong
We know (now!) that most sea level declines locally are due to land UPLIFT. There are other places where the land level is falling. These effects are due to geological forces not understood in the day of
De Maillet. The modern theories of geology were over a century away, and almost
250 years before plate tectonics.

Comte de Buffon (1749)
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Measured the rate of cooling of iron spheres of various diameters—found a nearly linear relationship between diameter and cooling time.
Extrapolated this to a molten Earth of known diameter—96,700yrs.
Why It Was Wrong
Buffon himself didn’t trust his results and noted that the thickness of sedimentary rocks would affect cooling rate. In other calculations, he estimated ages up to 3 Gyr.

William Thomson, Lord
Kelvin (1860)
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Postulated Sun was receiving energy from infall of meteoric material.
Observations showed too little material to maintain the Sun’s temperature—
Sun must be cooling.
Variously estimated between 10-500 million years for the age of the Sun
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Why It Was Wrong
There was no way to measure the Sun’s temp. at the time (Kelvin put it at
2.3 times the actual value)
He assumed the specific heat of the Sun was the same as water
He disregarded any possible INTERNAL heat sources.

Kelvin, round 2
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Applied similar reasoning to the Earth’s cooling.
Used data collected from mines, showing increasing temp. with depth.
Using various guesses and supplementary evidence, the result was about 98
Myr.
Wrong, again!
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Kelvin used inadequate theory of heat conduction
Data available was sparse, unreliable.
Dismissed INTERNAL sources of heat, specifically lunar tidal friction, chemical action, etc.
Radioactivity was still unknown.

Moon’s Origin (1890)
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Hypothesis was that Moon was formed by rapid rotation of Earth in Early history—mat’l flew off
Tidal friction slowed
Earth’s rotation at rate which could be measured—resulted in
Earth less than 1 Gyr.
Why Wrong
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Moon probably originated from impact of planetoid sized body
Hypothesis could not account for inclination of
Moons orbit with Earth’s rotation
Moon would have been 3 times larger under this hypothesis than it really is.

Various proponents (1900)
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Assume that the influx from rivers, precipitation of salts and other minerals into the oceans simply accumulates
Measures the present rate of influx and assume constant—results in dates of order of 100 Myr.
Why Wrong
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Ocean crustal material is recycled by the oceanic conveyor belt—unknown at time. The ocean
“consumes” material influx
Rates of mineral influx are poorly known over large periods of time. Not constant at all.

Various (late 1800’s, early
1900’s) Why Wrong
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Typically assumed constant average rates of erosion and deposition
Computed ages based on known thicknesses of, for example, Cambrian strata
Yielded ages on order of
100 Myr.
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Rates of erosion highly variable
Thicknesses of Cambrian strata varied widely depending on location

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Most estimates in the pre-radioactive scientific era relied on theoretical models that were incomplete and had too little reliable measurements as inputs.
The consensus ages collected around predictions of a few hundred Myr—over 10 times too small!
What was needed was a “clock” that was unaffected by ordinary physical processes, including geological, solar, mechanical, etc. events that could affect chemical reactions, weather patterns, erosion and sedimentation that could change age estimates dramatically.

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Radioactivity was discovered by Henri Becquerel in 1896 in uranium salts.
Radioactivity was thought to be like X-rays, a form of electromagnetic radiation initially.
Further experiments showed it was mostly composed of charged particles—subatomic in nature
Subatomic processes are insulated from all ordinary physical processes, i.e. chemical interactions, including explosions!

Basic picture Relative sizes
 The nucleus is very small
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If the nucleus was a pea at the 50 yd. line in a football field, the nearest electron would be in the endzone.
Chemical interactions involve only electrons, not the nucleus.

 Over many years, the properties of radioactive elements were catalogued and experiments were able to determine the statistical laws that governed their decay.
 It became possible to know how long the sample had been present by looking at how much decay product had accumulated over time.
 A CLOCK HAD BEEN FOUND!

Change in Nucleus Radioactive decays
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Beta minus decay
 n 0 → p + + e − + ν e
Alpha decay
 238 U → 234 Th + α
Fission
Various
A
B
Z
(A, Z+1)
(A–4, Z–2)
A=Z+N, mass number
N=# neutrons
Z=# protons

N

N o e
 t /


N o


# of
atoms
( originally
) present

All radioactive elements have average lifetimes, but often we speak of half-lives. A half-life is defined as the amount of time it takes for a radioactive material to decay to ½ of its original amount, i.e. when we have:
N

1
2
N
0
This implies that the half-life is:
1
2

 t
1 / e
2
 t
1

/ 2

/
 ln 2

Rb-87 decays to Sr-87 in a half-life of 48.8 billion years.
A long-lived radioactive element is needed to date something that is very old.
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Carbon dating is only good for relatively short time periods, about 70,000 yrs.
C-14 has a half-life of about
5700 yrs.
In about 5-10 half-lives, any radioactive element in terrestrial material falls below minimum detectable amounts (MDA)

What we need to know
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The amount of radioactive material originally present
Has the rock been disturbed in any way to add/subtract material since formation?
The decay series of the radioactive element
How We Know It
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Sometimes unknown, but can be deduced assuming that its decay products were not originally present or it can be determined.
Re-melting can often be detected via crystallization.
Decay series is known from laboratory experiments.

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“The Age of the Earth,” G. Brent Dalrymple,
Stanford University Press; 1 edition (February 1,
1994)
“Introductory Nuclear Physics,” Kenneth S.
Krane,Wiley; 1 edition (October 22, 1987)
“Finding Darwin’s God,” Kenneth R. Miller, Harper-
Collins; (1999)