Transcript The Modern Age of the Earth Richard J. Lloyd Nov. 18, 2008 Edinboro University of Pennsylvania.

The Modern Age of the Earth
Richard J. Lloyd
Nov. 18, 2008
Edinboro University of Pennsylvania
Objectives
 History of dating methods, pre-radioactive
era
 The era of radioactive discovery
 Dating methods and their applicability
 How to date a rock
 The Age of the Earth
History of Dating Methods (Western)
Biblical Chronologies
a) Chronologie de l’historie
sainte (1738) collected
over 200 computations
with creation dates
ranging from 3483 B.C.
to 6984 B.C.
b) 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
History (cont.)
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.
Progress of the Sun
The Modern, Pre-Radioactive Era
Credibility of biblical
chronologies eroded in
17th and 18th centuries
when observations of
nature were becoming
more precise in concert
with the theories that
explained natural
phenomenon.
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
Declining Sea levels
De Maillet (1720’s)
• Based on assuming Earth was
once entirely covered with
water
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.
Cooling of Earth and Sun
Comte de Buffon (1749)
 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.
Cooling, cont.
William Thomson, Lord
Kelvin (1860)
 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
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.
Cooling, cont.
Kelvin, round 2
 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!
 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.
Other Methods
Moon’s Origin (1890)
Why Wrong
 Hypothesis was that Moon
 Moon probably originated
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.
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.
Salt Clock
Various proponents (1900)
Why Wrong
 Assume that the influx
 Ocean crustal material is
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.
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.
Sedimentation
Various (late 1800’s, early
1900’s)
Why Wrong
 Typically assumed constant
 Rates of erosion highly
average rates of erosion
and deposition
 Computed ages based on
known thicknesses of, for
example, Cambrian strata
 Yielded ages on order of
100 Myr.
variable
 Thicknesses of Cambrian
strata varied widely
depending on location
Summary
 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.
The Radioactive Era
 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!
Atomic structure
Basic picture
Relative sizes
 The nucleus is very small
 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.
Radioactivity
 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!
Radioactivity Basics
Radioactive decays
Beta minus decay
 n0 → p+ + e− + νe
Alpha decay
 238U → 234Th + α
Fission
 Various
A
Z
B
Change in Nucleus
(A, Z+1)
(A–4, Z–2)
A=Z+N, mass number
N=# neutrons
Z=# protons
Mathematics of Radioactive Decay
N  No e t / 
N=# of radioactive atoms left
No # of atoms originallypresent
  mean(average) lifetime
Half-lives
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:
1
N  N0
2
This implies that the half-life is:
1
 e t1 / 2 / 
2
 t1 / 2   ln 2
Analyzing the Rubidium-Strontium
Clock
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.
 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)
How We Date Old Stuff
What we need to know
How We Know It
 The amount of radioactive
 Sometimes unknown, but
material originally present
 Has the rock been
disturbed in any way to
add/subtract material since
formation?
 The decay series of the
radioactive element
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.
References
 “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, HarperCollins; (1999)