The Modern Age of the Earth

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Richard J. Lloyd

Nov. 18, 2008

Edinboro University of Pennsylvania

The Modern Age of the Earth


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
17
th

and 18
th

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)

Why It Was Wrong


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.

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)

Why It Was Wrong


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


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

Wrong, again!


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
.


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
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
.


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.

Salt Clock

Various proponents (1900)

Why Wrong


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
.


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.

Sedimentation

Various (late 1800’s, early
1900’s)

Why Wrong


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
.


Rates of erosion highly
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

Change in Nucleus

Beta minus decay


n
0





p
+


+


e



+


ν
e

Alpha decay


238
U





234
Th


+


α

Fission


Various






(
A
,
Z
+1)



(
A

4,
Z

2)



A=Z+N, mass number

N=# neutrons

Z=# protons





B
A
Z
Mathematics of Radioactive Decay



N=# of radioactive atoms left





/
t
o
e
N
N


present
originally
atoms
of
N
o
#

lifetime
average
mean
)
(


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:



This implies that the half
-
life is:



0
2
1
N
N

2
ln
2
1
2
/
1
/
2
/
1






t
e
t
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
material originally present


Has the rock been
disturbed in any way to
add/subtract material since
formation?


The decay series of the
radioactive element


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.

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
, Harper
-
Collins; (1999)