Article: 9231 of talk.origins Subject: Calibration of K-Ar dating. Date: 26 Jul 91 15:12:2
Article: 9231 of talk.origins
From: hyde@ac.dal.ca
Newsgroups: talk.origins
Subject: Calibration of K-Ar dating.
Message-ID: <1991Jul26.121222.847@ac.dal.ca>
Date: 26 Jul 91 15:12:22 GMT
References: <1991Jul24.174058.6673@galileo.cc.rochester.edu> <5650@tekfdi.FDI.TEK.COM>
Organization: Dalhousie University, Halifax, Nova Scotia, Canada
Lines: 101
In article <5650@tekfdi.FDI.TEK.COM>, bobb@tekfdi.FDI.TEK.COM (Robert Bales) writes:
>
> 2) The citing of rocks which date to 3.9 billion years as evidence that the
> earth is actually at least that old. The problem here is that no
^^
> verification of the accuracy of the dating method was given, while I had
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> shown cases in which the same types of measurment methods (determination of
> isotope ratios) had given wrong results.
With regard to dating, bob, you never answered this post.
I have not been wise enough to keep track of the dates when
you have been asked to address these points, but after each
previous posting you left Talk.origins for some months.
[original post begins]
Bob, while you're at it, you can try to explain the
similar dataset from the Antarctic ice sheet (they've drilled
a little deeper there, 160,000 + years). Curiously, isotope
ratios in these cores vary with periodicities which exactly match
(within fairly narrow error bars) the periodicities of certain
astronomical parameters (such as the obliquity of the earth's
orbit, with a periodicity of 41,000 years, and the precessional
periods of about 19,000 and 23,000 years.
Now, there are good theoretical reasons to expect such a match,
(these orbital wobbles affect received sunlight)
and indeed it is observed both in ice and sea-bottom cores.
[insert july 91]
These sea bottom cores now go back
several million years and the astronomical periodicities
are still there, and K-Ar dates are still in reasonable
agreement (better than 10%) with astronomical dates.
The question for you, Bob, is this: Since the astronomical
periods do *not* depend in any way on radiometric dating, and
since these same periods show up in cores dated *by* radiometric
dating (the dreaded K-Ar and uranium series dating) is this not
an *independent* test of radiometric dating? If not, why?
The predictions as to time made by calculation of planetary
orbits and by K-Ar dating agree very well - for a long time you've
complained that K-Ar is not calibrated - well here it is.
[july 91 insert]
In recent months a 25 million year long record from the
triassic (about 200 million years ago, for those of us
who believe such things) has been obtained. The rock
is banded, and the bands form quite regular groupings.
The smallest bands contain about 20,000 varves (annual
layers) - and the precession cycle at that time was
about 20,000 years long. Coincidence? Well, the
precession cycle is modulated by the 100,000 year
eccentricity cycle so the bands should occur in groups
of five, with slightly different characteristics within
the group. They do. Not enough? There is also a
400,000 year eccentricity cycle, so the large bands
should be bunched in groups of four. And they are.
Well before this result was obtained (it hasn't even
been published yet) a simple climate model was used
to estimate the power spectrum of maximum annual
temperature at a similar site [see ref]. The low
frequency end of this model's output agrees
entirely with the observations. The cores
have not yet been measured accurately enough to
compare the high frequencies.
This is quite clear evidence that these bands are
astronomical in origin, and thus *astronomy*, not
radiometric dating, tells us that this sample of
rock was laid down over 25,000,000 years.
So the earth is at least that old. Furthermore, since
K-Ar dating gives the same length to this record we
have no reason for not trusting within a few percent the
K-Ar absolute age for this stratum, which is about
200 million years.
Well, Bob?
Bill Hyde
References
The geological evidence was presented by Paul Olsen of
Lamont-Doherty at a recent workshop at Johns Hopkins.
Preprints should exist in a few months. The theoretical
paper is
Short, D. A., J. G. Mengel, T. J. Crowley, W. T. Hyde and G. R. North 1991:
Filtering of Milankovitch Cycles by Earth's Geography.
Quaternary Research. 35, 157--173.
############################################################################
From: jwm@sun4.jhuapl.edu (James W. Meritt)
Subject: The age-of-the-Earth debate. (three centuries of debate)
Date: Mon, 22 Jul 91 19:59:37 EDT
From: Scientific American v261 p90(6) August, 1989
As the sun's first rays of thermonuclear light blazed across the
galaxy 4.5 billion years ago, the primal earth emerged from a pinning,
turbulent cloud of gas, dust and planetoids that surrounded the new star.
During the next 700 million years the cloud settled into a more tranquil
solar system, and the sun's third planet began to solidify.
On these figures for the age of the earth rest all of geology and
evolution. Indeed, they seem to be part of humankind's permanent store of
facts. Yet this chronological structure is quite new. In fact, two earlier
estimates have toppled during the past 150 years as the descriptive
sciences of biology and geology deferred to the more exact science of
physics.
The first estimate fell during the 19th century. To the great
displeasure of Charles Darwin and the geologists of the period, the
physicist William Thomson (later Lord Kelvin) performed a seemingly
flawless calculation to show that the earth had not existed throughout
eternity, as many thought then, but had formed 100 million years ago. That
chronology collapsed at the turn of the century, when the advent of
radioactive dating techniques showed the earth's age to be a few billion
years. After a fierce debate between geologists and physicists, radioactive
dating prevailed. Above all, the age-of-the-earth controversy illustrates
that emotion, intuition and vested interests can direct the course of
science almost as much as logic and experimentation.
Intuitively, one might think questions about the earth's age were as
timeless as speculation about the structure of the universe and our role in
it. Actually, many of the earliest civilizations treated the earth's
creation as part of the question of the origin of the universe. The
resulting cosmologies tended to be cyclical. The Greeks, for example,
believed natural history repeated itself perpetually. The Maya recorded
3114 before the common era (B.C.E.) as the year during which the universe
had been most recently re-created. In the first century of the common era,
many Han Chinese held a similar view. They believed the universe was
destroyed and re-created every 23,639,040 years.
The Judeo-Christian tradition also combines the earth's and the
universe's birth in a single event. The story of Genesis led scholars to
calculate the number of human generations since Adam and Eve. In 1654 John
Lightfoot refined Archbishop Ussher's famous calculation of the moment of
creation to an ultimate degree of precision: October 26, 4004 B.C.E., at
nine o'clock in the morning in Mesopotamia, according to the Julian
calendar.
Mikhail V. Lomonosov was one of the first scientists to suggest (in
the mid-18th century) that the earth formed independently of the rest of
the universe; he set the interval at hundreds of thousands of years. In
1779 the Comte de Buffon tried to determine the age of the earth
experimentally. He believed the earth was slowly cooling from an initial
hot state, and he estimated that the earth was 75,000 years old by creating
a small globe that resembled the earth's composition and then measuring the
rate at which it cooled.
Lomonosov and Buffon were virtually alone in their rigorous pursuit of
the absolute age of the earth. When other 18th-century naturalists pondered
the question at all, they either placed everything in the hands of the
Creator or else supposed that the earth and its living things had simply
taken a long time to reach their present condition. James Hutton
characterized the long view in his classic Theory of the Earth in 1795. "We
find no vestige of a beginning, no prospect of an end."
The chronology of geological periods did, however, intrigue Hutton's
contemporaries. They inferred that the successive strata of rock and soil
at a particular site represent the order in which the layers formed. In the
1790's William Smith built on this perception: two layers from different
sites could be regarded as equivalent in age if they contained the same
fossils. Extrapolating from these ideas, the naturalists began to chronicle
the strata and to estimate the duration of geological periods. Their
estimates varied widely, since they could only make crude guesses about the
time required to build up the layers.
In 1830 Charles Lyell gave such work a theoretical boost. Lyell
insisted that rock formations and other geological features took shape,
eroded and re-formed at a constant rate throughout time. Virtually none of
the naturalists applied Lyell's notion to calculate the age of the earth's
features; the data on geological processes were just too meager. Lyell did,
however, persuade many naturalists to become uniformitarians--that is, they
rejected the idea that there had been catastrophic geological change or a
rapidly forming, young earth. After all, evidence from stones and bones
suggested that each geological period lasted for many years, perhaps even
hundreds of millions of years, and the age of the earth had to be several
times that.
Therefore, the naturalists were startled when Lord Kelvin (then the
physicist William Thomson from Glasgow) determined in 1862 that the earth
had formed somewhere between 20 and 400 million years ago. Thomson rejected
uniformitarianism as unprovable. He and many other physicists of the day
believed the earth was originally molten; its surface had cooled and
solidified, but the core remained hot. The deeper one descends into the
earth, they noted, the higher the temperature.
To derive the earth's age, Thomson calculated how long the earth
required to cool from its primordial to its present state. He conjectured
that the gravitational contraction that formed the earth had generated all
of the earth's heat (except for a small contribution from the sun). Then he
investigated how well the earth conducts heat and how much heat is
necessary for it to melt or to raise its temperature by a certain amount.
He knew that the earth had cooled steadily as energy radiated into the cold
vacuum of space, according to the second law of thermodynamics. Using
Jean-Baptiste-Joseph Fourier's theory of heat conduction, he predicted how
the earth's temperature distribution might have evolved [see "The Fourier
Transform," by Ronalf N. Bracewell; SCIENTIFIC AMERICAN, June]. He
corroborated his calculations by accounting for heat from the sun and the
effects of tidal friction. In time he refined his estimate of geological
history to from 20 to 40 million years.
Thomson's work distressed geologists, who were comfortable with the
idea of unlimited time. They resented this audacious physicist who meddled
in their field, but they could not fashion a counterargument, and they
produced few papers on geochronology.
Thomson's calculation seemed unassailable on the grounds of logic and
physics. His conclusion eventually proved to be inaccurate by a wide
margin. Still, Thrason had instigated a conceptual coup d'etal: qualitative
geochronology was overthrown in favor of quantitative methods. Until the
end of the century, Thomson's estimates were the standards against which
all others were compared.
Thomson's result shocked biologists just as much as it surprised
geologists. Darwin regarded Thomson as an "odious spectre" whose chronology
was one of the shy naturalist's "sorest troubles." Darwin and other
biologists had postulated that complex organisms would require much more
than 40 million years to evolve. But neither living nor fossilized
organisms offered a basis for an independent calculation of evolutionary
time. The biological calendar ultimately relied on geology.
Thomas H. Huxley, a strong supporter of Darwin, attacked Thomson's
most vulnerable position. Huxley's view epitomized the disdain that
geologists of the late victorian period felt for the physical sciences and
the reluctant respect the workers held for quantifiable data. In his
presidential address to the Geological Society of London in 1869, Huxley
argued that no modern geologist would insist on absolute uniformitarianism
but that its principles could be applied. Then Huxley directed his rhetoric
at Thomson. The admitted "accuracy of mathematical processes [must not be
permitted to] throw a wholly inadmissible appearance of authority over the
results, [for] pages of formulate will not get a definite result out of
loose data." Perhaps, Huxley suggested, heat radiated from the earth more
slowly than Thomson supposed. Thomson thought he had estimated
conservatively, but he could not be certain of his values.
Thomson no longer battled alone, however. Both the American astronomer
Simon Newcomb and the German physicist Hermann von Helmholtz calculated the
time needed for a nebular cloud to condense gravitationally to the present
size of the sun. Their independent results of 100 million years established
an upper limit for the age of the earth (presuming that the earth did not
exist before the sun). George H. Darwin, son of the famous Charles and
professor of astronomy at the University of Cambridge, joined the
discussion. He posited that the moon broke loose from a rapidly rotating
molten earth and found that Thomson's original estimates corresponded well
to the time that terrestrial tidal friction would require to brake the
earth to its present 24-hour period of rotation.
A few geologists concurred with Thomson's estimate of the earth's
lifetime. Even before Thomson, John Phillips, Smith's nephew and pupil, had
claimed that the earth must have endured for 96 million years--a result
calculated from the admittedly imprecise rate of strata formation from
river-derived sediment. In 1868 Archibald Geikie, director of the
Geological Survey of Scotland, looked at evidence of erosion and concluded
that the earth was no older than 100 million years.
In 1899 John Joly of the University of Dublin devised the only truly
new geological technique for measuring the earth's age. He maintained that
all the salt in the oceans came from mineral deposits that had eroded and
dissolved. He also proposed that the salt concentration in the oceans could
not decline. Joly could therefore relate salinity to age. He obtained the
best available figures for the quantity of water that flowed into the
oceans each year and the amount of salt per volume of runoff. He then
calculated the annual increment of salt. He multiplied the salinity of the
ocean by its total volume and divided the product by the annual increase.
Joly thus determined that the brackish sea developed over 80 to 90 million
years.
At about the same time an increasing number of geologists swelled the
consensus that the earth had formed less than 100 million years ago. Yet
all attempts to measure the age of the earth rested on an assumption, an
analogy or a best guess about the rate of change of geological processes.
Such assumptions created room for doubt. Some critics protested
against the premise that only gravitational contraction explained the
earth's or the sun's heat; another energy source might be possible. Some
maintained that the earth had never been molten, whereas others suggested
that its interior was still molten. (A liquid interior would conduct heat
by convection--something that Thomson had not taken into account.) Still
others questioned the data on erosion, sedimentation and salinity.
As the century drew to a close, geologists generally agreed that
nearly 100 million years had passed since the earth was born. They did not,
however, reconcile their differences with Thomson, who had recently been
elevated to the peerage as Lord Kelvin for his scientific accomplishments.
Employing his heat calculations, Kelvin was urging ever-shorter geological
time scales, all the while high-handedly dismissing geological evidence.
By this time, however, geologists were wary of Kelvin's physical
techniques. They had greater confidence (perhaps unwarranted) in their own
methods than in the eminent physicist's collection of assumptions. After
all, they had discovered several approaches to the chronology that gave
concordant results. Geologists felt they had grandly completed their
apprenticeship in the quantitative sciences after several decades of
vigorous exploring, mapping, measuring and classifying.
Yet it was not long before physical scientists were once again
treading on geologists' turf and calculating its age. This time the study
of radioactivity gave momentum to the attack. In 1896 A. Henri Becquerel
discovered the phenomemon; in 1898 Marie S. and Pierre Curie first detected
the radioactive elements polonium and radium. Then in 1902 and 1903 Ernest
Rutherford and Frederick Soddy explained the process of radioactivity in
several papers. Radioactivity, they correctly stated, was the spontaneous
transmutation of atoms of one chemical element into another.
At first, the radiation of alpha, beta and gamma rays was more
important to geochronology than were the radio-elements themselves. (It was
later discovered that alpha particles are composed of two protons and two
neutrons, just like the nucleus of a helium atom; beta particles are
emitted electrons, and gamma rays are photons of electromagnetic
radiation.) Earlier, in 1900, Rutherford and R.K. McClung of McGill
University in Montreal showed that the various rays carry enormous amounts
of energy. Their paper made little impression beyond the small community of
physicists and chemists working on radioactivity.
The reception was entirely different in 1903 when Pierre Curie and
Albert Laborde announced that radium generates enough heat to melt more
than its own weight in ice in an hour. Public interest was aroused over
this apparently inexhaustible cornucopia of energy. Where did the energy
come from? Rutherford and Howard T. Barnes discovered the source. They
showed that the heat was proportional to the number of alpha particles
radiated. These relatively massive particles were emitted at great
velocity. If the particles collided with neighboring atoms, the particle's
kinetic energy was transformed into heat.
Geologists immediately recognized that the relation between heat and
radioactivity could significantly influence determinations of the age of
the earth. Kelvin had assumed that the earth's heat came from either the
sun or the original molten state of the earth. In both cases, gravitational
contraction was the only source of energy. If the earth and sun contained
quantities of radioactive materials sufficient to provide large amounts of
heat, however, then this discovery could invalidate all chronologies that
Kelvin had based on the earth's colling.
In 1903 George Darwin and Joly were the first to make this very claim:
radioactivity was at least partially responsible for the earth's and the
sun's heat. But was there enough radioactive matter within the earth to
make a measurable difference?
Part of the answer was at hand. Julius Elster and Hans F. Geitel, two
schoolteachers in Wolfenbuttel, Germany, detected radioactivity in the air
in 1901 and, soon after that, in the soil. Before long, many enthusiastic
amateurs as well as professional scientists were finding radioactive rain,
and snow and groundwater--even radioactive mist at the base of Niagara
Falls. Soon geologists had no doubt that radioactivity was widely
distributed .
As for its concentration, Robert J. Strutt of the Imperial College in
London found traces of radium in many rocks. Indeed, Strutt found too much
radium for it to be distributed uniformly throughout the globe (without
even considering the contribution from all the other radioelements). Its
radioactivity alone could account for the earth's internal heat. His work
suggested that geochronology could be extended by an indefinite time. He
found no vestige of a cooling, no prospect of an age.
The scientific community responded with ambivalence. Joly and William
J. Sollas of the University of Oxford worried that Strutt's work might
overturn their own calculations demonstrating an age of about 100 million
years. Kelvin's own feelings were divided: he privately acknowledged that
his estimates had been superseded, but in public he remained contentious.
Others were delighted to be liberated from Kelvin's earth age. It turned
out that radioactivity not only loosened Kelvin's theoretical shackles but
also held the key to determining the age of the earth.
During the first years of the century, scientific enthusiasm for
radioelements replaced enthusiasm for radiation when Rutherford and Soddy
proposed that radioactivity was actually spontaneous alchemy. A sample of a
radioelement, they said, decays at a regular rate into a different chemical
element. The rate of decay is expressed as a half-life: the time needed for
half of the atoms of a given radioelement in a sample to change into a
decay product.
Half-lives range from billions of years to millionths of a second.
Uranium, thorium and radium have long half-lives and therefore exists in
tangible quantities on earth, whereas those elements that have short
half-lives have a transitory existence. Hence, the presence or absence of
particular radioelements in rocks can imply an age; analysis of the
quantities of the radioelements can reveal an absolute age.
The radioelements form distinct decay series: one radioelement decays
into the next element in the series until a stable element is produced. The
uranium-radium, uranium-actinium and thorium series were known or suspected
in the early years of this century. The technique of radioactive dating of
rocks developed from the study of radioelements and their decay series.
Rutherford and the radiochemist Bertram B. Boltwood pioneered the work.
As a consulting chemist after his graduation from Yale University,
Boltwood examined numerous ore smaples, among them monazite, a mineral
containing uranium and thorium. When the charismatic Rutherford lectured at
Yale in 1904, Boltwood's curiosity about radioelements became a passion,
and he began to document the relations among the elements in the decay
series.
Later that year, Rutherford suggested a way to determine the age of
the earth from measurements of helium in minerals. Rutherford then believed
(and in 1908 proved) helium is not a product of any particular decay series
but is formed in all the series when two electrons bond to an alpha
particle. Sir William Ramsay and Soddy at University College in London had
just discovered the rate at which radium produced helium. If the
Ramsay-Soddy rate was accurate and no helium escaped from the mineral from
the time of its formation--both great leaps of faith--the amount of helium
would determine the age of the sample. Rutherford could boast an age of 40
million years for a fergusonite rock he owned.
Boltwood, on the other hand, thought to look for the end products of
the decay series. The amount of an end product would increase over the
years as the radioelements decayed. It was already known that radium was a
product of the uranium series; in 1905 Boltwood pointed to lead as the
final product. The uranium-to-lead hypothesis received additional support
from Rutherford. He argued that if uranium decayed to radium and if radium
(then thought to have an atomic weight of 225) and its daughter products
then emitted five alpha particles (which each have an atomic weight of
four), the decay would yield an element of an atomic weight of 205--not far
from lead's accepted value of 206.9.
Boltwood credited Rutherford for suggesting the lead method of dating
ancient rocks, but it was the chemist who demonstrated its feasibility. By
the end of 1905 he had calculated ages ranging from 92 to 570 million years
for 26 different mineral samples. Fortunately for the reputation of the new
technique, these results remained unpublished. Boltwood's radium-to-uranium
ratio was inaccurate both because Rutherford's scale for measuring
quantities of radium was badly calibrated and because the half-life of
radium was revised several times during 1905 and 1906. A rock's age rested
critically on both these values.
When Boltwood published his work in 1907, he reported a striking
constancy in the lead-to-uranium ratios for minerals from the same rock
layer, which were presumably of the same geological age. He also observed
that the amount of lead in a mineral increased as the relative age of the
mineral increased. Minerals from which lead had apparently been leached
gave lower ratios than did other minerals from the same layer. All this fit
together well. Boltwood could find, however, no constancy in
lead-to-thorium ratios from several minerals; the end product of thorium
remained a mystery. He was inclined, therefore, to ignore lead-to-thorium
ratios; an error that affected his measurements of minerals that contained
both uranium and thorium.
To determine the absolute age of minerals, Boltwood examined the
uranium-radium decay series. The latest value for the half-life of radium
was 2,600 years, which Rutherford had deduced from the number of alpha
particles emitted from radium each second. (The figure accepted today is
1,620 years.) Given that the decay of radioactive materials is exponential,
the fraction of radium decaying in one year would be 270,000 parts per
billion, based on Rutherford's half-life. Rutherford and Boltwood found
that almost all rocks contained 380 parts of radium per billion parts of
uranium. Thus, the fraction of radium decaying each year multiplied by the
fraction of radium in uranium yields one part of radium decaying each year
for every 10 billion parts of uranium.
Boltwood correctly assumed that the decay series of the rocks he
collected were in an equilibrium state. The uranium-to-lead series, for
instance, is in equilibrium when the number of uranium atoms decaying per
unit of time is equal to the number of radium atoms decaying, or lead atoms
forming, in that time. To maintain this equilibrium, radioelements that
have long half-lives must exist in greater quantities than those that have
short half-lives. (Although the supply of uranium will slowly decrease over
time, Boltwood realized that the amount lost is negligible.)
Boltwood deduced that if one part of radium decays each year for every
10 billion parts of uranium, then one part of lead forms each year for
every 10 billion parts of uranium. Boltwood expressed this relation in a
formula: the age of the rock equals 10 billion years multiplied by the
ratio of lead to uranium. He then calculated that a sample of uraninite,
which had a ratio of .041, was 410 million years old and a sample of
thorianite, which had a ratio of .22, had formed 2.2 billion years ago.
Actually, when the accurate value for the half-life of radium was
applied, the age of Boltwood's samples was found to range from about 250
million to 1.3 billion years. Even with this correction, his thorianite
measurement was invalid because the decay of thorium contributed some lead
in addition to the lead that derived from uranium. Nonetheless, these
results were spectacular: they demonstrated that the earth was about a
billion years old.
Oddly, this enormous accomplishment was met with indifference.
Although Boltwood's paper appeared in America's foremost geological
journal, no one was inspired to duplicate or extend his work on the lead
method. Nor did Boltwood's result sway geologists' opinion that the
significance of radioactive was overrated. They not only discounted the
heating effect of radioactive decay on the earth but also "refined" their
geological and physical data to show that Kelvin's range of time was
correct after all!
Boltwood wrote no more on the lead dating method. He returned to the
study of decay series and discovered ionium, the immediate parent of
radium. Rutherford retained a light hold on the age-of-the-earth topic,
publishing about one paper a decade--hardly the mark of a consuming
interest. Meanwhile, Strutt refined the helium method until 1910, when he
too departed for greener research pastures.
Strutt left a legacy, however. He had sparked an interest in
geochronology in a young English geology student, Arthur Holmes, who kept
the subject alive almost single-handedly. Indeed, Holmes ultimately forced
geologists to accept radioactive dating in the course of his long career in
industry and at the Universities of Durham and Edinburgh. Until 1930,
however, Holmes and Joly were the only geologists who were skilled in the
dating technique, and Joly, moreover, doubted its accuracy.
Holmes did not. He also considered the lead method to be more reliable
than the helium technique. In 1911 he examined many rock samples and
calculated that the most ancient was 1.6 billion years old. He maintained
(with more faith than justification) that his samples had contained no lead
when they were formed, that all the lead came from the decay of uranium and
that external mechanisms had not removed or added any lead or uranium.
Two years later, however, his critics could crow in the light of two
new advances. The first was the discovery of isotopes: atoms that have the
same chemical propertties but different atomic weights because the number
of neutrons varies. Lead, for example, has a nucleus that contains 82
protons and can have an atomic weight from 195 to 214. The second advance
was the discovery of the physical laws that specify the decay products of
each radioelement. These laws indicated that the thorium series did after
all end in a particular isotope of lead.
Although for many earth scientists these new discoveries made
radioactive dating seem more difficult and unreliable, Holmes forged ahead,
publishing in the years before and after World War I a steady stream of
papers on geochronology. He incorporated information about isotopes into
his work and sharpened his results. Although his success wore down overt
resistance to radioactive dating, the method gained little support.
An exception was Joseph Barrell, a professor of geology at Yale, who
in 1917 reinterpreted geological history to conform with the results of
radioactive dating. Barrell emphasized that geological processes vary in
intensity in a cyclical rather tahn a uniform fasion. Thus, current rates
of geological change could not, as uniformitarians claim, be a guide to the
past.
Finally, resistance began to falter. By 1921, at a meeting of the
British Association for the Advancement of Science, the speakers,
representing geologists, botanists, zoologists, mathematicians and
physicists, seemed to agree that the earth was a few billion years old and
that radioactive and geological dating techniques could be reconciled. But
no plan was drafted for reconciliation. Not surprisingly, the old guard
remained skeptical. Sollas would accept no age for the earth greater than
100 million years. "Geologists," he said, "are not greatly concerned over
the period which physicists may concede to them; they do not much care
whether it is long or--in moderation--short, but they do desire to make
reasonably certain that it is one which they can safely trust before
committing themselves to the reconstruction of their science, should that
prove to be necessary."
The battle was won finally in 1926 when in the U.S. the National
Research Council of the National Academy of Sciences appointed a committee
to examine the status of the age-of-the-earth problem. Holmes, as one of
the few experts on the subject, was a committee member and wrote almost 70
percent of the report. The committee agreed unanimously that radioactivity
provided the only reliable geological time scale. The report presented an
overwhelming amount of clear and detailed evidence. The constants of
radioactivity were firmly established, lead isotopes were easily
incorporated into the calculations, and the mineral samples were carefully
chosen to ensure that decay products had not been lost over time. The
radioactive dating methods pioneered by Rutherford and Boltwood and
enhanced by Holmes had at last received the blessing of geologists. Not
only had they found a vestige of a beginning, but they also had a prospect
for dating all of geological history.
During the past six decades, application of the lead dating method has
become more and more sophisticated, and current techniques reveal that the
oldest rocks on the earth were formed as much as 3.8 billion years ago.
This would date the minimum age of the earth's solid crust but not
necessarily the period when the spiral cloud of gas and dust condensed to
form the solar system. In 1955 Clair Patterson of the California Institute
of Technology and his colleagues first determined the age of the solar
system by dating meteorites. The most recent measurements of meteorites
place the age of the primal earth at 4.5 billion years.
##############################################################################
Article: 9363 of talk.origins
From: hyde@ac.dal.ca
Newsgroups: talk.origins
Subject: A question Bob has never answered.
Message-ID: <1991Aug7.135944.963@ac.dal.ca>
Date: 7 Aug 91 16:59:44 GMT
Article-I.D.: ac.1991Aug7.135944.963
References: <1991Jul31.143934.10585@infonode.ingr.com> <2655@anaxagoras.ils.nwu.edu> <5680@tekfdi.FDI.TEK.COM>
Organization: Dalhousie University, Halifax, Nova Scotia, Canada
Lines: 112
In article <5650@tekfdi.FDI.TEK.COM>, bobb@tekfdi.FDI.TEK.COM (Robert Bales) writes:
>
> 2) The citing of rocks which date to 3.9 billion years as evidence that the
> earth is actually at least that old. The problem here is that no
^^
> verification of the accuracy of the dating method was given, while I had
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> shown cases in which the same types of measurment methods (determination of
> isotope ratios) had given wrong results.
With regard to dating, bob, you never answered this post.
I have not been wise enough to keep track of the dates when
you have been asked to address these points, but after each
previous posting you left Talk.origins for some months.
[original post begins]
Bob, while you're at it, you can try to explain the
similar dataset from the Antarctic ice sheet (they've drilled
a little deeper there, 160,000 + years). Curiously, isotope
ratios in these cores vary with periodicities which exactly match
(within fairly narrow error bars) the periodicities of certain
astronomical parameters (such as the obliquity of the earth's
orbit, with a periodicity of 41,000 years, and the precessional
periods of about 19,000 and 23,000 years.
Now, there are good theoretical reasons to expect such a match,
(these orbital wobbles affect received sunlight)
and indeed it is observed both in ice and sea-bottom cores.
[insert july 91]
These sea bottom cores now go back
several million years and the astronomical periodicities
are still there, and K-Ar dates are still in reasonable
agreement (better than 10%) with astronomical dates.
[original post resumed]
The question for you, Bob, is this: Since the astronomical
periods do *not* depend in any way on radiometric dating, and
since these same periods show up in cores dated *by* radiometric
dating (the dreaded K-Ar and uranium series dating) is this not
an *independent* test of radiometric dating? If not, why?
The predictions as to time made by calculation of planetary
orbits and by K-Ar dating agree very well - for a long time you've
complained that K-Ar is not calibrated - well here it is.
[july 91 insert]
In recent months a 25 million year long record from the
triassic (about 200 million years ago, for those of us
who believe such things) has been obtained. The rock
is banded, and the bands form quite regular groupings.
The smallest bands contain about 20,000 varves (annual
layers) - and the precession cycle at that time was
about 20,000 years long. Coincidence? Well, the
precession cycle is modulated by the 100,000 year
eccentricity cycle so the bands should occur in groups
of five, with slightly different characteristics within
the group. They do. Not enough? There is also a
400,000 year eccentricity cycle, so the large bands
should be bunched in groups of four. And they are.
Well before this result was obtained (it hasn't even
been published yet) a simple climate model was used
to estimate the power spectrum of maximum annual
temperature at a similar site [see ref]. The low
frequency end of this model's output agrees
entirely with the observations. The cores
have not yet been measured accurately enough to
compare the high frequencies.
This is quite clear evidence that these bands are
astronomical in origin, and thus *astronomy*, not
radiometric dating, tells us that this sample of
rock was laid down over 25,000,000 years.
So the earth is at least that old. Furthermore, since
K-Ar dating gives the same length to this record we
have no reason for not trusting within a few percent the
K-Ar absolute age for this stratum, which is about
200 million years.
Well, Bob?
Bill Hyde
References
The geological evidence was presented by Paul Olsen of
Lamont-Doherty at a recent workshop at Johns Hopkins.
Preprints should exist in a few months. The theoretical
paper is
Short, D. A., J. G. Mengel, T. J. Crowley, W. T. Hyde and G. R. North 1991:
Filtering of Milankovitch Cycles by Earth's Geography.
Quaternary Research. 35, 157--173.
E-Mail Fredric L. Rice / The Skeptic Tank