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Author Topic:   Introduction To Geology
JonF
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Message 241 of 293 (687611)
01-14-2013 1:53 PM
Reply to: Message 240 by Dr Adequate
01-14-2013 6:49 AM


Re: U-Th, Th-Pa, And Ra-Pb
Some interesting applications of U-Th disequilibrium include Radio-dating backs up biblical text: Siloam Tunnel located and dated to 700 BC wherein they carbon dated a leaf in the plaster and a speleotherm growning from the plaster. Also The diffusion-adsorption model dating bone that's been exposed to groundwater.
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Dr Adequate
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Posts: 15987
Joined: 07-20-2006
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(1)
Message 242 of 293 (687726)
01-15-2013 9:37 PM


Erratum
Foreveryoung has pointed out that most cosmic rays do not in fact come from the Sun, and I have amended the relevant articles accordingly.

I will now write out a hundred times "I have never actually studied astronomy, and should not presume that I know anything about it."


  
Dr Adequate
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(1)
Message 243 of 293 (687734)
01-16-2013 6:04 AM


Paleomagnetic Dating
Paleomagnetic dating

Introduction

In this article we shall discuss how we can use the paleomagnetism in rocks to attach dates to them (paleomagnetic dating). The reader may find it useful to go back and read the main article on paleomagnetism before continuing.

Polar wander and dating

One we have dated a sufficient number of rocks and measured the orientation of the magnetism they contain, we can build up a picture of how the position or apparent position of the poles over time.

So if we are then faced with a rock the date of which we do not know, then we do know (of course) the latitude and longitude at which we found it, and we can measure the orientation of its magnetism, and so we can look at the global picture we've built up of continental drift, and so figure out when the rock must have formed in order to have its magnetism oriented in just that direction.

Magnetic reversals and dating

Once we have dated a sufficient number of rocks and found out whether they have normal or reverse polarity, we can likewise build up a timeline for the occurrence of the reversals.

As noted in a previous article, magnetic reversals come at irregular intervals. This means that the pattern of normal and reverse polarity in an assemblage of rocks can be distinctive in the same way (though for a completely different reason) that growth rings in a tree can be distinctive. We might, for example, see a long period of reverse polarity, followed by six very quick switches of polarity, followed by a long period of normal polarity; and this might be the only time that such a thing occurs in our timeline.

So if we are presented with an undated rock, and we find a really distinctive pattern of paleomagnetic reversals within it, we may be able to identify the one time at which such a sequence of magnetic reversals took place.

Strengths and weaknesses of the method

The reader will observe that it is necessary to be able to date some rocks, in fact a lot of rocks, before paleomagnetic dating can be brought into play. You may therefore be wondering why, if we have perfectly good dating methods already, we don't just use them.

However, the advantage of paleomagnetic dating is that we can use it on different rocks from those susceptible to our ordinary methods of absolute dating: while most radiometric methods usually require igneous rocks, paleomagnetism can be measured in sedimentary rocks.

One problem which may arise is that the direction of the poles from a given location, or the pattern of magnetic reversals, may repeat over a long enough period of time, so that the paleomagnetic data we get when we measure these factors is not unique to a single time in the history of the Earth.

It is possible to get round this problem if we can find an approximate date of the rocks by other means. For example, if by considering their stratigraphic relationship to a datable igneous rock, we can establish that they are (for example) less than 20 million years old, then it may turn out that the paleomagnetic data, though not unique over the whole history of the Earth, are unique over the course of the last 20 million years, and then we can go ahead and use paleomagnetic dating.

Edited by Dr Adequate, : No reason given.


  
AZPaul3
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Posts: 3428
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(2)
Message 244 of 293 (687879)
01-17-2013 12:25 PM


Appreciation
Dr. A,

Just a little love.

An excellent set of articles. Lots of information concisely presented.

The feedback from the others strengthens the whole piece still more.

Great stuff.

Thank you.


  
petrophysics1
Inactive Member


(1)
Message 245 of 293 (687907)
01-17-2013 5:02 PM


Try Looking
Dr.A writes:

The picture below shows large-scale cross-bedding in sandstone. (To give you an idea of the scale, the small patches of green visible in the top right corner are not moss, as you might guess, but bushes.)


The scale of the cross-bedding depends on the process that produced it: this was produced by wind; smaller-scale cross-bedding would be produced by the formation of tidal ripples.

Source of the above quoted

This was not deposited by wind.

Take your cursor/arrow and put it in the lower left corner of the picture. Now slide it up the left side of the picture to the base of the reddish brown conglomerate. Look at the size of the rocks just above your cursor/arrow. Do you really think the wind blew those in there?

Move about 20% to the right, see those holes. Well rocks like the ones on the left used to be there but fell out.

Go to the center, does that stuff look well sorted to you? It looks poorly sorted to me which means there is no way this is wind blown.

See the cross-beds along the bottom, those are trough cross-beds. They form in fluvial deposits.

Geology is about the careful, accurate, objective (unbiased) description and identification of minerals, rocks and fossils and their position in 3 dimensional space. Without this nothing else is possible in geology. If you understood that you would have looked at the photo, instead of believing an interpretation.

Rock descriptions are reproducible scientific data, they never include someone's interpretation of how it was deposited.

Edited by Adminnemooseus, : Added source for quoted material, since the "reply to message" was not used.


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Dr Adequate
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Message 246 of 293 (687961)
01-18-2013 2:41 AM
Reply to: Message 245 by petrophysics1
01-17-2013 5:02 PM


Re: Try Looking
Those are good reasons, thank you.
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Dr Adequate
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(2)
Message 247 of 293 (687962)
01-18-2013 2:45 AM


Sclerochronology
Sclerochronology

Introduction

In this article we shall discuss the basis of sclerochronology, a method of dating shells and corals by analysis of their growth patterns.

Growth patterns in shells and corals

When shelly organisms grow, many types lay down bands of new growth in a way that regularly reflects the passage of time: for example, laying down one growth band per day (as many corals do), or one growth band every low tide, as mussels do.

Some corals lay down distinct bands of skeletal calcium carbonate on a daily basis and also display seasonal patterns, so that they keep count both of days and of years. In the same way, mussels deposit their growth bands every low tide, but also show variations according to the phase of the moon, so that they keep count both of low tides and of lunar months. The photograph below shows the clam Arctica islandica, a popular species with sclerochronologists.

It is possible to use these growth patterns to date recent sediments in a manner analogous to dendrochronology. However, there is a more interesting way of using this data, which we shall discuss in the remainder of this article.

Tidal braking

the friction of tides that the Moon causes on the Earth slows down the Earth's rotation: this is known as tidal braking. The effect, though small, is measurable by the high-precision clocks used by astronomers, and so can be established directly as well as on theoretical grounds: at present, the effect amounts to a day getting shorter by 2.3 milliseconds over the course of a century (see here for more details). This may not sound like much, but it adds up: over the course of 100 million years, that would add up to a change in the length of a day of 38 minutes.

This means that in the past days must have been shorter. As the length of a year is constant, this means that in the past there must have been more days per year: using the present rate of slowing as a basis, there would have been about ten more days per year a 100 million years ago.

We should note that in fact scientists do not simply extrapolate the present rate of slowing in a linear manner to calculate past rates of rotation, but rather calculate this from the physics of the Earth-Moon system. For the purposes of this article, it is not necessary to go into the details of the calculation.

Tidal braking and sclerochronology

This immediately suggests a way of dating corals and shellfish. Take corals as an example. As I have said, they lay down daily bands, and the way in which they do so displays seasonal fluctuations. This means that by counting the number of daily bands per year, we can find out how many days there were per year at the time when they were formed. In the same way, by looking at mussels we can find out how many low tides there were per lunar month when they grew.

So by calculating how tidal braking has changed the number of days in a year or a lunar month, we can put a date on the organisms: for example, a coral showing 375 daily growth bands per year must have grown around 100 million years ago.

Weaknesses of the method

The number of days per year or per lunar month changes so slowly over time that we cannot expect sclerochronology to be as precise as radiometric methods such as U-Pb. If a change of one day per year corresponds to the passage of 10 million years, then this limits the precision with which we can resolve the age of a shell or coral.

What is more, the change in day length is not as predictable as the decay of radioactive isotopes. The graph below shows changes in day length from 1860 to 1980.

As you can see, although there is a general tendency for the Earth to slow down, occasionally it has sped up.

Over the longer term, the magnitude of tidal braking will depend on the exact interaction of the Earth and Moon. Such things as the position of the continents and of mid-ocean ridges will affect tidal patterns, and these change over time as we have seen in our discussion of plate tectonics. Then again, the formation of polar ice-caps, and the concomitant fall of sea-levels would speed up the Earth's rotation as a consequence of the law of conservation of angular momentum.

Because of these considerations, geologists prefer to use radiometric methods rather than sclerochronology where it is possible to do so, even though radiometric dating is rather more expensive. However this sclerochronological technique, even if it is rarely used in practice, has a distinct theoretical significance: it acts as an check on the validity of radiometric methods. When we find approximately 400 days per year in the Devonian period and about 390 in the Carboniferous (see J. Wells (1963) Coral Growth and Geochronometry, Nature 197(4871), 948-950) then this is in line with the dates put on these periods by radiometric methods. Since the mechanisms of coral and shell growth are completely unrelated to the process of radioactive decay, this provides a completely independent check on radiometric dating. The agreement between sclerochronology and radiometric dating is therefore a good reason to have confidence in both.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.


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RAZD
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Posts: 19320
From: the other end of the sidewalk
Joined: 03-14-2004
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Message 248 of 293 (688013)
01-18-2013 2:08 PM
Reply to: Message 247 by Dr Adequate
01-18-2013 2:45 AM


Re: Sclerochronology
Well done again Dr Adequate

... When we find approximately 400 days per year in the Devonian period and about 390 in the Carboniferous (see J. Wells, Coral Growth and Geochronometry, Nature, March 1963) then this is in line with the dates put on these periods by radiometric methods. ...

There is additional material on this at Age Correlations and An Old Earth, Version 2 No 1 Message 10

quote:
A few more data may be mentioned: Lophophllidium from the Pennsylvanian (Conemaugh) of western Pennsylvania gave 390 lines per annum, and Caninia from the Pennsylvanian of Texas, 385. These results imply that the number of days a year has decreased with the passage of time since the Devonian, as postulated by astronomers.

I also found this graphic on this website although it was not used in the article:

Original at http://freepages.genealogy.rootsweb.com/...ogy/fig1wells.jpg (3)

This shows the smooth change in the length of days with time. The calculations based on just the astrophysics gives a 400 day/year figure for the Devonian and a 390 day/year figure for the Pennsylvanian, so there is very close accord between the predicted number of days, the measured number of days and the measured age of the fossil corals.

That curve would show a theoretical smoothed out running average days per year, without the spin up spin down effects of the magnetic core (and major earthquakes).

The photograph below shows the clam Arctica islandica, a popular species with sclerochronologists.

It is possible to use these growth patterns to date recent sediments in a manner analogous to dendrochronology. However, there is a more interesting way of using this data, which we shall discuss in the remainder of this article.

We also see similar layering in fossil brachiopods, such as are found on Mt Everest and other locations, showing the length of time that each organism lived. These would also show climate variations, and should be able to be aligned into a Sclerochronology (shell chronology) extending back millions of years ... if some grad student was looking for a research project ...

Enjoy

Edited by RAZD, : i


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Dr Adequate
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Posts: 15987
Joined: 07-20-2006
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(1)
Message 249 of 293 (688053)
01-18-2013 6:08 PM


Tidal rhythmites and dating
Tidal rhythmites and dating

Introduction

In this article we shall explain what tidal rhythmites are, how they are formed, and their implications for dating.

As this article re-uses ideas introduced in the previous article on sclerochronology, the reader will need to have read that article first.

Rhythms and rhythmites

Rhythmites are sedimentary rocks which display a repetitive vertical succession of types of sediment. We have already discussed varves, which are a kind of rhythmite. In this article we shall be interested in rhythmites produced by the action of the tide.

Tidal cycles include:

* A semidiurnal cycle. In most locations the moon produces two high tides and two low tides a day.

* A diurnal cycle. Some locations, such as the Gulf of Mexico, get only one high tide and one low tide per day.

* A mixed cycle. This can be seen in many locations on the west coast of America. In a mixed cycle, there are two high tides and two low tides per day, but one high tide is higher than the other, and one low tide is lower than the other.

* A fortnightly cycle. The highest high tides and lowest low tides (spring tides) occur at new moons and full moons, when the moon is either in line with or opposite the sun.

* Tidal cycles such as the apsidal cycle (presently lasting 8.85 years) and the nodal cycle (18.6 years) can also be distinguished; for the purposes of this article we may overlook them.

In additional to these tidal cycles, the rhythm of the seasons can also have their effect on sedimentary deposition. Varves are a special case of this, though typically varves are so thin that cycles of shorter duration are not discernible. Some sediments, however, will display a full range of cycles from semidiurnal to annual.

Rhythmites and dating

Any or all of the cycles mentioned above can be recorded in nearshore sediments. So it is possible to look at nearshore sedimentary rocks and, depending on which rhythms are recorded in the rock, to find out how many days there were in a month, or days in a year, or months in a year, or all of these facts, at the time when the rhythmite was deposited.

This allows us to subject these rhythmites to the same analysis as is used in sclerochronology, except that we can look at days in a month and months in a year as well as days in a year. The fact that there is close agreement between the number of days in a year as calculated on the basis of rhythmites and by the use of sclerochronology is a reason to have confidence in both methods, since it is hard to see how both could be wrong and yet coincidentally in agreement.

The same caveats apply to the use of this type of rhythmite for dating as apply to sclerochronology, and for just the same reasons. Also as with sclerochronology, the agreement of data from rhythmites with dates produced by radiometric dating is a reason to have confidence in radiometric methods.


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RAZD
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Message 250 of 293 (688070)
01-18-2013 8:30 PM
Reply to: Message 249 by Dr Adequate
01-18-2013 6:08 PM


Re: Tidal rhythmites and dating
* A mixed cycle. This can be seen in many locations on the west coast of America. In a mixed cycle, there are two high tides and two low tides per day, but one high tide is higher than the other, and one low tide is lower than the other.

At Victoria BC (Canada ... used to live there) the tides go through a cycle from two highs and two lows to one high high and one low low and back over a two week period. This is due in part to the geography affecting the tides coming around Vancouver Island in two directions.

Enjoy


we are limited in our ability to understand
by our ability to understand
Rebel American Zen Deist
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to share.


Join the effort to solve medical problems, AIDS/HIV, Cancer and more with Team EvC! (click)

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Dr Adequate
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Posts: 15987
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(2)
Message 251 of 293 (688233)
01-20-2013 11:42 PM


Fossils And Absolute Dating
Fossils and absolute dating

Introduction

In this article we shall discuss how fossils can be used for the purposes of absolute dating.

Fossils and dating

We have already discussed the construction of the geological column. If our stratigraphic methods show that fossil A was always deposited below fossil B whenever we are in a position to compare their dates of deposition, then we can conclude that species A is older than species B. We can apply the same sort of reasoning to the stratigraphic relationships of fossils and datable rocks.

For example, suppose that using stratigraphic methods, we can show that a particular fossil is always older than rocks which are 14 million years old or less, and always younger than rocks which are 16 million years old or more, whenever we are in a position to make a comparison.

Now, it is a fundamental principle of science --- arguably, the only fundamental principle of science --- that a rule that works every time we can test it must be taken as true unless and until we find a counterexample. So in this case we would have to conclude that this fossil species is between 14 and 16 million years old wherever we find it, even in those cases where there are no datable rocks that we can compare it to.

But this means that we can now use the fossil species to date the sedimentary rocks in which it is found; and we can say that those fossils found in the same strata as this species must be the same age; those species which stratigraphy tells us are older than it is must be more than 16 million years old; and those species which stratigraphy tells us are younger than it is must be less than 14 million years old.

Hence we can use datable rocks to put dates on fossil species; and then we can use the fossil species to put dates on other rocks which would otherwise be difficult to date.

Those fossils we have described as "index fossils" are particularly suitable for this purpose, since they have a wide geographical distribution but only inhabit a thin slice of time.

Advantages of the method

There are three main advantages of using fossils for dating in this manner.

First of all, we may want to date a stratum which is a long way up or down from any rocks we can date using radiometric methods. In this case, the use of fossils will be absolutely the best method available.

Second, it is much faster than any more technical method. Why send a rock to a laboratory and wait for a reply when you can just glance at the fossils it contains and say: "Ah yes, Early Ordovician"?

Third, by the same token, it's much cheaper. Radiometric dating requires specialized equipment: lasers, spectrometers, or in the case of Ar-Ar dating a small nuclear reactor. Even the humblest items of equipment come at a price: laboratories that carry out U-Pb dating wash the bottles they use for two years continuously to eliminate contamination. Rather than employ the services of such a laboratory, it is so much cheaper for the geologist to recognize a well-known species of ammonite, trilobite, foraminiferan, or whatever, the age of which is already known.

Edited by Dr Adequate, : No reason given.

Edited by Dr Adequate, : No reason given.


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RAZD
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Posts: 19320
From: the other end of the sidewalk
Joined: 03-14-2004
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Message 252 of 293 (688291)
01-21-2013 5:26 PM
Reply to: Message 251 by Dr Adequate
01-20-2013 11:42 PM


Re: Fossils And Absolute Dating
Hi again Dr Adequate

But this means that we can now use the fossil species to date the sedimentary rocks in which it is found; ...

The typical creationist complaint is that this is circular reasoning: rocks to date the fossils that then date the rocks ...

... but it is not circular: the first rocks are dated by radiometric methods the second rocks are usually of a type that cannot be dated by radiometric methods, sedimentary rocks, which is where the fossils are found.

It's linear:

dated rocks → date range for fossils → date range for undated rocks

The layers can also be tracked across the countryside and sometimes found between date-able rocks.

... well-known species of ammonite, trilobite, foraminiferan, or whatever, the age of which is already known.

And diatoms among others.

Enjoy


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This message is a reply to:
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Dr Adequate
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Posts: 15987
Joined: 07-20-2006
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(1)
Message 253 of 293 (688299)
01-21-2013 6:45 PM
Reply to: Message 252 by RAZD
01-21-2013 5:26 PM


Re: Fossils And Absolute Dating
The typical creationist complaint is that this is circular reasoning: rocks to date the fossils that then date the rocks ...

... but it is not circular: the first rocks are dated by radiometric methods the second rocks are usually of a type that cannot be dated by radiometric methods, sedimentary rocks, which is where the fossils are found.

Well, as I said:

Hence we can use datable rocks to put dates on fossil species; and then we can use the fossil species to put dates on other rocks which would otherwise be difficult to date.

This "circular reasoning" thing has always struck me as one of the odder creationist fantasies. They are surely not actually imagining some geologist saying: "I know this rock is ten million years old because I know the fossils in it are ten million years old; and I know the fossils are ten million years old because I know the rock they're in is ten million years old." I think as usual they aren't imagining anything at all, they're merely reciting words; if they tried to attach meaning to them they'd realize that what they're saying can't really be true.


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Dr Adequate
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(4)
Message 254 of 293 (689381)
01-30-2013 6:31 AM


Absolute Dating: An Overview
Absolute dating: an overview

Introduction

In this article, we shall take a look back at the methods of absolute dating, and see how we know that they can be relied on.

Basis of the methods

One argument in favor of the absolute dating methods presented in the preceding articles is that they should work in principle. If they don't, then it's not just a question of geologists being wrong about geology, but of physicists being wrong about physics and chemists being wrong about chemistry; if the geologists are wrong, entire laws of nature will have to be rewritten. Science, since it concerns just one universe with one set of laws, constitutes a seamless whole; we cannot unpick the single thread of absolute dating without the whole thing beginning to unravel.

Still, it has happened in the past that scientists have thought they'd got hold of a law of nature and then found out it was false. There is no particular reason to suspect that this will turn out to be the case when it comes to the laws underlying absolute dating; nonetheless, an argument from principle alone can never be entirely convincing. Let us therefore turn to the evidence.

Sea-floor spreading

You will recall from our discussion of sea-floor spreading that the sea-floor spreads out from mid-ocean rifts, and so ought to be younger nearer the rifts and progressively older further away from them.

What is more, we can measure the rate of spreading directly by GPS, SLR, and VLBI. This means that if we didn't have any other way of doing absolute dating, we would as a first approximation take the age of basalt on a spreading sea floor to be the distance from the rift divided by the rate of spreading.

Now if we estimate the age of the sea-floor like that, then we get a good agreement with the dates produced by radiometric methods. It is hard to think that this is a coincidence; it is also hard to think of any mechanism that could produce this agreement other than that the rocks are as old as radiometric methods tell us.

Marine sediment

We began our discussion of absolute dating by saying that sedimentation rates could not be relied on for absolute dating. If there is one possible exception to this, it would be the deposition of marine sediment, since it is not subject to erosion, and since we would expect the rates of deposition of the various sediments to be, if not actually constant, then not subject to such a degree of variation as (for example) glacial till. Based on the known rates of deposition, we may therefore at least say that the depths of marine sediment found on the sea floor are consistent with the ages of the igneous rocks beneath them as produced by radiometric dating.

Radiometric dating and paleomagnetism

The polarity of the Earth's magnetic field is a global phenomenon: at any given time it will either be normal everywhere or reversed anywhere. So if our methods of radiometric dating are correct, then we would predict that rocks dated to the same age would have the same polarity, which they do.

If this does not completely prove that radiometric dating is correct, it does at least show that (barring a wildly improbable coincidence) there is at least a one-to-one relationship between the dates produced by radiometric methods and the true dates, and so it must be taken as an argument in favor of these methods.

Comparison with historical dates

It is possible to test radiocarbon dating by using it to put a date on historical artifacts of known date, and to show that it is usually very accurate.

It has also been possible to test Ar-Ar dating against the historical record, since it is sufficiently sensitive to date rocks formed since the inception of the historical record. For example, Ar-Ar dating has been used to give an accurate date for the eruption of Vesuvius in 79 A.D, as recorded by Roman historians at the time. (See Lanphere et al., 40Ar/39Ar ages of the AD 79 eruption of Vesuvius, Italy, Bulletin of Volcanology, 69, 259263.)

Radiocarbon dating, varves, and dendrochronology

Because varves contain organic material, it is possible to compare the dates from varves with the dates produced by radiocarbon dating, and see that they are in good agreement. We also see close agreement between dendrochronology and uncalibrated radiocarbon dates.

Now, each of these three methods relies on a different underlying physical process: radioactive decay, outwash from glaciers, the growth of trees. We can hardly suppose that there is some single mechanism which would interfere with all three of these very different processes in such a way as to leave the dates derived from them still concordant.

But it is equally far-fetched to imagine that three different mechanisms interfered with the three processes in such a way as to leave the dates concordant; that would require either a preposterous coincidence, or for natural processes to be actually conspiring to deceive us: an idea which is, if anything, even more preposterous.

Now, preposterous things do happen occasionally. But in this case there is a perfectly reasonable and straightforward explanation for why the dates are concordant, namely that they are correct.

Radiometric dating, sclerochronology and rhythmites

Similar remarks may be made about the agreement between radiometric dating of rocks, sclerochronology, and dating by rhythmites.

Are we to believe that one single mechanism interfered with the decay of radioactive isotopes, the secretion of calcium carbonate by molluscs, and the action of the tide? Absurd. But are we instead to believe that three separate mechanisms interfered with these processes in such a way as to leave all the dates concordant? That would be equally absurd. The straightforward explanation for the concordance of the dates is that they are in fact correct.

Consider the following analogy: a clockmaker sells us an electric clock, a pendulum clock, and a spring-driven clock, and guarantees that they are shockproof. Skeptical of the clockmaker's claim, we subject the clocks to shock: we shake them, drop them, hit them with hammers and shoot them out of a cannon. Throughout this process, they all go on showing exactly the same time. Is it plausible that we have damaged their very different internal mechanisms in such a way that they are all running fast or slow but still in perfect synchrony? Or is it more likely that they are synchronized because nothing that's happened to them has affected their working?

Agreement with relative dating

Relative dating by definition does not produce actual dates, but it does allow us to put an order on the rocks, and so if absolute dating is to be trusted, it should agree with this order, telling us, for example, that Ordovician rocks are older than Triassic rocks; and it does.

It is hard to see this as a coincidence; it is equally hard to think of some alternate explanation of why we can correlate isotope ratios or sclerochronological data with the relative order of rocks as deduced from stratigraphic methods --- other than the straightforward explanation that absolute dating is producing the right dates.

Internal consistency of radiometric dates

In our discussion of radiometric dating, we have seen that many, indeed most, radiometric methods are self-checking.

So in the U-Pb method, we check that the two uranium isotopes produce concordant dates. In the Ar-Ar method, we check that step heating yields the same date at every step. In Rb-Sr, Sm-Nd, Lu-Hf, Re-Os, La-Be, La-Ce and K-Ca dating, we check that the data we plot on the isochron diagram lies on a straight line.

These precautions allow us to throw out most data that have been produced by confounding factors such as atmospheric contamination, weathering, hydrothermal events, metamorphism, metasomatism, etc.

It is, as we have explained, possible for the occasional incorrect date to slip through this filter, since it is possible for some of these confounding factors to accidentally change the isotope ratios in such a way as to produce something that looks like a good date: apparently concordant dates for Ar-Ar or U-Pb, or a false isochron for the various isochron methods.

It would indeed be remarkable if this never happened, since one-in-a-thousand chances do in fact occur one time in a thousand. But by the same token, the other 999 times they don't, and so although any particular date produced by these methods might be called into question, it must be the case that the vast majority of dates that pass through these filters must be good; for we can hardly suppose that the confounding factors are actively conspiring to deceive us, and so these long-shot events must be as rare as statistical considerations would lead us to expect.

Mutual consistency of radiometric dates

You might perhaps suggest that if some unknown factor, contrary to our present understanding of physics existed that sped up or slowed down radioactive decay in the past, then we would expect the radiometric dates to be concordant whether they were right or wrong.

This is, as I say, contrary to our present understanding of physics, and so is mere unfounded speculation. What is more, the reader should recollect that "radioactive decay" is not the name of one process; it is the name of any process that rearranges the nucleus. So to leave dates produced by different radiometric methods still concordant, nature would somehow have to conspire to fool us by changing the rate of alpha decay, of beta decay, of electron capture, in such a way that the different dating methods based on these different modes of decay come up with the same dates.

Another point to bear in mind is that a change in the rate of radioactive decay, even if it was carefully coordinated in this way, would still not change every radiometric date in the same direction: if, for example, radioactive decay sped up at some time in the past then this would make U-Pb or Ar-Ar dates older than they should be, but it would make the dates produced by cosmogenic surface dating younger than they should be.

Summary

It is possible to doubt any particular date obtained by absolute dating methods. But it would be bizarre to doubt the general picture they paint. For what we see is a massive agreement between the different radiometric methods, varves, dendrochronology, sclerochronology, rhythmites, paleomagnetic data, sedimentation rates, sea-floor spreading, and relative dating methods.

For the dates obtained by absolute dating to be wrong in general and yet wrong in such a way as to be in agreement with one another and with other observations, we would have to suppose either that we are looking at an inconceivably massive coincidence, or that the whole Earth is a fraud designed to deceive us.

Ideas to the latter effect have actually been proposed from time to time; most notably by the nineteenth century religious zealot Philip Gosse, whose eccentric work Omphalos proposed that the Earth was a mere few thousand years old, but that God had created it to look much older. To this the Reverend Charles Kingsley memorably answered: "I cannot believe that God has written on the rocks one enormous and superfluous lie for all mankind". That of course would be a theological rather than a geological question, and so is outside the scope of this textbook. What can be said is that geology is a science, and that in science it is necessary to proceed on the basis that the universe is not a lie; because if we believed that, we could believe that anything at all was the case and disregard all evidence to the contrary. The scientific method compels us, then, to disregard the possibility of divine malice; and mere natural processes, being mindless, cannot be actually malevolent.

What, then, of coincidence? Well, there are limits to the degree of coincidence we can believe in, otherwise again we could believe nearly anything. The scientific method requires us to discard such remote possibilities unless there is at least a hint of a shred of evidence for them.

We are left with the conclusion that the great majority of the dates produced by absolute dating methods must be reasonably accurate.

Edited by Dr Adequate, : No reason given.


Replies to this message:
 Message 255 by AZPaul3, posted 01-30-2013 9:17 AM Dr Adequate has not yet responded

  
AZPaul3
Member
Posts: 3428
From: Phoenix
Joined: 11-06-2006


Message 255 of 293 (689388)
01-30-2013 9:17 AM
Reply to: Message 254 by Dr Adequate
01-30-2013 6:31 AM


Re: Absolute Dating: An Overview
Dr. A,

Not bad for a math teacher.

Congrats.

And thanks.


This message is a reply to:
 Message 254 by Dr Adequate, posted 01-30-2013 6:31 AM Dr Adequate has not yet responded

  
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