Circular Reasoning

[RickG]

"Circular reasoning" is one of the claims made in the Creation Science community concerning "Relative" dating methods used by geologists. An example is: Rocks date the fossils and fossils date the rocks.

How is this "circular reasoning" claim justified, other than the misrepresentation of what "relative dating" is meant to describe?

 

[Hank]

If you over simplify, you can find circular reason in most measurements. If you measure a table, and say it is 42 centimeters long, I can ask how long a centimeter is. A centimeter is an arbitrary unit. One either accepts the length of a centimeter or not.

When it comes to rock dating, it's more complex. IIRC, when one dates a rock, one does not know what the data will yield. So you need to make some in depth researches to get an approximation. Make more comparisons, and slowly arrive at a rough date. Like anything else, rock dating is a progressive technology, and is as accurate as humans know how.

 

[RickG]

If you over simplify, you can find circular reason in most measurements. If you measure a table, and say it is 42 centimeters long, I can ask how long a centimeter is. A centimeter is an arbitrary unit. One either accepts the length of a centimeter or not.

I'm not sure you quite understand what the term "relative dating" is with respect to geochronology. Then again, I may just be misunderstanding your analogy.

When it comes to rock dating, it's more complex. IIRC, when one dates a rock, one does not know what the data will yield. So you need to make some in depth researches to get an approximation. Make more comparisons, and slowly arrive at a rough date. Like anything else, rock dating is a progressive technology, and is as accurate as humans know how.

For geochemist/geochronologist, it is not as evasive as you seem to have described. Then again, I'm not sure we are both on the same wavelength. Thanks for your reply and please expound if you wish.

 

[Hank]

Thanks for your reply and please expound if you wish.

I'll try, I am better with drawing stuff than words.

A centimeter is an arbitrary unit. It is as long as it is because most don’t question its lengths. It’s unlikely someone will claim circular reasoning when confronted with a length of an object, given in centimeter.

Measuring the age of a rock falls in the same tasks of measuring something. The proverbial centimeter equals an event in relative dating, or rock layers. It does not give you an actual age yet, only that a rock or fossil was at a given event. One also needs to make sure the rock layers were not turned upside down with earths’ relentless motions, are all accounted for and so on and so forth. One pieces this slowly together on how old a given rock might be. To arrive at the age, one needs an absolute read out. Rocks, well, they are not all that willing to volunteer for a good read out. Fossils embedded in a rock may give one a better absolute reading with radiometric dating than the rock itself. And this is where I think that phrase comes from. Yet the fossil was in the rock. So it's a one way street, nothing two way or circular, unless you challenge the entire unit of measurement.

Hopefully this is making more sense, if not, I have all weekend, to re-read my old books, and give a better answer with back ups.

 

[RickG]

I'll try, I am better with drawing stuff than words.

A centimeter is an arbitrary unit. It is as long as it is because most don’t question its lengths. It’s unlikely someone will claim circular reasoning when confronted with a length of an object, given in centimeter.

Measuring the age of a rock falls in the same tasks of measuring something. The proverbial centimeter equals an event in relative dating, or rock layers. It does not give you an actual age yet, only that a rock or fossil was at a given event. One also needs to make sure the rock layers were not turned upside down with earths’ relentless motions, are all accounted for and so on and so forth. One pieces this slowly together on how old a given rock might be. To arrive at the age, one needs an absolute read out. Rocks, well, they are not all that willing to volunteer for a gcystiphyllum niagarenseood read out. Fossils embedded in a rock may give one a better absolute reading with radiometric dating than the rock itself. And this is where I think that phrase comes from. Yet the fossil was in the rock. So it's a one way street, nothing two way or circular, unless you challenge the entire unit of measurement.

Hopefully this is making more sense, if not, I have all weekend, to re-read my old books, and give a better answer with back ups.

Okay Hank, I understanding what you are saying, though I do disagree with the difficulties you describe with radiometric dating, but let's not get sidetracked with dating methods just yet.

With the exception of organisms that uptake atmospheric carbon dioxide during their life span which are less than 50,000 years of age, lending themselves to radiocarbon dating, fossils are not directly dated. What is dated is the strata in which they are contained and that is where relative dating comes in use. There are specific fossil species that are only found in specific strata, and because they are only found in specific strata they are called "index fossils". Now, the strata in which they are contained of course spans a specific duration of geologic time. It could be a few million years or tens of millions of years. Thus by dating the strata in which they are first observed and the strata in which they are last observed, we have a range of geologic time with "absolute" dates of both the upper and lower ends. So when we find an index fossil we know that that fossil falls in the range of a geologic time span. For example, cystiphyllum niagarense is an invertebrate fossil found only in the Silurian Period, ranging in age from 443 to 419 million years of age. So when we find a cystiphyllum niagarense fossil, we know that the strata we are in is of that geologic age time span. There is nothing circular with this process.

 

[crjmurray]

Okay Hank, I understanding what you are saying, though I do disagree with the difficulties you describe with radiometric dating, but let's not get sidetracked with dating methods just yet.

With the exception of organisms that uptake atmospheric carbon dioxide during their life span which are less than 50,000 years of age, lending themselves to radiocarbon dating, fossils are not directly dated. What is dated is the strata in which they are contained and that is where relative dating comes in use. There are specific fossil species that are only found in specific strata, and because they are only found in specific strata they are called "index fossils". Now, the strata in which they are contained of course spans a specific duration of geologic time. It could be a few million years or tens of millions of years. Thus by dating the strata in which they are first observed and the strata in which they are last observed, we have a range of geologic time with "absolute" dates of both the upper and lower ends. So when we find an index fossil we know that that fossil falls in the range of a geologic time span. For example, cystiphyllum niagarense is an invertebrate fossil found only in the Silurian Period, ranging in age from 443 to 419 million years of age. So when we find a cystiphyllum niagarense fossil, we know that the strata we are in is of that geologic age time span. There is nothing circular with this process.

Pardon my ignorance, but once you've dated the strata using the relative age of the index fossil, don't you then date the strata using a secondary method?

 

[RickG]

Pardon my ignorance, but once you've dated the strata using the relative age of the index fossil, don't you then date the strata using a secondary method?

There are numerous methods in dating rocks. Also, there is very little exposed strata on earth that hasn't already been dated completely independent from their use with index fossils. Specific fossils can also be given an absolute date by dating the "site" containing the fossil. Of course those that is only specific for the site.