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Author Topic:   How the geo strata are identified as time periods
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Message 61 of 101 (344982)
08-30-2006 6:34 AM
Reply to: Message 57 by iano
08-29-2006 9:06 PM


Re: accusations
Hi Iano,
There's a thread reserved for members to express their concerns about moderation. Please use it and allow this thread to focus on the topic.

--Percy
EvC Forum Director

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Jazzns
Member (Idle past 3911 days)
Posts: 2657
From: A Better America
Joined: 07-23-2004


Message 62 of 101 (345059)
08-30-2006 2:06 PM
Reply to: Message 60 by Percy
08-30-2006 6:33 AM


Re: Igneous Rocks
Yes you are correct Percy.
I am digging pretty deep into my dusty geo knowledge but I am pretty sure you can use the time to cool of as one the the markers to help identify how MUCH older the layers it intruded into are than the igneous rock itself.
So lets say you can absolute date the intrusion at 100 MA. That is your first lower bound on the age of the surrounding rock. Then you look at the details of the intrusion and figure out that it took 2 MA to cool off. You have just shifted your lower bound of the date of the surrounding rocks to 102 MA. I have no idea if that is even a reasonable amount of time for a magma body to cool but you could insert a correct number in there just the same and the concept would still hold true.
It is true that the time to cool a batholith is one of many single pieces of evidence that immediatly points to the earth as ancient but the focus of this thread is more on ages as in "birthday of the rock" kind of ages. You simply cannot cool intrusive rocks faster and have them come out like they do without fundamentally changing the laws of nature. It is just like we might be able to bake a cake by heating it the equivalent of its bake time in under a nanosecond but what comes out of the oven will most certainly not be cake.
Edited by Jazzns, : Changed "upper bound" to "lower bound" Upper = oldest it could be, lower = youngest it could be.

Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)

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Jazzns
Member (Idle past 3911 days)
Posts: 2657
From: A Better America
Joined: 07-23-2004


Message 63 of 101 (345094)
08-30-2006 4:00 PM
Reply to: Message 59 by kuresu
08-30-2006 1:41 AM


Re: Igneous Rocks
1)what is it about igneous rocks over sedimenatary (can metamorphic
rocks be part of a layer?) that allows us to absolutely date them?
There are a number of questions there that require some potentially longs answer. I'll try to be as simple as I can.
Metamorphic rocks can be "layers" in the true sense and also not "layers". Metamorphic rocks are created in many different ways which is where much of the distinction lies. Simply burying regular sedimentary rock layer far enough down will cause it to metamorphose and keep its layered configuration. For example, limestone will metamorphose under heat and pressure into marble. Rocks can also metamorphose when they come into contact with something hot like the magma intrusions we have been talking about. This kind of metamorphism is called contact metamorphism. In these cases, the rock partially melts due to the heat and solidifies again as metamorphic rock. In the case of contact metamorphism, it really isn't a layer. Usually around the edges of an intrusion you have lots of rock that has been contact metamorphised.
The reason that igneous rocks can be absolute dated is because of their internal structure and chemistry. They are bulks of material from the same original non-rock (magma) source. Igneous rocks are essentially crystalized magma. The crystal structure traps other elements some of which happen to be radioactive. The crystal structure, while forming, also rejects the inclusion of other elements which happen to be the decay product of some of the radioactive elements that it DOES trap. More on this in question 2.
Sedimentary rocks cannot be dated because they are simply small chunks of igneous rocks that can potentially come from a wide array of original source. Two different sand particles in the same sandstone may have come from two orignal chunks of granite that are millions of years apart. Plus, even if you do date the individual grains all you are really dating is the age of the source material, not the date that the sandstone was laid down. A particular sand particle could have traveled far and long, even temporarily being part of another sandstone, before it eventually made it to its current place.
Metamorphic rock can be dated sometimes. Because the rock partially melts, there are some aspects of it that are similar to igneous rocks. When you can date these, they tell you the date of metamorphism and not the date the rock was originally laid down.
2)what methods allow for absolute dating? Is there more than one, in other words?
I don't know if there are more ways to absolute date then the broad category of radioisotope dating. I think there are but I am not educated in them. Maybe a real geologist can pipe in to enumerate some other absolute methods that do not involve radioisotopes.
Radioisotope methods use the trapped radioactive elements in the crystal structure of an igneous rock mentioned above.
We can experimentally verify that radioactive elements decay into other elements at a known constant rate. That rate is represented by the time it takes for 1/2 of any amount of the material to decay naturally. This is called the "half life" of the element.
I mentioned before that during its formation, the crystal struction of igneous rock rejects the decay product (called the daughter product) as it forms. This can be verified experimentally and is supported by the basic laws of chemistry. Therefore we know that from the moment it is solid, the original radioactive material (call the parent) has been completely seperated from any daughter product. But when the parent decays, THAT daughter product STAYS in the crystal because it only gets rejected as the crystal is forming.
So we can come along and measure how much of parent and daughter elements are in a sample of the rock. We know that all the daughter used to be parent and the ratio combined with our knowledge of how fast the elements decay means we can know with quite high accuracy how much time has passed since that rock solidified from the magma.
3) for relative dating, besides what you showed, are there methods for finding out about how old those inbetween layers are? if there are,
what are they?
This is a very complex issue that usually eats up many semester of college work. The basics involve some no-brainer principles:
Principle of Superposition - Unless something has been done to flip the stack, rock #1 on top of rock #2 is younger than rock #2.
Principle of cross cutting - An intrusion, fault, erosional surface, or anything else that disrupts a layer is by definition younger than the layer.
So essentially you can take a picture of the geographic column at a given location and reconstruct the order of events that things occurred.
For example, if there are three layers with a fault that splits the bottom two but not the top, then you know that the first two layers were laid down, the fault occurred, and then the top layers was laid down after.
Index fossils also help. If you know that elsewhere in the world you never see a particular fossil past a certain age that can be verified absolutly, if you find it in someplace where you cannot absolute date then you can MAYBE say that the other absolute ages are the youngest that the rock that contained the fossil could be. There are a lot of caveats when dealing with index fossils.
We could probably do a whole long thread about relative dating with nice examples/pictures etc if enough people are interested.

Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)

This message is a reply to:
 Message 59 by kuresu, posted 08-30-2006 1:41 AM kuresu has replied

Replies to this message:
 Message 64 by kuresu, posted 08-30-2006 10:02 PM Jazzns has replied

  
kuresu
Member (Idle past 2513 days)
Posts: 2544
From: boulder, colorado
Joined: 03-24-2006


Message 64 of 101 (345265)
08-30-2006 10:02 PM
Reply to: Message 63 by Jazzns
08-30-2006 4:00 PM


Re: Igneous Rocks
so you can absolute date igneous and certain metamorpich rocks, but you can not use the metamorphic rocks to date layers.
reason being, the rock was laid down first, and then metamorphised. And since metamorphic rock only comes from sedimentary rock, and since sedimentary rock cannot be dated absolutely due to coming from many different sources, you only get the date when the sedimentary rock metamorphised and trapped any radioactive isotopes.
correct?
this means that for all intents and purposes, only layers with igneous rocks can be absolutely dated.
1)can igneous rocks come from any other source?
2)can an igneous layer be inserted inbetween two pre-existing layers?
(note that I'm not asking if they can be put at the bottom by batholiths and plutons, hold on, new question)
3)in the case of batholiths and plutons contact metamorphising rocks:
could the newly metamorphised rocks be dated younger than those on top of them?

All a man's knowledge comes from his experiences

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 Message 63 by Jazzns, posted 08-30-2006 4:00 PM Jazzns has replied

Replies to this message:
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JonF
Member (Idle past 168 days)
Posts: 6174
Joined: 06-23-2003


Message 65 of 101 (345368)
08-31-2006 7:49 AM
Reply to: Message 64 by kuresu
08-30-2006 10:02 PM


Re: Igneous Rocks
this means that for all intents and purposes, only layers with igneous rocks can be absolutely dated.
Just a nitpick; that's mostly but not 100% true. There are structures that form when sedimentary rocks lithify (turn into rocks), and in some cases those structures can be dated. (E.g. SHRIMP U-Pb dating of diagenetic xenotime in the Stirling Range Formation, Western Australia: 1.8 billion year minimum age for the Stirling biota. But it ain't easy, and requires recently depeloped instrumentation to handle incredibly tiny sample sizes. The vast majority of dates are not from sedimentary rocks.
1)can igneous rocks come from any other source?
No.
2)can an igneous layer be inserted inbetween two pre-existing layers?
Yes, but not without the insertion being easily detectable. The effects of the heat, you know, and the shape of the insertion.
3)in the case of batholiths and plutons contact metamorphising rocks:
could the newly metamorphised rocks be dated younger than those on top of them?
I'm not sure, but perhaps.

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Jazzns
Member (Idle past 3911 days)
Posts: 2657
From: A Better America
Joined: 07-23-2004


Message 66 of 101 (345403)
08-31-2006 11:01 AM
Reply to: Message 64 by kuresu
08-30-2006 10:02 PM


Re: Igneous Rocks
John had some good answers but I would like to clarify on some others.
For your first question. Igneous rocks are simply rocks that come from crystalized magma. The only source is magma and the only distinction is above or below ground. Igneous rocks are pretty easy to understand IMO since they are sort of the "first" rocks that all kinds of other complicated rocks are derived from.
2)can an igneous layer be inserted inbetween two pre-existing layers? (note that I'm not asking if they can be put at the bottom by batholiths and plutons, hold on, new question)
There are two types of igneous intrusions that come off of a larger magma body called sills and dikes. These look sort of like appendages of the larger magma body. When one of these appendages reaches the surface that is when you get a volcanoe.
A dike is different from a sill in the direction in intrudes. A dike cuts through layers. A sill though runs parallel to layers and can sometimes look like it is splitting inbetween the layers. The thing to note though is that diagnostically you can still tell that a sill is an intrusion and therefore younger than the layers it has split. There will usually be tell tale signs of metamorphism where the sill comes into contact with the other layers.
3)in the case of batholiths and plutons contact metamorphising rocks: could the newly metamorphised rocks be dated younger than those on top of them?
This depends. The metamorphic rock is always younger than the immediatly surrounding rock that was the source rock for the metamorphism. By "younger" too I mean simply that the metamorphic event happened after the other layers were laid down. Really that is a no brainer becaused how could it not. The layers had to be there to be metamorphised. Beyond that it a factor of the surrounding area. The next layer just above the metamorphic rock may or may not be older. If you can tell that the same magma body intruded into THAT layer somewhere else then you know it is older. If there is some other cross cutting relationship that affects the metamorphic rock but not the higher layers it would mean that the higher layers are younger than the metamorphic rock.

Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)

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 Message 64 by kuresu, posted 08-30-2006 10:02 PM kuresu has replied

Replies to this message:
 Message 67 by kuresu, posted 08-31-2006 11:13 AM Jazzns has replied

  
kuresu
Member (Idle past 2513 days)
Posts: 2544
From: boulder, colorado
Joined: 03-24-2006


Message 67 of 101 (345406)
08-31-2006 11:13 AM
Reply to: Message 66 by Jazzns
08-31-2006 11:01 AM


Re: Igneous Rocks
okay, so what I'm getting is:

The flood is OFF TOPIC here! Thank you.


the geology presented here busts the Flood.
unless the Flood also produced the igneous rock layers,
there's no way that the flood happened at the global scale.
1)only magma produces igneous rocks
2)any intrusions of magma that are parallel can still be found to be an intrusion, and thus younger than what had been laid down before
3)the radiometric dating is done with the igneous rocks
even if radiometric dating is wrong (which I don't think) there's still no way for a flood to deposit layers of igneous rocks, expecially those due to lava flows and ash, right?
Edited by AdminNosy, : Topic warning

All a man's knowledge comes from his experiences

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Replies to this message:
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jar
Member (Idle past 394 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 68 of 101 (345408)
08-31-2006 11:19 AM
Reply to: Message 67 by kuresu
08-31-2006 11:13 AM


Re: Igneous Rocks
There is one other question I'd like you to consider.
How do you get clastic sedimentary rock?

Aslan is not a Tame Lion

This message is a reply to:
 Message 67 by kuresu, posted 08-31-2006 11:13 AM kuresu has replied

Replies to this message:
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kuresu
Member (Idle past 2513 days)
Posts: 2544
From: boulder, colorado
Joined: 03-24-2006


Message 69 of 101 (345410)
08-31-2006 11:26 AM
Reply to: Message 68 by jar
08-31-2006 11:19 AM


Re: Igneous Rocks
what the??
I've heard the name before, (no clue what it is) and since you're just asking for it,
how?

All a man's knowledge comes from his experiences

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 Message 68 by jar, posted 08-31-2006 11:19 AM jar has replied

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Jazzns
Member (Idle past 3911 days)
Posts: 2657
From: A Better America
Joined: 07-23-2004


Message 70 of 101 (345437)
08-31-2006 12:53 PM
Reply to: Message 67 by kuresu
08-31-2006 11:13 AM


Ages
I think the thing to notice is even the very topic of this thread runs counter to any kind of young earth scenario not necessarily the flood. Magma activity can still happen during a flood. All you can say about the existence of an intrusion is just that it happened after the intruded upon rock was laid down. There are millions of other specific reasons why intrusions invalidate a flood but like Nosy said, that is for another thread.
This thread is about age. In particular, because we can absolute date a rock we can determine the minimum age of the earth by finding the oldest rock that we can absolute date. I am not sure but I think the oldest one we have found is only a little over 4 GA (billion years). The date often used of 4.5 GA is using other evidence such as dating moon rocks, meteors, etc that are objects as old as the earth since they formed form during the same stellar phenomenon that formed the earth.
On another topic regarding the OP, the various ages (as in birthdates) of the earth are determined by absolute dating but the names and durations of the ages (as in eras) are determined by other factors. For example, the Carboniferous is named as such because it was a period of time where land plants pretty much covered the earth. That age ended when something else notable occurred. Another good example is how the Mezozoic ends with the major extinction event that wiped out the dinosaurs. Mezozioic means "belonging to the reptiles" and it marks the period in Earth's history when reptiles and dinosaurs were the most prominent.
Edited by Jazzns, : No reason given.

Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)

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jar
Member (Idle past 394 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 71 of 101 (345448)
08-31-2006 1:29 PM
Reply to: Message 69 by kuresu
08-31-2006 11:26 AM


Clastc sedimentary rocks.
Well, I'm trying to stay in the background pretty much but I think this is important because it shows the reasoning that led to the idea of a Young Earth only 6000 or 10,000 or even 1000,000 years old being discarded long before we could do any absolute dating.
The important thing is that Clastic Sedimentary rocks are made up of other rocks. They began life as something else, igneous or metamorphic or other sedimentary rocks or often a combination of all those. They began life somewhere else, were formed, then were worn down, disturbed, moved to where they are now and then transformed.
A short lesson plan is here

Aslan is not a Tame Lion

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NosyNed
Member
Posts: 8996
From: Canada
Joined: 04-04-2003


Message 72 of 101 (345451)
08-31-2006 1:38 PM
Reply to: Message 71 by jar
08-31-2006 1:29 PM


Chemistry of formation
Thanks for the link Jar. Now I fell compelled to go out and bite rocks.
It raises another dating question in my mind: What is the chemistry (or whatever) of the lithification of various clastic rocks?
What happens when mud turns to mudstone, sand to sandstone etc?
PS --- ONE MORE (AND ONLY ONE MORE TIME) the flood is NOT on topic here. Not a word, not even typing fxxxd. My friend with the nose will suspend the next person who brings it up.
Edited by NosyNed, : corrected one of the spelling errors

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jar
Member (Idle past 394 days)
Posts: 34026
From: Texas!!
Joined: 04-20-2004


Message 73 of 101 (345457)
08-31-2006 1:54 PM
Reply to: Message 72 by NosyNed
08-31-2006 1:38 PM


Re: Chemistry of formation
PS --- ONE MORE (AND ONLY ONE MORE TIME) the flood is NOT on topic here. Not a word, not even typing fxxxd. My friend with the nose will suspend the next person who brings it up.
Well, it will likely come up but only in the lowercase fashion, water as one of the transport and shaping methods. I don't see how we can get around it and still describe some of the things seen.

Aslan is not a Tame Lion

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 Message 72 by NosyNed, posted 08-31-2006 1:38 PM NosyNed has not replied

  
JonF
Member (Idle past 168 days)
Posts: 6174
Joined: 06-23-2003


Message 74 of 101 (345496)
08-31-2006 4:56 PM
Reply to: Message 70 by Jazzns
08-31-2006 12:53 PM


Re: Ages
I am not sure but I think the oldest one we have found is only a little over 4 GA (billion years)
The oldest known rocks (assemblages of minerals) are from the Slave Lake region of Canada, and are indeed just over 4 GA. Bowring, S. A. & Williams, I. S., 1999. Priscoan (4.00-4.03 Ga) orthogneisses from northwestern Canada, Contrib. Mineral. Petrol. v134 #1 pp 3-16; three rocks have ages of 4.0020.004 billion years (sample SAB91-63), 4.0120.006 billion years (sample SAB91-37), and 4.0310.003 billion years (sample SAB94-134). The Tera-Wasserberg concordia-discordia plot for sample SAB94-134 is at http://i2.photobucket.com/...10/JonF/SlaveLake_SAB94-134.png.
The oldest known terrestrial minerals are zircons from Jack Hills in Australia, found in sedimentary rocks (so the rocks are not that old). Wilde SA, Valley JW, Peck WH and Graham CM (2001) Evidence from Detrital Zircons for the Existence of Continental Crust and Oceans on the Earth 4.4 Gyr Ago. Nature. 409: 175-178. The zircons are 4.4040.008 GA. Lots more on these zircons at Zircons Are Forever.
The date often used of 4.5 GA is using other evidence such as dating moon rocks, meteors, etc that are objects as old as the earth since they formed form during the same stellar phenomenon that formed the earth.
Indeed the dates of many moon rocks and meteors are around 4.5 GA, and that is good evidence that the Earth is about that old. (See Part II. Radio-isotopic Dating; at the end are several tables of such ages reproduced from Dalrymple's "The Age of the Earth").
But the 4.55 GA age of the Earth is derived from a lead-lead isochron analysis of almost solely terrestrial sources. Lead-lead isochrons are unlike other isochrons in that they are not "anchored"; the intercepts of the isochron line with the axes is meaningless, and you need to obtain an "anchor point" for the isochron line from something other than the lead content of your samples. We need to figure out the primordial lead isotope ratios of the Solar System; and that we get from the Canyon Diablo meteorite. This meteorite is primordial and contains essentially zero uranium (less than 10 ppb). So its current lead composition is also primordial to within a lot better than one percent. This anchors the isochron, and then terrestrial samples define the slope of the isochron, which determines the age; 4.55 billion years.
There's a lot more to it than this; Dalrymple devotes an entire chapter to it, and that's not the really technical version. In particular, the primordiality of the Canyon Diablo meteorite has been checked many ways and passed all tests.

This message is a reply to:
 Message 70 by Jazzns, posted 08-31-2006 12:53 PM Jazzns has replied

Replies to this message:
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Jazzns
Member (Idle past 3911 days)
Posts: 2657
From: A Better America
Joined: 07-23-2004


Message 75 of 101 (345501)
08-31-2006 5:03 PM
Reply to: Message 74 by JonF
08-31-2006 4:56 PM


Re: Ages
Facinating that the CD Meterorite would have such low levels of Uranium. Is it just luck that we found a sample that has been spared inclusion of any uranium during formation?
What I find interesting is that the lead/lead method you mentioned agreed very closely with the direct method on other glactic material. It is my understanding that there is no reason to expect that they should unless they ARE actually that old. Is that a fair characterization?

Of course, biblical creationists are committed to belief in God's written Word, the Bible, which forbids bearing false witness; --AIG (lest they forget)

This message is a reply to:
 Message 74 by JonF, posted 08-31-2006 4:56 PM JonF has replied

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