How the geo strata are identified as time periods

You are correct faith for the most part. A thicker layer can be deposited quicker than a thinner although what constitutes a layer gives us some evidence as to comparitive speed of deposition. I think jar's question was more appropriate assuming a constant rate of deposition. I would think this is obviously true. In order to have sandstone, you have to have sand. That sand had to come from somewhere either created by God or eroded from some other larger grained silicate material. For the purposes of this thread we can ignore the former.We don't have this kind of question concerning non-sedimentary rocks like volcanic rocks. There source there is always magma or lava. Although there you do have another relative dating principle which is how long it took the magma to cool. In the case of rocks formed from magma, sometimes there are large crystals of consolidated material that can only form if the magma cools slowly enough to allow the crystals to grow. This can give us an indication of the time it took to get from liquid to solid but not the age of the rock, but it helps in some relative dating.{As an aside, unless there was something fundamentally different about the laws of nature, the creation of the above mentioned crystals actually by itself falsifies a young earth as some of them require a magma body to cool slowly over millions of years. If the magma body cools faster then the crystals dont form or dont form of a given size. This can be shown directly. Another thread if you are curious. }The main way layers get dated is by association to layers that CAN BE dated absolutly. Anything igneous can be radioisotope dated and sometimes volcanoes are nice enough to leave a very large continuous layer of ash. The ash can be dated and then it gives one notch on the ruler from which you can start using relative dating above and below it to fill in the gaps.So if you have layers like:Lava flow
Sedimentary rock type 1
Sedimentary rock type 2
Sedimentary rock type 3
Ash layerThe lava and ash are dated absolutly {another aside, they always give dates that match older is deeper which is good evidence for the correctness of the method}. Then you can put other evidence into play to sandwhich the 3 sedimentary layers inbetween the two absolute dates.Other things that come into play are cross cutting relationships, index fossils, and unconformities. These again are used to get evidence based best guess dates for things that we cannot date absolutly.It would be good for you to read up on unconformities. It seems like you dont think they exist or something. This seems to be an opportunity for you to learn something that may help you when talking about this stuff. It is hard to discuss the geo column if you deny the existence of unconformities just like it would be hard to discuss genetics of the ToE if you denied the existence of mutations. These things DO exist and it would be wise not to deny it. Their existence is problematic for a scheme of global deposition because they represent a clear stoppage of deposition, removal of material, and continuance of deposition.How unconformities help in dating is that if you identify one, they can speak to the relative newness of the layers above and the oldness of the layers below. This depends on how extensive the unconformity is and if we can find the column in a surrounding area where the unconfirmity is different to compare. For example, in another area the unconfirmity might not exist and you can see the layers that had been removed. All this helps fine tune the picture created by relative dating.The main thing though is the cornerstone dates created by absolute dating. If I could use a ruler as an analogy, absolute dates are the inch marks. The relative dating would be the smaller subsections of it although my anaolgy breaks when you consider that the subsections in the geo column are not going to be regular like they are on a rule. That should be obvious though. Without absolute dating things get a lot fuzzier as to assigning a date to a rock.

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

I have a standing open door to Faith to discuss what I know about geology as long as it stays civil. If this is to be a learning thread, I think that would be quite nice actually.Let me know where you want to take this if you want me to participate. I can simplify my first post into the one main topic of absolute dating. Everything concrete about how we identify layers starts from there. That is if I understand correctly that you want to know how layers are named/dated by mainstream geology. From abolute dating we can talk about the other things you seem to be interested in such as index fossils, layer continuity, etc.Just let me know what you want.Edited by Jazzns, : No reason given.

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

I'll try to KISS but I can't promise it won't be a little lengthy. Probably the most illuminating answer to this is to describe the three main types of rock. They are:Igneous Rocks: Rocks formed by the cooling of molten material from the mantle of the earth.Sedimentary Rocks: Rocks made from compacted and lithified (glued together) sediment. Sediment is loose material that comes from the weathering of other Igneous, Sedimentary, or Metamorphic rocks.Metamorphic Rocks: Rocks that have been altered in some way usually by heat and pressure. These rocks have different properties than their original form due to the metamorphism.A layer may constitute any of these rock types. Also each rock type can be broken down into further categories. For example, there are two basic types of Igneous rocks being intrusive and volcanic. Settling really only applies to particles held in suspension. For the most part this only applies to sedimentary rocks because they are constructed of loose material that is transported. During transport the material is in some state of suspension in either the air or water.Settling follows the basic property of physics concerning the amount of energy required to keep an object of a certain mass in motion. If there is not enough energy in the wind or water flow to hold a particular particle in suspension it will fall out of suspension. This is very basic.3 is answered above. It really doesn't make much sense to talk about how the three major rock types settle unless you want to get into the specifics of each kind of sedimentary rock.To keep this line of questioning on topic, you may want to further your questions onto the subject of igneous rocks because they are the main type of rock used to identify time periods in the geo strata per the OP.

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

Settling does have much to do with layers forming. It is just isolated to sediment or ash falling out of suspension. It just does not seem to call the creation of the other rock types by that name.1) What is an intrusive igneous rock?An intrusive igneous rock is a rock that is created by magma cooling in the crust. Because it is allowed to cool slowly, larger crystals of similar elements form {these crystals have a name that I forgot}. A popular example of an intrusive igneous rock is granite.2) What is an extrusive igneous or volcanic rock?Volcanic or extrusive igneous rock is created by the action of magma coming to the surface and cooling there. This includes ash that is ejected from some of the more explosive volcanoes. Because the magma is at the surface it cools quickly and the structure is uniform. This is in contrast to the intrusive igneous rock with its large crystals. Good popular examples of volcanic rock are obsidian and pumice.4) How do the igneous layers form?It is hard to call an intrusive igneous rock a "layer" because it is created when a magma body rises up intruding into other rock and then cooling. This leaves behind a large non-uniform bulk of igneous rock right in the middle of other "stuff" which may be layers or not. Depending on the size of the solidified magma body there are two names for these object. A large one is called a batholith while a small one is called a pluton. When these are pushed to the surface or the surface is eroded down to these, often these object stick out creating large granitic mountain ranges such as the Rockies. The Sandia mountains where I live are a pluton that was pushed up to the surface by tectonic activity.Just because intrusive igneous rock though does not leave a "layer" behind does not mean it cannot be used to help give a difinitive age of the surrounding rock. Hopefully this will spawn some nice questions about how. =)Other igneous rocks can form "layers" in the more basic sense in that they leave behind material that is spread out over an area. A lava flow will cover whatever layer happens to be at the surface. Ash also eventually falls down over a quite wide area in some cases which is extrodinarily helpful becuase it gives a single continuous layer over a wide are that helps sync up the column. For example, you can tell by the ash from two different locations if the ash samples came from the same eruption. Therefore you can tell that the layer immediatly beneath the ash, even if they are of a different type, are of the same age.

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

Yes. How very sloppy and unclear of me.To clarify, how things settle does matter when talking about sedimentary rocks. Where the particles settle, how far and under what conditions they travel, what medium they are suspended in originally, how fast they go, all matter in the formation and identification of sedimentary rocks.I am not sure how this really relates to the discussion though because how things "settle" does not really play too much into what age it is given AFAIK.

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

Reading back in the thread and I caught this. I did actually answer this in my first post when I talked about absolute dating. Maybe we will get there again so we can go deeper into it.First and foremost a given layers is examined in comparison to other layers above it and below it that can be absolute dated.Index fossils, which you seem to be curious about, do help but often the identify a very wide span of time that that particular fossil is present. For example, if you find a trilobite you know you are looking at the bottom quarter of the column. From there you can look at other factors to confirm and correlate with that such as absolute dating.

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

Sometimes yes it can help lean you one way or another if a fine grained layer is reasonably thick or thin.You have to be careful though because the thickness could be because of time or relative density of the particles and I am not sure if you can distinguish between the two. Maybe a real geologist could speak up but we may be getting to specific for this thread.We really need to get to absolute dating to properly address the OP.

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

The only thing that settling may be able to tell you is some very rough information about how long it may have taken that particular layer to have formed. The best example is using a very fine grained sedimentary rock. Superfine sediment does not tend to stick around on land so it almost always ends up in a large body of water as wind and water runoff carry it down. As long as the water is agitated though, the very fine particles of sediment will not settle very quickly at all. So if you have a very thick layer of very find sediment you can guess that it took quite a while for all those tini grains to settle out but you can't be totally sure. It may also just be that there was a whole lot of them. Overall settling of sediments just isn't a good lead when talking about identifying the age of rocks. Not all igneous rocks are tougher than sedimentary rocks though. The previously mentioned pumice and obsidian are pretty fragile. This is an aside though.Now to the questions.1) Is a single layer uniform (composed of the same material)?When you get down into the really fine details of a sedimentary rock then answer is no. The Grand Canyon is a good example where some layers are described as a "Shaley Limestone" or "Sandy Limestone". Within each layer there may be finer gradients of differing materials indicating a slight change in how things "settled" although overall it is still one layer.As for "extrusive rocks" (I assume you mean extrusive igneous rocks) being mixed into a layer; if you all of a sudden see igneous rock showing up then you know a volcanoe has erupted somewhere nearby. It should interrupt the layer rather than be mixed into it. If the circumstances of "settling" don't change much due to the eruption then you may find that things pick up with a new layer of the same material that was deposted before the volcanic material was laid down. A magma chamber is an intrusive igneous rock but plutons and batholiths are WAAAY bigger than that. Were are talking whole mountains and mountain RANGES.Intrusive rock can help with dating because of the principle of cross cutting relationships. We know that the intrusion is younger than the "stuff" it has intruded into. If the surrounding rock wasn't there, it would not be an intrusive rock. So since we can date the intrusion absolutly, we know that the rock it intruded into is older than the date given by the intrusion.So lets say a magma body intrudes into some sandstone. The magma body cools into granite and we come in later and absolute date it to 300 million years old. We don't know exactly how old the sandstone layer is, but we know it is older than 300 million years. The intrusion gives us an upper bound to the age of the sandstone. It is more than just the relative relationship. Obviously if the igneous rock is layed on top of another rock it is older. The big main point to take hope is that we have a method of assigning an absolute date to the formation of igneous rocks. We can give an igneous rock a birthday.From there we can use that date to say the rocks above it are younger than THAT PARTICULAR AGE and the rocks below it are older than THAT PARTICULAR AGE.So in a simplistic example:----> Igneous Rock absolute dated at 10 MA
====> Sedimentary rock
~~~~> Igneous Rock absolute dated at 20 MAEven though we can't date the sedimentary rock directly, we know it is between 10 and 20 MA.The reality is of course that there are few circumstances on earth that are that easy to sandwhich between two things that we can date absolutly. Even if we just make it 1 order more difficult.----> Igneous at 10MA
====> Sedimentary #1
~~~~> Sedimentary #2
++++> Igneous at 20MAWe can say that both Sedimentary #1 and #2 are between 10 and 20 MA but just knowing the ages of the igneous rocks does not tell us what time sedimentary #1 represents compared to sedimentary #2. That is when we have to start looking at other factors that may give us clues as to what span of time those layers represent in between the dates given by the igneous layers.

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

I am curious about this. I feel like I may not be understanding what you are looking for.There are very few things in the column that are global and even when things in the column do relate they only do so locally. A particular relationship or set of relationships might help me pin down very narrowly the age of a rock in a particular area. This may not be true for a rock layer a few above or below or a short distance away.Absolute dating give us some big lines that we can draw across a particular column where we have distinct and actual ages. But that may not work for another column where that layer which could be dated does not exist and there is no way to correlate layers in that other column to the first.In reality, when you see an age given to a non-igneous layer you are getting an evidence based estimate of the age that takes into account the sometimes quite complex geological relationships of the locality of the layer.Again the key point to take home is that the primary demarcation of the ages is based on absolute dating. There are plenty of other reason just examining relative dating situations and the content of the rocks to show that the earth is ancient, but in terms of giving a rock a birthdate there is only absolute dating. I don't have a good vocabulary of specific ACTUAL examples to deconstruct. Maybe someone can entice rox, IRH, or the likes out of hiding to point us at some good examples and let us baby geologists work them out.

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

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.

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

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. 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. 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.

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

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. 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. 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.

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

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.

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

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?

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

It may be interesting to point out that non-clastic sedimentary rocks also exhibit characteristics that attest to their status as ancient.Limestone for examples requires trillions upon trillions of sea creatures to be born, live, die and slowly sink to the bottom of the ocean.Evaporite deposits require the deposition area to be continuously flooded (oops bad word) with "salty" water and then evaporate. This will leave behind a thin film of evaporite material. Some evaporite deposits are 10s if not 100s of meters thick.These are just some examples out of many of processes required to create these rocks that, unless the laws of nature fundamentally change, cannot happen except over very long spans of time. I mentioned the cooling of an intrusion as one. The actual process of metamorphism is another. These things simply cannot happen fast. If they did, the outcome would not be the same. I'll repeat my previous analogy. You can try to bake a cake by applying the same amount of heat it would normally get over its entire bake time in a nano-second; but the outcome will most certainly not be cake.I'll also point out that although these testaments to the earth being ancient, they don't help as much as other dating techniques when trying to assign actual ages. For a given layer, you can get an idea based on this knowledge to get a rough estimate of how long it might have taken all that sand to pile up, but there is much more potential for error in those kinds of estimates.

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