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How did the geologic column form/develop? (old-earth)

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  • How did the geologic column form/develop? (old-earth)

    I'm trying to find out what is the (old-earth) theory for how all the geologic column (all the layers/strata) came to be. When I try to use google, the results seem to consist entirely of links to YEC sites, and I'd like to hear the old-earth theory from its advocates.

    I assume that the story is a complex one, with different processes occurring at different times and places. But I'm hoping someone can give me an introduction or an overall idea.

    I seem to have several fundamental questions I'm trying to understand. Like is it possible to give me an idea regarding where these layers are? It seems unlikely that the layers are uniform around the globe, because that would seem to imply that the layers were build up by adding new mass (extraterrestrial?) to the earth. I'm assuming the processes primarily involve matter repeatedly taken from some places on earth and piled up repeatedly over time in other places on earth. So I'm curious what places (or kinds of places?) on Earth was the matter taken from and what places where layers built up over time?

    Another question is what makes them distinct layers? That is, presumably if the process were perfectly uniform over the whole time, there would not be identifiable layers at all but one uniform mass? So what kinds of changes over time creates various layers of different kinds?

    I've gathered that there are some layers created by lava or volcanic ash. But it seems that this is not the majority of the bulk of the strata?

    A few places I've found seem to indicate that the sediments were all deposited under water. That is, that multiple times the sea level rose and covered major portions of the continents (or entire continents?), and then receded. The idea I gathered is that sediments were deposited while under the ocean, and then tended to be weathered down during times when exposed (when the oceans were receded). Is this correct? If so, then I have additional questions: Wouldn't that mean that the layers do not represent a complete chronology of time because they would represent only those durations of time when the land was covered by the ocean (and even then the top layers of each of those sequences may be missing because they were weathered away when exposed)? And if the sediments were only (or primarily) deposited when the land was covered by the ocean, then how did all those land animal fossils get in there?

    Directing me to resources is helpful but even better if someone can explain any of these things to me themselves. Thanks.

  • #2
    Sounds like what you really need is an introductory text on geology, especially one with a good description of the rock cycle. I poked around a bit on line and found this at the Idaho State U. web site. Looks like a good starting place for the basics.

    Basics of Geology

    Read up on the rock cycle and on stratigraphic columns in particular.

    HTH

    ETA: The U. California Museum of Paleontology has a good overview of geology and stratigraphic columns too

    UCMP Geology Wing
    Last edited by HMS_Beagle; 02-24-2014, 07:09 PM.

    Comment


    • #3
      Originally posted by Joel View Post
      I'm trying to find out what is the (old-earth) theory for how all the geologic column (all the layers/strata) came to be. When I try to use google, the results seem to consist entirely of links to YEC sites, and I'd like to hear the old-earth theory from its advocates.
      Joel, just an introductory text on this issue would, literally, take a book. You're talking about a multi-billion-year process that, in turn, is made from several other processes, some local, some regional, some worldwide. Just to list a basic glossary would take multiple posts.

      HMS has pointed you to a good online source, and I would recommend starting there and coming back here with any questions you may have.

      Comment


      • #4
        Than's for the links. All very interesting. But most of my questions in the OP are still unanswered. And in the second link, I'm mostly just seeing conclusions about what life existed in different eras. Maybe I'm missing it, but I'm not finding information there about how sediments were deposited.

        In the meantime I've also tried doing some looking around on Wikipedia, and read about some things like unconformities, and their different kinds:
        http://en.wikipedia.org/wiki/Lithostratigraphy

        It occurred to me that it might help to start answering my questions (and/or motivate discussion) if I were to learn about a concrete example, so I searched and found this page on the geologic column at the Grand Canyon: http://en.wikipedia.org/wiki/Geology...nd_Canyon_area

        I have not read through it all yet. I've just read about the first two 'groups', and thought I'd summarize my understanding of what it says so far (and someone correct me if I'm understanding incorrectly).

        2.5-1.8by ago: A large amount of western North America was under the ocean building up sediments.
        1.8-1.6by ago: Plate tectonics lifted this land up above the ocean.
        <1.75by ago: This land was buried under rock 12 miles deep.
        1.75-1.73by ago: Being buried under so deep, the pressure turned the ocean sediments into metamorphic rock. Later, lava intruded into this metamorphic rock.
        1.73-1.2by ago: Erosion completely removes the 12 miles of rock and some top parts of the metamorphic rock. Sometime in this time, plates moving apart stretch and thin the inner North America making it a basin that becomes an enormous sea covering perhaps half of North America.
        1.2b-740my ago: Sediments build up on the bottom of this sea, forming the "Grand Canyon Supergroup" of layers of sedimentary rock.
        800my ago: Plate tectonics tilted the Supergroup and pushed up mountains (presumably up out of the inland sea?)
        800-525my ago: These mountains (and any additional deposits) eroded away to small hills, in some places all they way down to the basement metamorphic rock.
        525my ago: the next extant sediment layers (the start of Group 3) were deposited.

        Does this seem right so far?

        How is it known about the burial under 12 miles of rock (and its erosion)? Where did all that 12 miles of rock (covering presumably most of western North America?--that's a lot of matter!) come from and how did it get deposited there? (in a relatively short amount of time) Where did it go? Into the ocean? Does any of it remain elsewhere in North America?

        Where did all the Supergroup sediments come from?

        This does answer one of my OP questions. The column in this location has gaps in time (1.8by-1.2by and 800-525my) due to alternating periods of deposition and erosion. And, for another of my questions, so far all the sediments were deposited under water (except for the missing 12miles of rock, which burial it says happened after being lifted up out of the ocean).

        For convenience, I've summarized the remaining unanswered questions from the OP, below:

        - Where are these layers (and where not)? What places (or kinds of places?) on Earth was the matter taken from and what places where layers built up over time?
        - What kinds of changes over time creates various sediment layers of different kinds?
        - Lava or volcanic ash is not the majority of the bulk of the strata?
        - All/most sediments were deposited under water?
        - And if the sediments were only (or primarily) deposited when the land was covered by the ocean, then how did all those land animal fossils get in there?

        Comment


        • #5
          Originally posted by Joel View Post
          Than's for the links. All very interesting. But most of my questions in the OP are still unanswered. And in the second link, I'm mostly just seeing conclusions about what life existed in different eras. Maybe I'm missing it, but I'm not finding information there about how sediments were deposited.
          That's fine. Joel, I doubt there could be any single website on the web, or even a good college-level book, that would answer all of your questions.

          2.5-1.8by ago: A large amount of western North America was under the ocean building up sediments.
          1.8-1.6by ago: Plate tectonics lifted this land up above the ocean.
          <1.75by ago: This land was buried under rock 12 miles deep.
          1.75-1.73by ago: Being buried under so deep, the pressure turned the ocean sediments into metamorphic rock. Later, lava intruded into this metamorphic rock.
          1.73-1.2by ago: Erosion completely removes the 12 miles of rock and some top parts of the metamorphic rock. Sometime in this time, plates moving apart stretch and thin the inner North America making it a basin that becomes an enormous sea covering perhaps half of North America.
          1.2b-740my ago: Sediments build up on the bottom of this sea, forming the "Grand Canyon Supergroup" of layers of sedimentary rock.
          800my ago: Plate tectonics tilted the Supergroup and pushed up mountains (presumably up out of the inland sea?)
          800-525my ago: These mountains (and any additional deposits) eroded away to small hills, in some places all they way down to the basement metamorphic rock.
          525my ago: the next extant sediment layers (the start of Group 3) were deposited.
          Without going through the text item by item, that looks like a decent summary.

          How is it known about the burial under 12 miles of rock (and its erosion)?
          It takes a certain depth of rock to induce one type of metamorphosis. I don't know the details, but I would recommend looking at this article for an introductory discussion.

          Where did all that 12 miles of rock (covering presumably most of western North America?--that's a lot of matter!) come from and how did it get deposited there? (in a relatively short amount of time)
          If I'm reading the article correctly, this was an area of the continental plate that was subducted under an unstable area of the North American Craton.

          And it should be noted that this was not a "relatively short amount of time." You're talking 50 million to 500 million years, depending on what part of the plate you're talking about.

          Where did it go? Into the ocean? Does any of it remain elsewhere in North America?
          Some of it was doubtless eroded into the ocean.

          Where did all the Supergroup sediments come from?
          It says in the article:

          Source: Wikipedia: Geology of the Grand Canyon Area

          In late Precambrian time, extension from a large tectonic plate or smaller plates moving away from Laurentia thinned its continental crust, forming large rift basins that would ultimately fail to split the continent.[5] Eventually, this sunken region of Laurentia was flooded with a shallow seaway that extended from at least present-day Lake Superior to Glacier National Park in Montana to the Grand Canyon and the Uinta Mountains.[2] The resulting Grand Canyon Supergroup of sedimentary units is composed of nine varied geologic formations that were laid down from 1.2 billion and 740 million years ago in this sea.[10] Good exposures of the supergroup can be seen in eastern Grand Canyon in the Inner Gorge and from Desert View, Lipan Point and Moran point.

          © Copyright Original Source



          The sediments that formed the Supergroup eroded from the areas of Laurentia that surrounded this shallow inland seaway.

          And, for another of my questions, so far all the sediments were deposited under water (except for the missing 12miles of rock, which burial it says happened after being lifted up out of the ocean).
          Sediment is only one of several broad categories of how a layer can be added, There are several others, ranging from lava flows and over-run (a nearby area subducts the area in question) to igneous intrusion in a buried layer, to tectonic uplift. You can even have wind-blown sediment, loess, that if subsequently buried can metamorphosize into a layer of rock.

          - Where are these layers (and where not)? What places (or kinds of places?) on Earth was the matter taken from and what places where layers built up over time?
          Very complex area, since your question covers information for the entire planet. However, the Wikipedia article on Geologic time scale provides a condensed explanation. There are also several links from that article that discuss specific time periods.

          - What kinds of changes over time creates various sediment layers of different kinds?
          Erosion runoff from a different base substrate.

          - Lava or volcanic ash is not the majority of the bulk of the strata?
          Nope. Volcanoes are relatively localized phenomena.

          - All/most sediments were deposited under water?
          Sediments are. But remember, sediment is not the only way to add material.

          - And if the sediments were only (or primarily) deposited when the land was covered by the ocean, then how did all those land animal fossils get in there?
          Actually, that's an interesting point. Many of the land animal fossils that we see in fossil layers lived in the shallows, laid their eggs in riverine mud, and spent most or all of their lives wading. This is about the only way you can get a fossil at all: if the body is not covered relatively quickly, scavengers can scatter the bones or even break them for marrow, and bacteria and insect activity will destroy bones as well as soft tissue. However, if the body is under water (especially if it is in a high-sediment area), the carcass is covered relatively quickly, and many of the scavengers that would attack an exposed carcass can't get to the body that is under water.

          But that's bonus trivia. There will NOT be a quiz.
          Last edited by Outis; 02-25-2014, 08:47 PM.

          Comment


          • #6
            Joel I can't speak for anyone else but I'm not inclined to answer a dozen questions sprayed out shotgun style. As Outis pointed out plate tectonics makes the Earth's crust a very dynamic place and the geology of any given area can be quite complicated. Strata can be buried miles deep, exposed, then buried miles deep again many times. Sediments come from the erosion of metamorphic, igneous, or other sedimentary rocks. Sediment layers can be deposited by water, or wind, or through chemical precipitation. They can also be organic like the chalk from shells in the White cliffs of Dover.

            If you're really interested come back and provide a nice description of the rock cycle in your own words so I can see you understand it. Do that and then we can tackle specific questions one at a time.

            Comment


            • #7
              Prefacing this with the fact that I'm nowhere close to a geologist and so welcome corrections to anything below.

              Sticking with the Grand Canyon, many of the layers above the schist were deposited in marine conditions (like most limestones) but many were semi-aquatic or dry (like some sandstones). What we see in the Grand Canyon stratigraphic column is not a series of deposits that were laid down only in a marine environment but rather periods of time when marine gave way to shallow ocean, which gave way to shoreline, dry land and then back through the progression to marine. Looking at the larger picture, the Grand Staircase, shows that the exposed sediment in the Grand Canyon was actually hidden under many more layers of sediment prior to uplift and erosion.

              So, to the question of how different layers form, the answer is a mixture of long-term environmental changes due to plate tectonics and mid-term environmental changes regarding the local geography.

              Lava does make up most of the lithosphere, though it's not necessarily the area of focus here. Most of the lithosphere is igneous rock, which is magma that has been cooled. In plate tectonics, magma from the earth's mantle flows through a fault, depositing lava on the ocean's surface and pushing the plates on either side of fault away from the zone. Over time, these areas that were at the bottom of the ocean push upward onto the continental shelves. By then, they are considerably denser than they were when deposited and have become the bedrock of various strata that are deposited in the ocean. This process continues on land as the plates continue moving. Eventually, the plates collide and can form subduction zones, where one plate is pushed underneath the other's bedrock and into the mantle again.

              For the most part, the matter that comprises the various strata come from earth: material is constantly being recycled from the mantle onto the lithosphere and back again into the mantle. It all changes form, of course, as rocks are ground into finer particles like sand . . . but it's virtually all terrestrial at this point. On the earth, material is constantly being moved around — by wind, by erosion, by ocean — and, as I believe you said, accumulation in one place generally corresponds with erosion in others.


              —Sam
              "I wonder about the trees. / Why do we wish to bear / Forever the noise of these / More than another noise / So close to our dwelling place?" — Robert Frost, "The Sound of Trees"

              Comment


              • #8
                Originally posted by HMS_Beagle View Post
                Joel I can't speak for anyone else but I'm not inclined to answer a dozen questions sprayed out shotgun style.
                Sorry if I'm asking too many questions. Some I'm just putting down as I think of them so I don't forget them.
                You have been very helpful so far, thank you.

                If you're really interested come back and provide a nice description of the rock cycle in your own words so I can see you understand it. Do that and then we can tackle specific questions one at a time.
                Okay, here goes:

                Igneous rocks are formed by lava (melted rock) cooling to a solid (above ground or under ground).

                Any rock (igneous, metamorphic, or sedimentary) can weather, and the pieces can be deposited somewhere else. These sediments can then combine into rock (lithify) by compaction or cementation (e.g., by calcium carbonate).

                Any rock under heat and pressure can be chemically altered, and we call the result metamorphic rock.

                Any rock can be pushed deep underground via plate tectonics, where it might melt to lava.

                Originally posted by Outis View Post
                If I'm reading the article correctly, this was an area of the continental plate that was subducted under an unstable area of the North American Craton.

                And it should be noted that this was not a "relatively short amount of time." You're talking 50 million to 500 million years, depending on what part of the plate you're talking about.
                You know better than I. My only basis of comparison was comparing to the other periods of sedimentation that I listed, and that the whole column of the Grand Canyon seems to be about 1 mile and is a couple billion years worth of accumulated layers.

                Subduction makes sense. I assume subduction under an already-existing 12-mile thick plate of rock is typically faster burial than, say, being buried by 12 miles of sediments from wind erosion.

                I suppose someone doing stratigraphy has to keep a watch for evidence of subduction, because that can cause older strata to end up on top of younger strata.

                The sediments that formed the Supergroup eroded from the areas of Laurentia that surrounded this shallow inland seaway.
                That kind of process would tend to level things out over time, as the seaway rose and the surrounding areas eroded, right?
                Unless some other process (volcanoes or plate tectonics) continued to raise the surrounding areas at the same time.


                So, I have a new thought: areas (such as in the area around the Grand Canyon) where we find sedimentary strata from erosion from elsewhere have, on average, accumulated more sedimentary matter on the surface than has been eroded away (otherwise we would see no sedimentary strata there). That would seem to imply that there are other places on the earth where there has, on average, been more erosion of sediments than sedimentary deposition, and so we should expect such areas to have no sedimentary strata, right? (I'm keeping in mind, however, that things can get pushed up from underneath too. And layers can be created by lava. For example, I could imagine a volcanic mountain as a spot that grows because it accumulates layers from lava faster than they are eroded away, but at the same time does not on average accumulate sediments from elsewhere (so we don't find sedimentary strata on the volcano), whereas a nearby basin might, on average, accumulate sediments eroded from that volcanic mountain.)

                I'm curious if there exists some map of the world showing these different areas (showing which areas where there exists accumulated sedimentary strata, and which areas have none (but may themselves be built up by lava flows or being pushed upward from underneath)?

                Comment


                • #9
                  Originally posted by Joel View Post
                  Subduction makes sense. I assume subduction under an already-existing 12-mile thick plate of rock is typically faster burial than, say, being buried by 12 miles of sediments from wind erosion.
                  It would certainly make more sense to me. Subduction can leave some odd results, such as folding and corrigating. Fortunately, I'm given to understand that subduction is relatively easy to detect because of the chemical changes and physical stresses upon the rock. Not my area of expertise, I fear, so my explanation is quite vague.

                  I suppose someone doing stratigraphy has to keep a watch for evidence of subduction, because that can cause older strata to end up on top of younger strata.
                  Yup. And as you noted in discussions before, sedimentary layers cannot be directly dated radiometrically.

                  That kind of process would tend to level things out over time, as the seaway rose and the surrounding areas eroded, right?
                  Unless some other process (volcanoes or plate tectonics) continued to raise the surrounding areas at the same time.
                  It could--provided that all else remained stable. The one thing you can count on, however, is that over that span of time, "stable" is at best a joke.

                  You mentioned one possible process (plate tectonics), and one that is probably only possible on a local scale (volcanoes). Sediment layers can be lain down, then eroded away again--usually not over the entire layer, but rather through a narrower area through the middle, at least at first. Like I said before, it can get complicated, but unless an area is completely eroded away, there's evidence of what has gone on.

                  So, I have a new thought: areas (such as in the area around the Grand Canyon) where we find sedimentary strata from erosion from elsewhere have, on average, accumulated more sedimentary matter on the surface than has been eroded away (otherwise we would see no sedimentary strata there). That would seem to imply that there are other places on the earth where there has, on average, been more erosion of sediments than sedimentary deposition, and so we should expect such areas to have no sedimentary strata, right? (I'm keeping in mind, however, that things can get pushed up from underneath too. And layers can be created by lava. For example, I could imagine a volcanic mountain as a spot that grows because it accumulates layers from lava faster than they are eroded away, but at the same time does not on average accumulate sediments from elsewhere (so we don't find sedimentary strata on the volcano), whereas a nearby basin might, on average, accumulate sediments eroded from that volcanic mountain.)
                  Quite possible.

                  I'm curious if there exists some map of the world showing these different areas (showing which areas where there exists accumulated sedimentary strata, and which areas have none (but may themselves be built up by lava flows or being pushed upward from underneath)?
                  To be useful, such maps are done not on a continent-wide scale, but on a county-wide scale. Oh, some regions can be fairly simple, but for others there is an admixture of rock types that may measure no more than a few feet across to other strata that may be hundreds of miles wide.

                  Now, the USGS has a database of maps here that may be useful, but I do not know what level they go to, nor what the access terms are (I think it's free, but beyond that, I don't know much about it).

                  Comment


                  • #10
                    Originally posted by Joel View Post
                    Sorry if I'm asking too many questions. Some I'm just putting down as I think of them so I don't forget them.
                    You have been very helpful so far, thank you.
                    No worries and you're quite welcome. Sometimes you learn more if you sip the knowledge rather than being fire-hosed.

                    I suppose someone doing stratigraphy has to keep a watch for evidence of subduction, because that can cause older strata to end up on top of younger strata.
                    Yes. Here is one famous example, the Lewis Overthrust.

                    That kind of process would tend to level things out over time, as the seaway rose and the surrounding areas eroded, right?
                    Unless some other process (volcanoes or plate tectonics) continued to raise the surrounding areas at the same time.
                    That's what happens. For example, the entire Colorado Plateau which includes the Grand Canyon is still being slowly uplifted by plate tectonic forces.

                    So, I have a new thought: areas (such as in the area around the Grand Canyon) where we find sedimentary strata from erosion from elsewhere have, on average, accumulated more sedimentary matter on the surface than has been eroded away (otherwise we would see no sedimentary strata there). That would seem to imply that there are other places on the earth where there has, on average, been more erosion of sediments than sedimentary deposition, and so we should expect such areas to have no sedimentary strata, right? (I'm keeping in mind, however, that things can get pushed up from underneath too. And layers can be created by lava. For example, I could imagine a volcanic mountain as a spot that grows because it accumulates layers from lava faster than they are eroded away, but at the same time does not on average accumulate sediments from elsewhere (so we don't find sedimentary strata on the volcano), whereas a nearby basin might, on average, accumulate sediments eroded from that volcanic mountain.)
                    You can find almost any sort of geologic formation you want somewhere in the world. One example of an area with very little sedimentation is the Canadian Shield, an area of very ancient igneous and metamorphic rock in the Eastern part of Canada.

                    The Earth's geologic history is very diverse (and very interesting!) There is no one-size-fits-all answer.

                    Comment


                    • #11
                      There are two points that are important for the layman to understand the rocks in the geologic column: (1) all the older different strata in the geologic column can be found in different places on the earth being deposited and forming today in sediments eroding from uplands, marine environments forming limestones, wind blown sand and silt, organic deposits forming muck, peat and coal, igneous extrusion/intrusion under mountain ranges and in the sea floor, and volcanic deposits. (2) Most of the sedimentary strata form in cycles of sandstone, (coal), shales, limestone, and evidence of these cycles can be seen in the young sediments forming today.
                      Glendower: I can call spirits from the vasty deep.
                      Hotspur: Why, so can I, or so can any man;
                      But will they come when you do call for them? Shakespeare’s Henry IV, Part 1, Act III:

                      go with the flow the river knows . . .

                      Frank

                      I do not know, therefore everything is in pencil.

                      Comment


                      • #12
                        Originally posted by Outis View Post
                        And as you noted in discussions before, sedimentary layers cannot be directly dated radiometrically.
                        Not quite true, although dating igneous and metamorphic rock is much more common. It is sometimes possible to date the "cement" that binds the grains. E.g. SHRIMP Uranium-Lead Dating of Diagenetic Xenotime in Siliciclastic Sedimentary Rocks (full article available with free registration).

                        Comment


                        • #13
                          Originally posted by JOnF View Post
                          Not quite true, although dating igneous and metamorphic rock is much more common. It is sometimes possible to date the "cement" that binds the grains. E.g. SHRIMP Uranium-Lead Dating of Diagenetic Xenotime in Siliciclastic Sedimentary Rocks (full article available with free registration).
                          Ah! Thank you for the clarification.

                          Comment


                          • #14
                            Originally posted by JOnF View Post
                            Not quite true, although dating igneous and metamorphic rock is much more common. It is sometimes possible to date the "cement" that binds the grains. E.g. SHRIMP Uranium-Lead Dating of Diagenetic Xenotime in Siliciclastic Sedimentary Rocks (full article available with free registration).
                            The grains within rocks can often be dated. For the most ancient zircons date the oldest known rocks as ~4.4 billion years.

                            http://www.cnn.com/2014/02/24/world/...arth-fragment/
                            Glendower: I can call spirits from the vasty deep.
                            Hotspur: Why, so can I, or so can any man;
                            But will they come when you do call for them? Shakespeare’s Henry IV, Part 1, Act III:

                            go with the flow the river knows . . .

                            Frank

                            I do not know, therefore everything is in pencil.

                            Comment


                            • #15
                              Originally posted by shunyadragon View Post
                              The grains within rocks can often be dated.
                              That only dates the specific grain, not the sedimentary stratum that the grain is found in.

                              Comment

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