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Thread: Question about geologic column

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    But how can it be different ages in the different strata if it was formed billions of years before the different strata was even formed?

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    New Member Minnemooseus's Avatar
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    Oversimplified version.

    You don't radiometric date sedimentary rocks. You do date igneous rocks. Sedimentary rocks are made of fragments of igneous rocks of various ages.

    When an igneous rock crystallizes from a magma, the radiometric clock is set to zero. The radiometric daughter elements (those formed from radioactive decay) are excluded from the initial crystal structure because of chemistry reasons. But the new daughter elements are trapped in the crystals once the igneous rock is solid.

    The age of a sedimentary rock (let's call it a sandstone) is the time that it was deposited, the earliest at the bottom, getting younger as you go up. The sandstone is made of of fragments of various aged older rocks, and thus a radiometric dating of the sandstone will be some pretty meaningless date prior to the deposition. About all you can say is that the sandstone is younger than that date.

    An old sandstone will be made up of old fragments and will whole rock radiometric date some average of those fragments ages. A younger sandstone will be made up of possibly also younger fragment, and would tend to whole rock radiometric date younger.

    I gotta go and get something else done.

    Moose

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    Okay. That makes sense now that I reflect on it.

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    Contributor barbos's Avatar
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    Quote Originally Posted by BH View Post
    If all the uranium came from a supernova several billion years ago why does it show different ages in the various rock strata.
    They measure how much time has passed since rock was molten. When rock solidifies tiny crystals of pure uranium are grown inside rock. After a while some of the uranium inside crystals decays into other elements but they stay inside the crystals. By measuring amount of these elements you can measure the age of the crystals and rock containing it. Asteroids are dated like that too.

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    Veteran Member Wiploc's Avatar
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    Quote Originally Posted by barbos View Post
    Quote Originally Posted by BH View Post
    If all the uranium came from a supernova several billion years ago why does it show different ages in the various rock strata.
    They measure how much time has passed since rock was molten. When rock solidifies tiny crystals of pure uranium are grown inside rock. After a while some of the uranium inside crystals decays into other elements but they stay inside the crystals. By measuring amount of these elements you can measure the age of the crystals and rock containing it. Asteroids are dated like that too.
    Thanks. That helps.

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    Super Moderator Bronzeage's Avatar
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    The oldest principle of modern geology is "The rocks at the bottom of the pile are older than the rocks at the top of the pile. The second oldest principle is "Sometimes the pile is on its side and sometimes it's upside down." What we can observe of the Earth's geology is such a very thin layer and what we see actually gives very little clue as to how it got to be what it is.

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    Sapere aude Politesse's Avatar
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    Quote Originally Posted by Bronzeage View Post
    The oldest principle of modern geology is "The rocks at the bottom of the pile are older than the rocks at the top of the pile. The second oldest principle is "Sometimes the pile is on its side and sometimes it's upside down." What we can observe of the Earth's geology is such a very thin layer and what we see actually gives very little clue as to how it got to be what it is.
    Steno's Laws.

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    Contributor Cheerful Charlie's Avatar
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    Quote Originally Posted by BH View Post
    If all the uranium came from a supernova several billion years ago why does it show different ages in the various rock strata.
    Because when Uranium is trapped in a mineral, as it decays, it's daughter elements remain there. Generally. Some will actually decay themselves yielding other daughter elements. Knowing how these elements decay gives us a good measuring stick. Sometimes some minerals trap daughter elements in very stable ways. And are very tough and don't easily change their composition. Zircon for example. So grains of zircon may become part of sedimentary minerals. Such rocks then must be younger than the zircon minerals. Thus volcanic material can be used to date sedimentary materials trapped between them. When zircons form, they stop the clock and start radioactive decay in a consistent manner, trapping their daughter elements that do not leach out.

    Historical geology is intensely logical and has built up large catalogs of techniques for dating geological assemblages. Including being aware of how one can be mislead if one is not careful.
    Cheerful Charlie

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    Administrator lpetrich's Avatar
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    The relative ages of the rocks were determined long before radiometric dating became practical. Those ages were determined with the help of 17th-cy geologist Nicholas Steno's principles of stratigraphy (Steno's Principles of Stratigraphy)
    • Superposition: In a sequence of strata, any stratum is younger than the sequence of strata on which it rests, and is older than the strata that rest upon it.
      "...at the time when any given stratum was being formed, all the matter resting upon it was fluid, and, therefore, at the time when the lower stratum was being formed, none of the upper strata existed." Steno, 1669.
    • Initial Horizontality: Strata are deposited horizontally and then deformed to various attitudes later.
      "Strata either perpendicular to the horizon or inclined to the horizon were at one time parallel to the horizon." Steno, 1669.
    • Stratum Continuity: Strata can be assumed to have continued laterally far from where they presently end.
      "Material forming any stratum were continuous over the surface of the Earth unless some other solid bodies stood in the way." Steno, 1669
    • Cross Cutting: Things that cross-cut layers probably postdate them.
      "If a body or discontinuity cuts across a stratum, it must have formed after that stratum." Steno, 1669\

    He also stated:
    1. "If a solid body is enclosed on all sides by another solid body, of the two bodies that one first became hard which, in the mutual contact, expresses on its own surface the properties of the other surface."
    2. "If a solid substance is in every other way like another solid substance, not only as regards the conditions of surface, but also as regards the inner arrangement of parts and particles, it will also be like it as regards the manner and place of production . . ."
    3. "If a solid body has been produced according to the laws of nature, it has been produced from a fluid."

    Steno considered some odd structures called glossopetrae or tongue stones that were present in some rocks of Tuscany in Italy (roughly west central Italy). He noticed that they had a strong resemblance to sharks' teeth, and he concluded that they were the teeth of long-ago sharks that got buried in sediment - sediment that got turned into rock and then pushed upward.

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    Administrator lpetrich's Avatar
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    With these principles of stratigraphy, one could work out which order the Earth's crust's rocks had been deposited in, and that was done long before radiometric dating became feasible. Before that, geologists had used estimates from sedimentation rates and the like - much less precise.

    The first division of strata was by 18th cy. Italian geologist Giovanni Arduino, who divided the rocks of the southern Alps into Primary, Secondary, Tertiary, and Quaternary ones -- 1, 2, 3, 4. But British geologists of the early 19th cy. went into much more detail, naming many of the stratigraphic divisions that we use today. Primary and Secondary were eventually dropped, though Tertiary and Quaternary survived. I'll summarize our present-day division. The divisions have this hierarchy:

    Supereon, eon, era, period, epoch, age, stage

    Here they are:

    Supereon, eon:
    • Precambrian
      • Hadean
      • Archean
      • Proterozoic
    • (Phanerozoic)
      • Phanerozoic


    Phanerozoic Eon: era, period:
    • Paleozoic
      • Cambrian
      • Ordovician
      • Silurian
      • Devonian
      • Carboniferous
      • Permian
    • Mesozoic
      • Triassic
      • Jurassic
      • Cretaceous
    • Cenozoic
      • Paleogene
      • Neogene
      • Quaternary

    Paleogene + Neogene = Tertiary

    Cenozoic Era: period, epoch
    • Paleogene
      • Paleocene
      • Eocene
      • Oligocene
    • Neogene
      • Miocene
      • Pliocene
    • Quaternary
      • Pleistocene
      • Holocene

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