How can layers of rock differ in age by millions of years?

January 28, 2005 4 Comments
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Tags: , , Earth Sciences & Geology
4 Comments to “How can layers of rock differ in age by millions of years?”
  1. michael971 says:

    It is called an unconformity.

    If a sedimentary rock is exposed and possibly eroded then millions of years later it is covered by water and more sediment deposited and turned to stone then there will be a gap in time when the rock was exposed.

  2. Alex G. says:

    Organic material is great, but Carbon dating only works for the most recent 50,000 to 75,000 years because the half-life of that is so short

    Anyway, your question highlights the reason you can only date certain types of rock. For example, sedimentary rocks are composed only of pieces of other rocks, and so are very difficult to date and are generally dated indirectly.

    Igneous rocks are easy to date because they are formed from a molten source. Minerals don’t form until the lava reaches certain temperatures cool enough for the specific minerals to form.

    The idea is that the newly formed mineral has a certain ratio of atoms. But, as that mineral, and the rock, ages, the amount of radioactive atoms will decrease as they decay, and the number of the daughter atoms it turns into will increase. But because this is all happening within a mineral, you hopefully don’t have anything leaving the system so that change in the amount of parent to daughter atoms is locked in.

    However, melting that rock will “reset” the ratios, so you can date an igneous rock even though it is formed from melting older rocks.
    Similarly, if you have minerals that get reformed during a metamorphic event that will also “reset” the ratio so you can date that event, but perhaps not the rock that originally existed.

  3. SallyC says:

    Rocks that are heated (metamorphosed) or melted will force a re-setting of the radiometric dates to the date of crystallisation of the minerals. Sedimentary rocks can’t be dated radiometrically, except to say they must be younger than any contained minerals. You can sometimes date zircons extracted from sediments, but the date you get will be the date of crystallisation (metamorphic or igneous) of the zircon, not the age of the sediment. So since sedimentation, metamorphism and igneous activity have been going on throughout geological time, layers of rock will have very different ages.

  4. bustersmycat says:

    Rock dating is based on the principal of superposition. That is, it is impossible to deposit on top of something, or break something, or intrude or melt something, unless that something already existed. There are other basic assumptions as well, of course, such as a given sequence of sediment has the same age everywhere it is found and that fossils that show changes over time display the same features only at the same time.

    This “law” allows rocks to be dated relative to each other. Rock A is older than Rock B is older than Rock C, so rock C must be older than rock A even if rock C is found where there is no rock A.

    Absolute dates are generally obtained from radiometric dating. This can only work for certain types of rocks under certain conditions, and then relative dating is used to age the rocks that are found in the inbetween dates. When there is a huge amount of information available that allows both relative and absolute dating of rocks, which is the case today after some 150 years of very detailed study by thousands upon thousands of geologists, the age relations are fairly well defined.

    The concept of “deep time”, that the earth is very old, millions upon millions of years old, was developed very early on in the study of geology. It became obvious that sedimentary rocks formed in essentially the same way that sediments are now depositing on earth, and the thick sequences of sedimentary rocks therefore could only have formed over very very long times because sediments take an extremely long time to accumulate even today.

    that is about as simple as I can make it. A lot more to it than that of course, but that is the fundamental idea.

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