I just found a video on sedimentary rock formation, and to my surprise, it stated that this only could occur in water! I always thought that any layer of matter, if buried deep underneath more matter, could form a sediment and become new rock. Am I completely wrong, or was the video simply not covering all bases?

I did Google it and check Wikipedia, but with my limited knowledge of the field, the results were inconclusive.

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    $\begingroup$ For starters, aeolian (wind blown) sandstones are common. The video is not covering all bases. $\endgroup$
    – Siv
    Apr 23 '16 at 18:30
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    $\begingroup$ Could you post a link to the video? Possibly it was stated that this specific type of sedimentary rock could have been only formed/shaped under water? Sorry for sounding pedantic but do you mean by 'to wiki' that you looked it up in one or several Wikis on that topic or do you mean Wikipedia? $\endgroup$ Apr 23 '16 at 21:04
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    $\begingroup$ Oh, on Wikipedia. I have not yet found a good wiki for earth science, I am all up for suggestions. And I looked over the video again, it is more an oddity of the script that makes it seem as if he is talking about all the sedimentation, when he is only talking about the evaporative / precipative sedimentation. Goes to show the value of clear language (and attention, I guess). I think it was this one: youtube.com/watch?v=ZkHp_nnU9DY $\endgroup$ Apr 23 '16 at 22:22

Not all sediments are deposited in water, but water is important in the formation of most sedimentary rocks.

If we're just thinking about the deposition of the sediment, then we don't necessarily need water. Some counterexamples are:

  • Aeolian sandstones, such as the Lower Permian Rotliegend sandstone of the North Sea. These are deposited by wind, not water.
  • Some types of sedimentary breccia, which are chiefly deposited by gravity, not water.
  • Tuff, which are deposited by gravity and wind, not water. They also undergo substantial compaction and lithification with or without water.

But deposited sediment does not a rock make. Once deposited and if buried, most sediment undergoes compaction and diagenesis, eventually lithifying — a fancy word for turning into a rock. Below the water table, the shallow crust is saturated with (mostly) saline water, and processes like dissolution and cementation are necessarily aqueous. So it's fair to say that water is essential in the formation of sedimentary rocks, on Earth anyway.


You were right to be skeptical, by the way; the video is introductory material apparently intended for grade school audience, so you can't treat it like a textbook. And you can't even take a textbook as 'truth', especially when it comes to slippery things like definitions. Sometimes generalizations and simplifications help, sometimes they don't.

  • $\begingroup$ I just want to underline that "No sedimentary rocks without water" is nonsense. Whilst I agree that water is by far the dominant agent of sedimentation, one can think of many exceptions. What about waterless Mars, which is largely covered in sediment in various stages of becoming sedimentary rock. Yes, there was some water in the past but not enough to account for the existing sand seas. $\endgroup$ Apr 26 '16 at 9:49
  • $\begingroup$ Gordon, I tried to allow for some edge cases in the less-than-emphatic language of the first sentence. But sand (sediment) is not sandstone (rock), so I stand by my generalisations, at least as far as terrestrial geology goes. $\endgroup$
    – kwinkunks
    Apr 26 '16 at 10:30

Because there are only three types of rock; namely sedimentary, igneous and metamorphic, all rock materials we observe must fall into one of these categories. For example, when a planet is forming, the process by which dust accumulates through gravitational accretion must be considered sedimentary. Eventually, this material will be compressed and may become igneous as the gravitational pressure increases, but they are indeed sedimentary layers of dust and larger particulate matter. This material is not igneous, until it reaches sufficient temperatures to melt, and is not metamorphic until an outside force is applied to cause deformation of the stratified layers.


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