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Why are pebbles round?

  1. Apr 22, 2004 #1
    This seems to be a very dumb question. :biggrin: Just google and you'll find something like:

    So the answer seems to be polishing by water. So why bother us with this thread?

    Because so many things in geology are totally different.
    Why are pebbles rounded?
  2. jcsd
  3. Apr 22, 2004 #2
    Human definition. Think about it, you walk around the beach, look at the nice round pebble, but when you see something square or whatever, thats not a pebble, thats a "rock".
  4. Apr 22, 2004 #3
    so it is simply human classification. A pebble is round, a rock is square like. no matter the size or anything.

    Go here, which is the rock, and which is the pebble?
  5. Apr 22, 2004 #4


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    The pebbles become rounded, or rather they become polished and smoothed due to not just water but the tumbling effect, similar to the tumbling that is done to polish gem stones.

    The original shape of the stone will dictate the shape of the pebble, with the more exposed surfaces receiving the most abrasion first.
  6. Apr 22, 2004 #5
    Remarkable observation, Atrius. yet, I'm afraid it doesn't help explaining why pebbles are round.

    We could assume that pebbles start as "rocks". could we? Then what happenes. Let's make it mulptiple choice, to narrow down the possibilities.

    Pebbles are rounded in the course of many million years because they:

    a - are polished by water
    b - are sand polished by winds
    c - are formed in earthquake faults
    d - grow that way
    e - a combination of a. and b. is true
  7. Apr 22, 2004 #6
    Sorry Evo I missed your post. Good suggestion, lets substitute the most dumb answer with your suggestion:

    Pebbles are rounded in the course of many million years because they:

    a - are polished by water
    b - are sand polished by winds
    c - are tumbling and expose different sides to the erosive effects
    d - grow that way
    e - a combination of a. and b. and c. is true
  8. Apr 22, 2004 #7
    Last edited: Apr 22, 2004
  9. Apr 22, 2004 #8
  10. Apr 23, 2004 #9
    Very impressive, rock queen :smile:

    Unfortunately the pebble that I found contained no treasure. But it holds a clue.
  11. Apr 24, 2004 #10
    Now this pebble. That weary pencil (can't affort new ones) indicates that the pebble is about an 3/4 inch in diameter. We see clearly grow rings following the outline of the stone exactly, where as polishing and wearing out could not have any effect on the interior of the stone. So at least for this pebble:

    Now looking at Atrius post, do we call more irregular shaped stones that are clearly shaped by erosion pebbles or rox? Do all pebbles (defined perhaps as highly regular rounded and symmetric boulders/stones/rocks) grow that way? How do they grow anyway?

    I guess the lesson is:
    Take nothing for granted even seemingly very simple things like eroding stones. Observe and analyse and keep wondering about what is going on.
    It's always different.
  12. Apr 26, 2004 #11
    Yes that is a very sorry looking pencil. I hope you have a pencil sharpener for the poor thing.

    As to your pebble, it looks ovoid rather than spherical. And it does look like the inside formed first and then another layer was added and cooled and then another layer was added.. Well molten rock is very hot. I think if you get a certain mixture of molten rock, perhaps the minerals will separate and maybe form these layers but of course I havent observed this in axion.
  13. Apr 26, 2004 #12


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    But how can you tell whether it formed from the inside out ("grew") or the outside in? If it stated life as a bubble in viscous rock mass which cooled quickly, and later had lots of nice, hot, mineral-laden water flow through it, 'depositing' lovely crystals as the temparature cooled, wouldn't it also end up as round, with more-or-less concentric layers?
  14. Apr 26, 2004 #13
    If it starts as a bubble and forms a nodule, it can't have water running through it. It will be sealed up. The crystals may precipitate out and grow, as in the case of the geodes...

    I think it would depend on the composition. Which minerals and what temperatures and what rates... because we see André's pebble with concentric rings, but there are also the hollow geodes and the filled thunder eggs of Australia.
  15. Apr 27, 2004 #14


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    See: "oolite(s); and, stromatolites."
  16. Apr 27, 2004 #15
    But his is only the beginning. Look how stones can really grow: MOERAKI BOULDERS

    and perhaps do the Costarican Stone spheres have a similar story to tell, instead of being man made.
  17. Apr 28, 2004 #16
    Thunder eggs are interesting.
    "Thunder Eggs
    Thunder eggs are round agate-filled nodules which form in some types of silica-rich volcanic rocks eg. rhyolites. As the molten volcanic lava cooled, steam and other gases trapped in the lava formed an expanding bubble. Silica minerals often crystallise around the bubble or grow crystal fibres which radiate outwards from the its centre. These mineral-filled bubbles with a radiating structure are called spherulites.

    Sometimes gas pressure can forced the spherulite apart to form a central hollow which is later filled with more minerals. Later drying out, shrinkage and cracking of silica gels or clays filling the cavity can produced the star-shaped pattern typical of many thunder eggs.

    Later, silica-rich solutions may enter the cavity and fill it with banded agate, chalcedony, clear quartz crystals or amethyst. Solutions of different composition seep in at various times, leaving behind several layers of different minerals. Well-known localities for Australian thunder eggs are Mt. Hay, Eumundi, Agate Creek and Mt. Tamborine, Queensland"

    Yes Bystander, see also "concretions"
    Last edited: Apr 28, 2004
  18. Jun 20, 2004 #17
  19. Jul 21, 2004 #18
    Hmm the story about those moeraki boulders sounds a bit like the story of (andre whats the translation in english of Mangaan knollen?) *srry, can't come up with the proper translation*.
  20. Jul 21, 2004 #19
    Good point Marijn,

    how about Manganese nodules

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