What causes the round shape of pebbles?

[Andre]

This seems to be a very dumb question. Just google and you'll find http://www.newscientist.com/lastword/article.jsp?id=lw618

While walking along a beach recently, I noticed that most of the pebbles, whatever their size, tend to have an ellipsoidal shape, often egg-shaped with two small axes of similar length, and sometimes flat, with all of the axes different. ... I would expect all pebbles to tend to the spherical, the shape which takes the least energy to roll and to which particles will tend as their "corners" are eroded. Or are all pebbles still on their way to becoming spherical?
...
On entering the sea, each type will undergo the same processes, turning the first type into elliptical forms and the second type into flatter forms. Those starting as ellipses will become more elliptical.
...
The mechanism causing this is essentially a single force-the sea-pushing individual pebbles over the top of more pebbles. The steady state of a rotating body will be about its major axis, so that the moment of inertia is minimised.
...
We have smooth, deep, cylindrical potholes in the sandstone beds of local mountain streams. These produce smooth, well-rounded stones, sometimes startlingly spherical. In contrast, stones in open stream beds take on smoothed irregular shapes.

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?

 

[Atrius]

Human definition. Think about it, you walk around the beach, look at the nice round pebble, but when you see something square or whatever, that's not a pebble, that's a "rock".

 

[Atrius]

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?
http://library.thinkquest.org/3461/media/rock.gif

 

[Evo]

This seems to be a very dumb question. Just google and you'll find http://www.newscientist.com/lastword/article.jsp?id=lw618



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?

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.

 

[Andre]

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

 

[Andre]

Sorry Evo I missed your post. Good suggestion, let's 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

 

[NileQueen]

Oh, I found a very large cracked pebble...

http://antwrp.gsfc.nasa.gov/apod/ap021028.html

nearly 50 km across in fact, in Mauritania (below Morocco) in the Sahara desert

http://www.solarviews.com/huge/earth/richat.jpg
This picture shows tremendous detail...

 

[NileQueen]

And while we're at it, what about the pebbles known as geodes, with treasure within?
http://www.desertusa.com/magjan98/jan_pap/du_rock_geode.html

 

[Andre]

Very impressive, rock queen

Unfortunately the http://home.wanadoo.nl/bijkerk/pebble.jpg that I found contained no treasure. But it holds a clue.

 

[Andre]

I do not know what I may appear to the world, but to myself I seem to have been only like a boy playing on the sea-shore, and diverting myself in finding a smoother pebble or a prettier shell than ordinary, whilst the great ocean lay all undiscovered before me.

Sir Isaac Newton,

Now this http://home.wanadoo.nl/bijkerk/pebble.jpg . 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:

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
X d - grow that way
e - a combination of a. and b. and c. is true

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.

 

[NileQueen]

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 haven't observed this in axion.

 

[Nereid]

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?

 

[NileQueen]

Nereid:

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,

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...

'depositing' lovely crystals as the temparature cooled, wouldn't it also end up as round, with more-or-less concentric layers?

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.

 

[Bystander]

See: "oolite(s); and, stromatolites."

 

[Andre]

But his is only the beginning. Look how stones can really grow: http://www.pacificislandtravel.com/new_zealand/about_destin/dunedin/mouraki.asp

The Moeraki Boulders, geological curiosities each weighing several tonnes, and measuring up to 4 metres in circumference, lie on Hampden Beach much as if a giant, weary of his play, has walked away from a game of bowls - so much so that the early whalers named them The Ninepins and the location, Vulcan's Foundry. The spherical, grey, septarian boulders were formed on the sea floor about 60 million years ago by the gradual accumulation of lime salts around a small centre. Their shape is thus caused by chemistry and not by wave action.

and perhaps do the Costarican Stone spheres have a similar story to tell, instead of being man made.

 

[NileQueen]

Thunder eggs are interesting.
http://www.amonline.net.au/factsheets/geodes.htm
"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"

 

[NileQueen]

I found a nice addition for this thread--
http://www.utah.edu/unews/releases/04/jun/marsmarbles.html

The pictures show quite a collection of 'blueberries'...amazing.

 

[Marijn]

Hmm the story about those moeraki boulders sounds a bit like the story of (andre what's the translation in english of Mangaan knollen?) *srry, can't come up with the proper translation*.

 

[Andre]

Good point Marijn,

how about Manganese nodules

Manganese nodules are pebbles or stones about the size of walnuts that are built of onionlike layers of manganese and iron oxides. Minor constituents include copper, nickel, and cobalt, making the nodules a potential ore of these valuable elements. Mining of manganese nodules has been the subject of study and experimentation since the 1950s (see below Economic aspects of the oceans: Sources of minerals and other raw materials).