Wikipedia:Reference desk/Archives/Science/2010 October 8

= October 8 =

There are mentions of cross-linked polystyrene. Rexolite is a significant industrial example. Why no mention of it?
I clicked on the "ask a question" box for newbies and it took me directly here. This doesn't seem like the right forum, since I am NOT writing an article, but here goes--

The Wikipedia article on polystyrene probably belongs to somebody and should include a reference to a major industrial product called Rexolite (also known as Rexalite). http://www.rexolite.com/list.html Certain industries use a lot of it. Wikipedia has an article on PEEK, perhaps a newer and superior material, but Rexolite has a longer history. It is so obvious by its absence. Is it banned since it's also a trade name? FWB (talk) 00:32, 8 October 2010 (UTC)


 * Welcome to Wikipedia. You can edit any article you want, unless someone added bad things to an article and then new editors cannot edit it. No one owns articles; you can edit any article. But if you add false information, then people will remove it. You can write it, see WP:BOLD! --Chemicalinterest (talk) 00:45, 8 October 2010 (UTC)


 * To FWB: Nope. Wikipedia is a work in progress. If isn't mentioned in the Wikipedia article, it is not because of any grand conspiracy. It just means no one has written about it yet. If you have access to reliable, independant information about the product (i.e. NOT the promotional material the producers of the product have written, but peer reviewed scientific journals and that sort of thing), then you can use those sources to help you improve Wikipedia.  In fact, every single word of every single article of Wikipedia was written by people exactly like you, and you have every right to, within Wikipedia's principles and guidelines, improve articles.  In fact, it is expected that you do.  Nothing gets fixed if you don't fix it.  -- Jayron  32  00:48, 8 October 2010 (UTC)


 * Why should Rexolite be mentioned? As far as I can tell, Rexolite is a brand name, not the name of "major industrial product" - the product would be cross-linked polystyrene, and that's what the article should talk about. Sometimes brand names become popular enough than an item is named after it (and then they have a problem of trademark dilution). Other times there is a patent and the product is only available under that brand name. Other than that, brand names are not really mentioned. Is Raxolite the only source of cross-linked polystyrene or do other people make it too? You may be able to add a "Major manufacturers" section to the article, but you'll need to find a number of them, and wikipedia is not really intended as a replacement for ThomasNet. And speaking of ThomasNet, I don't see Rexolite or C-LEC listed so are you sure they are major? If you are trying to get your website linked into wikipedia your best bet is to add a section with a description and properties of cross-linked polystyrene and then use this website as a reference source for it. Ariel. (talk) 01:09, 8 October 2010 (UTC)


 * To my humble experience the Rexolite is a registered trade mark widely licensed and used in the industry for RF circuit substrates with very certain dielectric, physical and chemical properties. If you tell an engineer about something "of Rexolite" he would understand everything from a single word. So the Rexolite certainly should be mentioned in Wikipedia with dedicated article.

Which one of these will change color in a Biuret Reagent test?
will cystine, proline, leucine, water and egg albumin change color with the biuret reagent test? I know Biuret Reagent turns violet in the presence of protein, but amino acids make up proteins, so will they change color? —Preceding unsigned comment added by 24.86.167.133 (talk) 00:50, 8 October 2010 (UTC)


 * See Biuret test, especially the first sentance. Now, does a single amino acid contain any peptide bonds?  If you can answer that, you will be able to answer your question.  -- Jayron  32  01:07, 8 October 2010 (UTC)

No. But how do I figure out whether cystine, proline, leucine, water and egg albumin contain peptide bonds? —Preceding unsigned comment added by 24.86.167.133 (talk) 01:14, 8 October 2010 (UTC)
 * You could start with our articles on cystine, proline, leucine, water and albumin, comparing them to our article on peptide bond. Physchim62 (talk) 01:29, 8 October 2010 (UTC)

OK, i read the articles, and these are my answers, could you please review them and suggest anything that will help if they are wrong? Cystine: yes, Proline: no, Leucine: No, Water: No, Albumin: Yes Any help would be greatly appreciated, thank you —Preceding unsigned comment added by 24.86.167.133 (talk) 01:42, 8 October 2010 (UTC)
 * Where is the peptide bond in Cystine? -- Jayron  32  02:04, 8 October 2010 (UTC)

Between the Sulfur and the hydrogen —Preceding unsigned comment added by 24.86.167.133 (talk) 02:08, 8 October 2010 (UTC)


 * Nope. Read peptide bond again.  You obviously didn't.  -- Jayron  32  02:09, 8 October 2010 (UTC)
 * There's no sulfur–hydrogen bond in cystine: the OP is making the common mistake of confusing it with cysteine. Even if there was an S–H bond, it wouldn't give a positive reaction to biuret reagent. Physchim62 (talk) 02:16, 8 October 2010 (UTC)

CO-NH —Preceding unsigned comment added by 24.86.167.133 (talk) 02:12, 8 October 2010 (UTC)

No. cystine is not because it contains a disulphide bond. Proline is because it contains a peptide bond, leucine isnt because it has some other bond (the bond is not between N and C), water is no because it has a hydrogen bond and albumin is a protein
 * Where is the peptide bond in Proline? Note that a peptide bond is NOT just any bond between C and N.  It is a very SPECIFIC kind of C-N bond, and it exists between the C=O bit and the N, not any other combination of C and N.  -- Jayron  32  02:29, 8 October 2010 (UTC)

The peptide bond does not connect the two prolines together.. i think —Preceding unsigned comment added by 24.86.167.133 (talk) 02:33, 8 October 2010 (UTC)

OK i think I have got it. Cystine, proline, leucine and water will not change color during a biuret reagent test. And Egg Albumin will. None of the amino acids here are proteins (does this mean that no amino acids are proteins?) —Preceding unsigned comment added by 24.86.167.133 (talk) 02:39, 8 October 2010 (UTC)
 * You are very close on this. You may be getting it.  Amino acids are the building blocks of proteins,  This is how it works.  Protiens or Polypeptides are built of individual amino acids, which link up via peptide bonds whereby the N on one amino acid bonds to the C=O on the next amino acid, and so on and so forth, making a chain. The Biuret test looks specifically for this peptide bond, so if you have a protein or a polypeptide, then you will get a positive result.  If you have a single amino acid, like proline, or leucine, or any of the other 20 or so amino acids, then you don't get a positive Biuret test, because a single amino acid isn't part of a peptide chain, its just a single amino acid.  The Biuret test looks specifically for chains of amino acids strung together.  It's the connections between the amino acids that the test is looking for, so while albumin, which consists of a chain of many amino acids, will give a positive test, a single amino acid will not, because it isn't part of that peptide chain!  -- Jayron  32  02:43, 8 October 2010 (UTC)

So taking all of this in consideration I can conclude that Cystine, Proline, and Leucine are single amino acids —Preceding unsigned comment added by 24.86.167.133 (talk) 02:47, 8 October 2010 (UTC)

Wait cystine is not a single amino acid, but because it is joined by a disulphide bond, it is not a protein —Preceding unsigned comment added by 24.86.167.133 (talk) 02:49, 8 October 2010 (UTC)
 * It's not a protein because the bond is different. Most bonds in proteins are peptide bonds, but you also get disulphide bonds (maybe about one or two percent of the time) the bond in cystine (NOT cysteine) is a disulphide bond, and wont react in the same way as peptide bonds. Physchim62 (talk) 03:03, 8 October 2010 (UTC)

OK i understand now, this helped a lot. Thank you very much :) —Preceding unsigned comment added by 24.86.167.133 (talk) 03:07, 8 October 2010 (UTC)
 * No problem, it is nice to know that we've helped someone! Now you've understood it, just remember that Biuret reagent tests for peptide bonds ;) Physchim62 (talk) 03:14, 8 October 2010 (UTC)

While this is a useful theoretical exercise, it's important to remember that theory is not easily applied to chemistry, and never works reliably in biology. I didn't dig for the best sources, but to quote, "Dipeptides, single amino acids (except serine and threonine), urea, and ammonia give blue solutions, which are negative tests" and in a historical source, "According to Schiff, the red coloration is caused by the formation of a copper potassium biuret compound. This reaction is given by all compounds in which two CONH2 groups are linked in the molecule to a single atom of carbon, or of nitrogen, or directly to each other... one of the CONH2 groups may be replaced by a CH2NH2 group, or a CSNH2 group.  Substitution may be made for only one of the hydrogen atoms attached to the nitrogen atom, and the CONH2 groups must be free"... the author then goes on to describe later researchers who found that this wasn't strictly true, and some compounds like urobilin without any CONH2s would react. Wnt (talk) 16:29, 8 October 2010 (UTC)

Rocket science
If a rocket in free space carries its own fuel, is there a limit to how fast it can go? Intuitively, it seems to me that if you increase the amount of fuel to ultimately go faster, that fuel will slow down the initial acceleration, so starting with more fuel eventually adds less and less to the final speed. Is that right? Bubba73 You talkin' to me? 04:05, 8 October 2010 (UTC)
 * You got it in one. That's the fundamental limit of rocketry: see reaction mass. PhGustaf (talk) 04:09, 8 October 2010 (UTC)


 * thanks. Bubba73 You talkin' to me? 04:50, 8 October 2010 (UTC)


 * Think of it this way, though. You have two rockets, identical except that one has twice the mass of fuel on board to start with as the other.  The heavier one starts up its engine and accelerates until half its fuel has been consumed -- now both rockets have the same total mass, but one rocket is already going quite fast, and still accelerating.  The second rocket starts its engine right at that moment and begins accelerating at the same rate, until both rockets exhaust their fuel (at the same time).  Which rocket is going faster when they both run out of fuel...?   Wiki Dao  &#9775;  (talk)  04:32, 8 October 2010 (UTC)


 * The one that started with more fuel will be going faster,but it won't be going twice as fast. Bubba73 You talkin' to me? 04:49, 8 October 2010 (UTC)


 * Right -- adding more fuel can get you more speed, but not as much speed per unit of fuel as the original fuel gave you. But actually it's even worse because you need larger and heavier fuel tanks to contain all the fuel, so you some of the extra fuel is spent lifting those.  (Unless you could make a solid fuel that was strong enough to retain its shape under acceleration, but no such fuel exists for rockets of any size.)  The need for these heavier tanks is the main reason for the use of multi-stage rockets, but these also have drawbacks -- more complexity and the need to carry additional rocket engines for the upper stages.  --Anonymous, 04:51 UTC, October 8, 2010.


 * ... and if you solve all of these problems, it gets even worse around 20,000 miles per second because relativistic effects begin to be noticeable and you need even more fuel than you would expect from calculations using Newtonian mechanics. (You can think of this either as extra mass, or as extra terms in kinetic energy proportional to v4, v6 etc.)    D b f i r s   07:33, 8 October 2010 (UTC)


 * You can read more in Tsiolkovsky rocket equation
 * Very roughly you need to double the initial mass for every 2 km/s deltaV if you use conventional rockets. This includes the mass of fuel tanks and engines for each stage.


 * Saturn V had a initial mass of 3 000 000 kg and could lift 120 000 kg to low earth orbit. To reach low earth orbit you need almost a deltaV of 10 km/s that is 5 times 2 km/s, this gives a expected mass-ratio of 1/32, (25=32) and the real mass-ratio was 12/300.


 * The lunar Ascent stage had a deltaV of 2.2 km/s a start mass of 4700 kg and 2353 kg fuel.--Gr8xoz (talk) 09:56, 8 October 2010 (UTC)

Highest mountains in Europe and reliable sources
Hi all. Stemming from personal interest, I've started adding sources in my home wiki to the various "List of [superlative] [geographical feature]s in Europe" articles. I know the problems with such lists: the definition of "Europe" varies from source to source and e.g. absolute lengths of rivers are impossible to measure, but that's the sources' problem. I just need to find reliable (looking) ones and state the results according to them.

However, my problem is that although reliable-looking lists of largest lakes and longest rivers / largest catchment areas made by the European Environment Agency can be found rather easily, I can't find an "official" list of highest mountains anywhere. The best that I've come up with is this at peaklist.org, but I have no idea about the reliability of the values. Of course I can/will base the articles on multiple sources, but it would be nice to know if there are better lists out there, preferably made by government agencies and not just random guys from the internet:-) Any ideas? --Albval (talk) 06:25, 8 October 2010 (UTC)


 * Peaklist seems to be the best it gets. I've used them as a source for heights in northern China for an article on the Altyn Tagh fault,, taking note of the description of their methods, which make extensive use of data from the Shuttle Radar Topography Mission and appear to be well researched. Mikenorton (talk) 22:10, 9 October 2010 (UTC)


 * Thanks for the info! Seems like their data is rather sound, then. I think I'll go forward with peaklist. Albval (talk) 16:07, 10 October 2010 (UTC)

Lizard hibernation
Do lizards hibernate in the wintertime ? I live in Cyprus and see many types of lizard ,chameleons,geckos ,and am interested in what they do .Alan Wright213.7.152.158 (talk) 13:51, 8 October 2010 (UTC)
 * AFAIK lizards don't really hibernate, at least not in any true sense. See Hibernation - it certainly doesn't list lizards as being an animal that hibernates. Hibernation tends to be most common in cold climates where food is very scarce in winter. Firstly I'm guessing Cyprus winters are fairly mild, and secondly I think it's more a case that lizards simply don't live/survive in particularly cold climates rather than hibernating. Having said which I seem to vaguely remember something about some types of lizards going into a type of suspended animation in the Australian deserts during periods of extreme scarcity, but that well may have been something else, frogs or perhaps even fish (but that is a bit of a vague distant memory). --jjron (talk) 15:35, 8 October 2010 (UTC)
 * Lizards do well all the way to Arctic Circle. They just dig into the ground or find some other shelter and "freeze" (supercool, not really freeze) with it. And then they thaw and crawl out into the sun. See How do reptiles survive Canadian winters?. East of Borschov 16:27, 8 October 2010 (UTC)

CM1 is to DMEM as swing is to roundabout?
Is CM1 a cell culture medium in the same way as DMEM is a cell culture medium? Seans Potato Business 15:11, 8 October 2010 (UTC)


 * Can you give us the reference/PMID where you found this acronym used? In (for example) this paper it appears to just be used as a convenience acronym for conditioned medium (day one) in an experiment where they were using RPMI, but CM1 doesn't seem to be a widely-accepted term for any specific medium formulation.  TenOfAllTrades(talk) 15:44, 8 October 2010 (UTC)

Seiche
I'm having trouble understanding the concept of a seiche. I think this stems from not really understanding a standing wave. How exactly is it that a wave remains stationary and yet travels across a lake or some other body of water to produce events such as what happened at Vajont Dam? Given that a seiche (if my understanding is correct) is a standing wave, and standing waves (again if my understanding is correct) remains stationary, how is that a seiche moves from one end of the lake to the other, overtops the dam, and causes damage downstream? To me, just from what I'm grasping, this sounds much more like a tsunami...so also what's the difference between a seiche and a tsunami in such events as happened at the dam? Ks0stm (T•C•G) 17:09, 8 October 2010 (UTC)
 * My understanding of a seiche would not cover the wave generated by the landslide in the Vajont Dam reservoir, much more of a local tsunami, something associated with many historical landslides such as the Chungar rock avalanche in Peru or the Goldau rockslide in Switzerland. I think that the Vajont Dam page needs to be updated. Mikenorton (talk) 17:27, 8 October 2010 (UTC)


 * A standing wave still involves changes in water level — the point is only that you can't say which way the wave is going, because every so often the whole wave is flat for a moment (but the water keeps moving, making it un-flat again). Your confusion may be because standing waves are typically shown (like a jumprope) with nodes at either end, whereas here the standing wave must have a node near the middle of the lake.  Either edge of the lake has the water going sharply up and down. Wnt (talk) 23:43, 8 October 2010 (UTC)


 * I wasn't suggesting that seiches couldn't overtop a dam, that has been known to happen, just not in the case of Vajont - the water there was displaced by 260 million cubic meters of rock. Seiches are caused by resonance of a body of water that has been 'excited' most commonly by seismic waves, or wind. Mikenorton (talk) 08:47, 9 October 2010 (UTC)

Accuracy of Gas Regulators
I live in a community that has natural gas lights ignited 24 hours a day. Utility company provide the gas and we are billed based on meter usage. I notice that same billing periods reflect different gas consumption and consequent different charges. Gas is supposedly regulated and I wonder why the difference in reading. Facts are gas is on twenty four hours a day, the meter is theoretically regulated. The difference in reading is 20CCF and 29CCF for a 29 day period. To me this is a significant error. Why?? It would seem that regulators would be temperature compensated, but that's the only thing that I can think of. Help!! —Preceding unsigned comment added by 68.226.8.62 (talk) 18:22, 8 October 2010 (UTC)
 * Are you sure the lights are not set to slightly different levels? It would be very easy to do, and I don't think you would notice the difference. Ariel. (talk) 19:02, 8 October 2010 (UTC)
 * Have you contacted the gas company about this? There could be a billing error or idiosyncrasy; there could be a technical error; they may adjust the flow based on conditions; the regulator might only be accurate to ~ 50%; you might be billed for some other gas-use besides the light that fluctuates in gas consumption; there could even be a mysterious 9 CCF gas leak that somebody should have a look at and fix.  The utility company ought to be able to answer for it.  Nimur (talk) 00:03, 9 October 2010 (UTC)


 * Check the date on which the meter was read. It should be on your bill. Perhaps the meter readings are not exactly 29 days apart each time.--Srleffler (talk) 06:49, 12 October 2010 (UTC)

Painting a GarageFloor
Help........... 6 years ago I bought a house in Scotland with no garage, so I had a pre-fabricated concrete model built on my land with a 9 inch base of poured concrete. After the requisite amount of time to allow it to cure, I swept off all the dust and sealed the floor with a proprietary PVA Glue (diluted as per instruction on the container). I then painted it with good quality garage - floor paint (after the recommended time had elapsed to allow the PVA to dry in. And after 6 years, we had the worst winter in living memory, and large sections of the floor paint peeled off. This year, during our relatively poor Summer, I scraped off all the loose paint, wire-brushed the bare and surrounding areas, swept and vacuumed the debris away, re-sealed the floor with dilute PVA Glue, and then re-painted the whole garage floor, which now looks wonderful. I even left the car in the drive for a week to give the paint a chance to really dry. And then went on holiday for a week to give the floor an even better chance. And when we came home, we parked the car in the garage overnight, before going shopping next day. And the floor? 4 patches of paint had lifted where the tyres had been parked - down to concrete level. So what did I do wrong when the initial similar procedure had lasted 6 years? Any tips or advice will be greatly and humbly appreciated. Thanks. 92.30.139.42 (talk) 18:51, 8 October 2010 (UTC)
 * Epoxy floor paint sites make reference to resistance to "hot tire lift" or "hot tire pickup." If you came back from holiday, your tires were probably hot. What was the paint - latex, urethane, epoxy ... ?  Acroterion  (talk)  19:01, 8 October 2010 (UTC)


 * Just thinking aloud. First time around the cement could have been very dry because it was still hydrating. Now, it is fully cured and has had time to soak up water from below. So second application went on to a damp surface. Did you add a water proofing agent to the mix? Also I would use PVA to seal against dust but   EVA if it was going to be in a damp environment. The very cold weather suggests the moisture froze under the paint.   The only long term solution I can suggest is move south of the border where the climate is  warmer. --Aspro (talk) 19:18, 8 October 2010 (UTC)


 * Moisture is wicking up from the earth under the concrete, so the paint peels off because the surface is not perfectly dry. 92.29.123.232 (talk) 11:39, 9 October 2010 (UTC)


 * Many thanks to all above. I had entertained the suspicion that this current year's poor and wet weather here in not-sunny Scotland might have been instrumental in preventing the over-winter dampness from properly drying out, but did wait until we had at least a few consecutive dry days in August before commencing the repair and re-painting work - but it seems that to properly dry out the concrete between the earth subsoil and the concrete itself I should have taken Aspro's advice and moved not only my wife and myself further South - but also my garage. But thanks again. I am going to leave the floor alone until next year but as an aside, how do you feel about me putting a rubber car mat on the floor where the tyres come to rest (for cosmetic purposes only). Thanks. 92.30.155.122 (talk) 14:07, 9 October 2010 (UTC)
 * The rubber mats would trap moisture underneath, so they would make things worse. The moisture building up under the paint is making it peel off. Without the paint, the moisture would just diffuse into the air and the surface would stay dry/drier. If you must paint it, you need a permeable paint that lets moisture through. I'm not sure if masonary paint is permiable. 92.24.189.189 (talk) 11:49, 11 October 2010 (UTC)
 * Rather than rubber mats (after all this is not Brazil) I would have thought it more culturally appropriate  to lay down a couple of haggis skin rugs (have checked  the database of endangered species and they are not on it). However, should you keep their heads attached by copying  the tiger-skin-rug format, I would seriously recommend extracting all their teeth to avoid  any chance  of puncturing your tyres, for I hear: they can bite through Wellingtons as easily as if  they where made from liquorish. No wonder that the Romans, with their open toed sandals, never ventured that far north.--Aspro (talk) 19:33, 10 October 2010 (UTC)


 * AhAh - now not many people know this - so keep it to yourself - the ONLY way to eat Haggis and avoid it biting back - is to pour a glass of malt whisky over it first. Not only does that vastly improve the mouth-feel and taste experience, it also acts as an anaesthetic on the Haggis, so you can complete the meal before it recovers. The ancient Scots knew this to be true but they wisely didn't tell the Romans - so I am imploring you to keep it a closely guarded secret too. Sshhh. 92.30.211.79 (talk) 23:08, 11 October 2010 (UTC)

is empirical electronegativity strictly transitive?
Theoretically we assign atoms a bunch of transitive numbers, but in the electronegativity table we don't define the electronegativity of a bond, just the electronegativity of a participant. I feel that unlike a redox potential, electronegativity isn't a strictly transitive property.

The C-H bond is nonpolar and totally non-acidic. The C-S bond is basically nonpolar. But the S-H bond can weakly hydrogen bond (much weaker than water's of course) -- but the high boiling point of ethanethiol (35C! that's higher than HF!) means the S-H bond must be decently polar -- and afterall, thiols have a lower pKa than water.

For example, I believe that in the context of comparing ethylamine and ethanethiol, sulfur seems to be more electronegative than nitrogen in withdrawing electron density from hydrogen, even though a nitrogen-sulfur bond would be polarised in the direction of nitrogen. John Riemann Soong (talk) 19:15, 8 October 2010 (UTC)


 * The C-H bond is not nonpolar, it just that as a whole, hydrocarbons are nonpolar due to the fact that the C-H bond polarities cancel, much like they do in CO2. If you do simple trigonometry, you can see that the tetrahedral bond angle provides that, for something like methane, the bond polarities exactly cancel, and for all larger alkanes, the same basic mathematics also applies.  This also explains why the H is not terribly acidic, the effect of these cancelations in bond polarities tends to make the C-H bond more non-polar than the absolute electronegativities would tell you.  If you look at molecules where there is only a single C-H bond on a carbon atom, especially terminal alkyne, the C-H is quite acidic, and can be readily removed by strong bases such as sodamide.  Electronegativity is perfectly transitive, its just that electronegativity is not the only property to look at when figuring out bond polarities and acidity like this, molecular geometry also plays a big role.  The nitrogen-sulfur paradox you note is also due to differences in molecular geometry; in amines, the two hyrogens partially cancel out their polarities.  However, these problems with electronegatity that you note are still real problems, which is why chemists can't really agree on how to even calculate it.  See Electronegativity.  These all assume electronegativity to be transitive, they just don't all agree on what absolute values the electronegatity to have.  These sorts of problems are encountered with just about any atomic property, consider all of the different ways to define Atomic radius.  -- Jayron  32  03:04, 9 October 2010 (UTC)
 * No, electronegativity is not perfectly transitive. It's not perfectly very much at all really! There are several ways to calculate electronegativity because there are many physical properties which follow similar periodic relationships, and it's convenient to conceptualize these with a concept of the electron-withdrawing ability of an atom. But, if one were being strict, we would say that the tabulated values are mean electronegativities, and that the elctronegativity of each (non symmetry related) atom in each molecule is different. There are a few simple rules to expand the standard table a bit. For carbon, an sp-hybridized atom is more electronegative than an sp2-hybridized atom, which in turn is more electronegative than sp3: this is why you can easily deprotonate a terminal alkyne but not an alkane. Another general rule is that the electronegativity of an atom will increase with its oxidation state: you can even calculate that it's about 0.1–0.2 points on the Pauling scale per oxidation number, but it's quite rare to actually bother with calculating a number, you just have to remember that it happens. Chlorine dioxide, for example, has remarkably similar properties to ozone – why? because chlorine in the +4 oxidation state has an electronegativity that's almost exactly the same as oxygen. You can find similar examples all across the Periodic Table. Physchim62 (talk) 11:24, 9 October 2010 (UTC)

river rock

 * Pebbles on beach at Broulee -NSW -Australia-2Jan2009.jpg

I am what might be called a rock hound and I have become very interested in river rock. Most river rock appears to be igneous and metamorphic (very small percent). I presume sedimentary rock is so soft that it has all been eroded to dust by the action of erosion in the rivers. Many of the smaller rock (less than 6") seems to rounded and flattened, somewhat like a pancake. Occasionally one finds an almost perfectly round rock with flattened but slightly convex sides. One never finds a ball shaped rock. My question is why are there so many pancake shaped rocks and no ball shaped rocks. I have a theory and would appreciate your comment. I think these rocks are originally created by a volcanic cannon. I think varying sized chunks of molten lava are thrown  in the air. They would be spinning and therefore take on a globular shape. Then when they fall back to earth they would flatten into a pancake shape. But if that were true one would expect one side to be much flatter than the opposite side. Perhaps if the earth is covered by several inches of volcanic dust, this would cushion the impact so that the impacted side would not flatten completely and thus would remain somewhat convex. Certainly erosions removes angularity from the rocks and polishes them. I think, however, it is unlikely that erosion alone can account for the rocks being frequently "pancaked" and never globular. Yoiur comments will be appreciated. WSC —Preceding unsigned comment added by 66.27.177.127 (talk) 22:34, 8 October 2010 (UTC)
 * Your theory is inconsistent with the way that most geologists think river rocks form. First of all, mudstone and similar conglomerates are very common in rivers, and these are sedimentary rocks.  Regarding the "pancake" vs. "ball" shape, I think it's reasonable to say that erosive forces in a river will be acting from a preferential direction - the direction of water flow - so it makes sense that the rocks will be eroded anisotropically.  Spherical rocks would tend to form if the water flows so rapidly that the rocks constantly bounce around in all directions; but for the most part, the action of erosion is slow.  On top of this, the "grain" of the rock, because it is sedimentary, will take the form of horizontal layers, which also affects the erosion, favoring flat-shaped rocks.  As far as volcanic ejecta - well, if that were the case, these rocks would have clear geochemical evidence that they were volcanic in origin, which is not generally the case.  (Depending where you are, of course), most of the rocks in a river or streambed are not igneous, let alone volcanic.  Nimur (talk) 23:01, 8 October 2010 (UTC)
 * I posted the picture of beach pebbles. There is not a spherical one in sight. (OR) I think that a loose pebble is eroded on its underside by the gravel it is dragged over by the water current. The current is not fierce enough to roll it all the time so it settles among the other pebbles in a particular orientation and gets a flat bottom. An initially spherical pebble that has had a flat eroded on it has two stable resting positions: flat down or flat up. As all the pebbles lose weight their relative packing changes occasionally, which can tip a pebble from one position to the other. Hence the common pancake eroded shape.Cuddlyable3 (talk) 23:21, 8 October 2010 (UTC)
 * It all depends on the precise type of rock, but to give an example, in Pennsylvania one sees rivers filled with rounded, flattened stones of various pretty pastel shades of shale and fine-grained sandstone. It's easy enough to see why on the surrounding banks — the rocks flake apart in flattened triangles and quadrangles.  Because the natural grain defines one dimension in which the rocks come apart easily, it's hard to picture them ever taking on a rounded shape from random erosion.  In the picture shown above, I think you can see three or four pebbles in the process of splitting along this weak axis.  I think that the grain is also evidenced by the calcite veins in several of the rocks, which are perpendicular to the grain.  (This is not inconsistent with the idea above that a flat surface is perpetuated by erosion, since the stones start out this way, though I don't know it's true) Wnt (talk) 23:37, 8 October 2010 (UTC)


 * I think there are three different explanations:
 * A sphere are a specific shape, there are many more ways a pebble can be flat than spherical. If hight, with and length are to some degree independent random variables then it is unlikely that they should be the same.
 * A spherical pebble that got a flat spot will be more likely to lie on that side and since movements in the water is mainly horizontal most abrasion will be on the surfaces facing up and down. And similarly a pebble grinded between two larger stones will often become flatter.
 * Most rocks are anisotropic and split easier in one direction than in other directions. This affect both the initial formation when they break off from the rock and the abrasion processes.
 * --Gr8xoz (talk) 00:13, 9 October 2010 (UTC)


 * Some pages on this general topic: Roundness (geology), Spheroidal weathering, Abrasion (geology), Sediment transport, Bed load, Shingle beach, Erosion. Not all of these are great pages, but some are quite detailed and contain links to other pages that might be of interest. Erosion processes differ between rivers and beaches, and between different kinds of rivers and shorelines too. Where I live, near Seattle, there's lots of till, created by glaciers and ice sheets. Whenever I dig a hole in the yard I find many many cobble-sized stones. They are usually more sphere-like, or egg-shaped at least, than flat. As I understand it, the whole lowland region around Puget Sound is a Drumlin field. There's a number of glacier-fed rivers around here that flow through classic U-shaped valleys, recently formed by large glaciers. The beds of these rivers are choked with rock debris, most dramatically near the source glaciers, but the "choked" effect can continue many many miles downstream and into the lowlands; the Nisqually River and the White River, for example (those pages have photos showing the choked riverbed). I think the stones found in rivers around here are likely to be glacial till. The rivers wash away the soil and collect the till stones in their beds. So the shape of the stones shouldn't be explained by river action alone. For more photos of river and beach pebbles, cobbles, etc, there's a number of categories at The Commons, like commons:Category:Gravel. Here's a nice photo of beach pebbles ("shingle", or a shingle beach). Looks much like many of the beaches where I live. Lots of flat stones, but plenty that are more "egg shaped". Perfect spheres are of course rare, as are "perfectly flat" stones (you know, ideal skipping stones). Pfly (talk) 02:50, 9 October 2010 (UTC)