June 24

The continents of Africa, Australia, and South America are fairly convex in shape with little in the way of inland seas, peninsulas, and islands compared to North America and Eurasia (or Antarctica below the ice). Is there any geophysical causal explanation for this, or, at least, any geophysical terms for it outside of chance? Thanks. μηδείς (talk) 02:26, 24 June 2013 (UTC)[reply]

If you mean

convex in the sense of polygon or a convex hull (or any of the related mathematical definitions of "convex"), then every continent is non-convex. For one counter-example, the whole of West Africa juts westward, forming the Gulf of Guinea; Africa is non-convex (a straight line - or a geodesic great circle path - from Accra to Luanda crosses water). There are thousands of other easy examples of the non-convexity of these continents. So, it seems you're using the phrase "convex" more loosely and subjectively, and therefore there probably isn't a geological reason why you perceive the shapes as you do. Some other factors contributing to your perception might include the map projections that heavily distort geographic scale. If you're interested in scale-length conundrums - in other words, how large or small a coastal feature needs to be before it is "negligible," you might read coastline paradox to see some approaches that geographers use to deal with the fractal-like nature of geographic shapes. Nimur (talk) 06:15, 24 June 2013 (UTC)[reply

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I strongly suspect that glacial activity is largely to blame for the intricate shape of the shorelines of Northern Eurasia, Nunavut, and Antarctica. --Dr Dima (talk) 06:33, 24 June 2013 (UTC)[reply]

We have a nice little animation showing how the continents have evolved since

Pangea. The good fit between South America and Africa was part of the genesis of the Pangea theory. SemanticMantis (talk) 19:38, 24 June 2013 (UTC)[reply

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There is such a thing as convexity, and perhaps it can be measured [1] (though I didn't read that last source, and my prejudice is that moving-windows are indicative of a less than theoretical approach) Yet ... I would expect the net convexity of a continent to be, well, 360 degrees, no more, no less, right? (if that's how you even measure it) My feeling is there is a valid question here, if but we could grasp it. Wnt (talk) 21:59, 24 June 2013 (UTC)[reply]

Obviously I am using the term convexity relatively. (To point out that Africa has an armpit where Brazil used to be, and so is not a perfect circle, strikes me as laughably pedantic.) Australia, South America, and Africa are obviously not round, but they also obviously have coasts with a lower fractal dimension than do North America and Eurasia. Dr. Dima seems to be on to something with glaciation at least being relevant in the case of the Canadian north coast--although I don't understand the mechanism of those islands' creation. μηδείς (talk) 22:18, 24 June 2013 (UTC)[reply]

Wnt, convexity is a property of a set or a function; convexity is not a scalar quantity; convexity is not measured in degrees. One boundary-curve is not more or less convex than another. Evidently, the OP is attempting to describe some subjective or heuristic property of these shapes, but is not using the correct terminology.

For example, we could say that

fjords have more structure than bights at short length-scale. This has nothing to do with whether either coastal feature is convex. I would also avoid guessing which coastline or continent obviously has lower fractal dimension, because that is not a parameter that is easy to estimate by visual inspection. Nimur (talk) 22:37, 24 June 2013 (UTC)[reply

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I'm sorry if my answer struck you as "laughably pedantic", but I have no way of knowing what you, or any other posters or readers, may know. You were indeed using "convex" in a way that is unfamiliar to me, and in contrast with the definitinos that I do know. I also agree with Nimur's objection above, there is nothing obvious about the claim that Africa's coastline has lower fractal dimension than NA. I also agree with Wnt that there might be an interesting question here, but it will be very challenging to specify exactly what geometric or topographic feature you want to get at. SemanticMantis (talk) 00:40, 25 June 2013 (UTC)[reply]

Probably there are as many answers to the question as there are ways that the features in question are formed. In terms of peninsulas, the Delmarva Peninsula was formed from deposits from the flooding of the Susquehanna River at the end of the last ice age (not likely to happen with the Congo River, but the Amazon?), Florida began as a piece of Africa that went with North America when the two continents separated (maybe just chance that a piece of NA didn't split off and go with Africa?), and Kamchatka appears to have been formed by volcanism. I believe the west coast and southern tip of South America have quite a few islands (and fjords), and glaciation works as an explanation there as well. Apparently there was an

inland sea formed by the Amazon about 15 million years ago. I'm not sure of the exact definition of an inland sea, but Lake Makgadikgadi (now the Okavango basin) might count and I expect the Great Rift Valley will be one in the future.--Wikimedes (talk) 02:44, 25 June 2013 (UTC)[reply

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I am really interested in features at a larger scale than river deltas--but your points are take. μηδείς (talk) 03:25, 25 June 2013 (UTC)[reply]

Thinking about this a little further, it does seem relevant that Africa was at the center of Pangaea, and hence its relatively smooth coastlines were formed by rifting that proceeds along the relatively straight mid-ocean ridges, the same applying for the opposing coasts of the continents that moved away from Africa. The relatively smooth west coast of South America seems to be the converse, a long line of subduction in which any detail has basically disappeared under the Andes. And Australia is actually the exception that proves the rule, since New Guinea is actually integral to it, and the very complex geography to its north is due to mountain folding and volcanism, which doesn't currently exist on, say, the east coast of South America. Pangaea seems to be trhe relevant factor. μηδείς (talk) 03:26, 25 June 2013 (UTC)[reply]

Coast is indeed turning out to be an interesting article. μηδείς (talk) 04:33, 25 June 2013 (UTC)[reply]

This is an interesting question. For Africa I wonder if it is in part due to the continent being largely a plateau (or a series of plateaus) without a lot of low lying coastal plains. [2], Africa as a whole is a vast plateau of ancient rocks. Also our article Geography of Africa (I note the map shown under Geography of Africa#Plateau region is strange as it shows higher elevations as green and lower as brown—opposite from the way elevation maps are normally done). I also wonder how plate tectonics relates. Plate boundaries are involved with things like the Gulf of California, the Caribbean Sea, as well as a lot of the complexities along the eastern edge of Asia—and of course the Mediterranean Sea the Red Sea, both of which involve Africa. But I am just speculating. Pfly (talk) 07:09, 25 June 2013 (UTC)[reply]

(ec)Emergent coastlines tend to be smoother than submergent coastlines. On a submergent coastline hills and valleys formed above sea-level become drowned and thus the coastline becomes more convoluted. On an emergent coastline the beach used to be below seal-level where there is no riverine erosion to form complex hill and valley topography. Roger (Dodger67) (talk) 07:23, 25 June 2013 (UTC)[reply]

I took this on a trip a few days ago, and don't know what it's called. If anyone can identify it from the picture and rename the file, that would be great. →Σσς. (Sigma) 07:33, 24 June 2013 (UTC)[reply]

It's a male

Widow Skimmer, Libellula luctuosa. A male dragonfly, not a male widow skimmer :) --Dr Dima (talk) 07:56, 24 June 2013 (UTC)[reply

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This book (while stating that B³⁺ isn't known) gives an estimated value for the ionic radius of the 4-coordinate B³⁺ ion. It gives a negative value(?!) What is that supposed to mean? Double sharp (talk) 13:22, 24 June 2013 (UTC)[reply]

There is no such thing as a generic tetracoordinate B³⁺. Not as in it hasn't been detected yet, but it doesn't make sense to refer to one. 'Tetracoordinate' means that three ligands are bonded to the boron centre, and changing the ligand will change the hypothetical radius of the ligated ion. Plasmic Physics (talk) 13:49, 24 June 2013 (UTC)[reply]

Lucky I didn't take and use their ionic radius info, then... Double sharp (talk) 14:33, 24 June 2013 (UTC)[reply]

Based on the pattern, maybe the negative-sign is a typo (should be 25 not –25)? But we can't speculate like that, especially since the value itself is said to be estimated without further reference. As Plasmic Physics notes,

Tetracoordinate would be four ligands not three. DMacks (talk) 16:55, 24 June 2013 (UTC)[reply

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Sorry about that, I crossed my thoughts. I meant four, not three. Plasmic Physics (talk) 23:32, 24 June 2013 (UTC)[reply]

(ec) I'm not sure why the website the OP references says the B³⁺ is unknown, as this is not the case. I expect that the negative sign somehow migrated from the row below, but however it got there, -25pm is a typo and not some new type of matter or a mathematical formalism carried too far. Inorganic Chemistry by Shriver an Atkins (p.25) gives 12pm (actually 0.12A) as the ionic radius for 4-coordinate B³⁺, citing Shannon, Acta Crystallogr. A32, 751 (1976). Advanced Inorganic Chemistry by Cotton and Wilkinson (p.45) gives 20pm as the radius estimated by both Goldschmidt and Pauling (no reference given), but this is for 6-coordinate B³⁺, which seems unrealistic (having said that, I'm sure someone will come up with an example of a 6-coordinate boron ion). 20pm is also given in the General Chemistry textbooks by Pauling (p.175, again mentioning the 6-coordinate rocksalt structure) and McQuarrie and Rock (p.253). I'm a solid state chemist by education, and would go with the Shannon radius unless I knew that the boron ion in question was a strange 6-coordinate species. Defending my choice on a Wikipedia article talk page against someone who supported 20pm with the other 3 sources might take some work however (including checking to make sure that 12pm wasn't a typo in Shriver-Atkins or the original Shannon paper).--Wikimedes (talk) 23:45, 24 June 2013 (UTC)[reply]

Can

Well, our article says "excess water in cool conditions may lead to rot". The average minimum cold-season temperature requirement (12C according to our article) might also be a problem in some Mediterranean climate zones. Where are you exactly? Looie496 (talk) 15:14, 24 June 2013 (UTC)[reply]

I am arguing over this with a friend who claims that the species can survive a winter in Dalmatia, Herzegovina and Montenegro, which I doubt. I too looked at our article first, but it's hardly helpful; there are very few cacti (and indeed very few plants in general) that would not have a rot problem due to "excess water in cool conditions". If the cactus cannot stand a temperature approaching 0 C (no matter how briefly), then it surely cannot survive in the said regions. Surtsicna (talk) 15:44, 24 June 2013 (UTC)[reply]

Having lived in Tucson, I can tell you that these cacti can definitely stand brief exposure to 0 C, or even significantly below that. There is no type of barrel cactus that can survive a long-lasting hard freeze, though. It seems very dubious that they could survive in the harsh mountainous conditions of the areas you mention. Looie496 (talk) 17:19, 24 June 2013 (UTC)[reply]

Those aren't entirely harsh mountainous areas. There's the Neretva valley, the seaside, and the urban areas in general. Frost is very rare there, but does happen sometimes. It even snows once in a few years. If the plant could survive brief frost combined with heavy rainfall or a few snowflakes (as opuntias and rebutias do), he might be right. Surtsicna (talk) 22:08, 24 June 2013 (UTC)[reply]

I am unfamiliar with Dalmatian and Montenegran geography and climate, however, I'm fairly confident that this cactus will grow in some places that have the mediterranean climate type. This page [3] says it is "hardy, to 10 F, at least." Also, usually managed gardens are a little warmer and more protected than the surrounding countryside (e.g. urban heat island, garden wall, etc). So often, plants can survive in gardens, even in locales where they might die in the wild. I've seen these growing all over California, including in the mediterranean zone. All the other sources I see indicate hardiness well below 0 C, and seem to agree that 10-20 F is fine [4], [5]. Also I believe larger, mature plants will be hardier. By the way, the key term that got me these results was "hardiness." SemanticMantis (talk) 16:21, 24 June 2013 (UTC)[reply]

Bear in mind that an "average minimum cold-season temperature" will refer to an average of both day and night temperatures over the season in question. Being mostly (here I generalise somewhat) "hot" desert plants, many (gen.) cacti can stand "brief" (i.e. night-time) temperatures of zero or below because many (gen.) "hot" deserts, being cloudless, sometimes-to-often experience such night-time temperatures. {The poster previously known as 87.81.230.195} 212.95.237.92 (talk) 17:08, 24 June 2013 (UTC)[reply]

Thanks! English is not my first language, so I found the term hardiness a bit misleading when I first encountered it. You both seem to agree with my friend, then? Surtsicna (talk) 22:21, 24 June 2013 (UTC)[reply]

Yes, I think your friend was right, that this plant could be kept in a garden in e.g. Dalmatia, and successfully overwinter. I don't think doing so would always' succeed though! I'd be much more confident at lower elevations, in a city, with a garden wall. Remember, microclimate can be very important in these cases. I must admit I have not personally grown that cactus in that climate, but thanks for the interesting question! SemanticMantis (talk) 00:07, 25 June 2013 (UTC)[reply]

I have learned that beach nourishment can cause the temperature of the beach sand to change,distorting the sex ratio of sea turtle populations (e.g cooler sand causes more embryonic sea turtles to become male). If you have an unbalanced ratio of females to males, would this cause the population of sea turtles in an area to decline?99.146.124.35 (talk) 17:17, 24 June 2013 (UTC) — Preceding unsigned comment added by 99.146.124.35 (talk)

Yes, and it is actually occurring now with other species as well. Plasmic Physics (talk) 23:29, 24 June 2013 (UTC)[reply]

You may be right about either claim, but ^{[citation needed]} SemanticMantis (talk) 00:24, 25 June 2013 (UTC)[reply]

Unclear. I don't have time to dig into the academic literature, but searching google scholar for /sex ratio sea turtle/ would be informative, especially with the Latin name. In general the

selective pressures that a given species experiences. Turtles have evolved to reproduce viable populations, in conditions with variable temperatures. So in a sense, they are very used to shifting sex ratios. Actually, the temperature-dependence of sex gives them some ability to control sex ratio by behavior, and that could potentially help them adapt to changing conditions. Still, sea turtles are broadly in decline and endangered, and what you suggest may be a factor. For more interesting results, you could try to add /climate change risk/ to the previous search. SemanticMantis (talk) 00:24, 25 June 2013 (UTC)[reply

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PMID 21246989

provides some relevant information. It states:

• Egg temperature affects sex ratio for a variety of sea turtle species, citing Standora 1985
• Relocating nests results in temperature-related shifts in sex ratio, citing Spotila 1987
• Quote: A one-half degree deviation from a 29.5°C pivotal incubation temperature can change the sex ratios of leatherback hatchlings to nearly 100% female (warmer temps) or male (cooler temps) (Binckley et al. 1998). citing Binckley 1998

Sorry I have not answered the second part (and really the heart) of your question, i.e. the impact of skewed sex ratio on population size. That seems like a very complex issue. -- Scray (talk) 03:46, 25 June 2013 (UTC)[reply]

I see that this question is basically a duplicate of "Sex ratios" above, where the discussion reflects some of the complexities I was considering. -- Scray (talk) 11:27, 25 June 2013 (UTC)[reply]

Since sea turtles do not appear to be monogamous - one male turtle could impregnate large numbers of females. So providing the sex shift is in favor of females, and providing it's not so extreme as to impact genetic diversity, there might not be a huge problem. However, if the shift is in favor of more males, then the direct reduction in the number of eggs laid would have a huge impact on the population. Sea turtles rely on vast number of hatchlings appearing at more or less the same time in order to overwhelm the local predators - when the numbers decline only slightly, the resulting population crash can be spectacular. Fortunately, global climate change is warming the beaches, resulting in more females. But there is a limit to that. When there are vastly fewer males, reduced genetic diversity and the inability of females to find a male partner will be a serious matter. Another saving factor is that turtles are long lived. They have a reproductive span of 50 or more years. Since the temperature of the beaches will occasionally be lower just because of natural variations in the local weather, there should continue to be years when substantial numbers of males are still produced.

However, this is a complex matter - and any disturbance to the ecology is to be feared because the consequences are hard to forsee.

SteveBaker (talk) 16:17, 25 June 2013 (UTC)[reply]