Why are coastlines crinkly near the poles but smooth in the tropics?

Submitted by emsot t3_1098uax in askscience

I've noticed from playing Worldle that you can instantly tell how close an island is to the poles by how crinkly its coastline is.

Everything in the Arctic or Antarctic has intricate crinkly edges: Svalbard, Ellesmere, the Falklands, the Kerguelen Islands.

Tropical islands look totally different, smooth and rounded: Sri Lanka, Barbados, Nauru.

Why's that?

Edit: I'm getting notifications every few minutes about glaciers, erosion and Slartibartfast, and almost all of the comments vanish so no one but me can see them. But thank you for all of the answers, I am feeling thoroughly educated!

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Bob_Skywalker t1_j3x05qd wrote

Jagged coastlines near the poles are caused by glaciation cutting through the landmass and isostatic rebound.

Smooth coastlines in the tropics and as you near the equator are due to heavier weathering from rain and liquid water.

Ice cuts, water erodes.

This is just the simple explanation from what I remember. Credentials- B Sc. Geology.

Edit: Additionally, I'd also like to point out that there are exceptions to this. Abundance of "jagged" toward the poles and "smooth" near the equator is just describing prevalence. Citing a smooth coast near a pole or a jagged pole near the equator doesn't discredit prevalence. For example, Hawaii, being relatively recently created by magma plumes it will take lots of time due to the mineral composition and youth of the islands for them to either smooth out or erode away. Another example is the tectonic uplift along the US west coast.

Edit 2: There are some top level comments that are more descriptive than mine with some good additional information. Don't just read mine because its higher and forget to scroll down for the more in depth comments.

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dittybopper_05H t1_j3x91zz wrote

That's not entirely accurate.

For example, the Marquesas are just south of the Equator and they have crinkly edges.

Meanwhile, Wrangel Island up in the Arctic Ocean is pretty smooth and rounded. Same with Bouvet Island down by Antarctica.

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HeartwarminSalt t1_j3x9b5s wrote

Coasts can be erosive or depositional., old or young. Young coasts of any type will likely be jagged (look at pacific islands when they first break thru sea level). Then if the coast is old, it can be jagged if it’s erosive or smooth if it has sediment (coral reefs count too!) being deposited.

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Captainbhusta t1_j3xcb1q wrote

Because of the way that glaciers and waves shape the land.

In the polar regions, glaciers advance and retreat over thousands of years, carving out fjords and other features in the coastline. The glaciers erode the land, creating valleys, and deposit the rock and sediment they have picked up in the process, building up landforms like moraines. This process creates a crinkly or jagged coastline.

In tropical regions, waves are the main agent shaping the coast. The waves erode the land primarily through a process called longshore drift, in which waves hit the coast at an angle and push sediment along the shore. This process creates a smooth coastline because the waves tend to erode the land evenly, not carving out fjords or creating other distinct landforms.

Climate and sea level changes also play a role. In warmer climates, sediment is transported more quickly, leading to a less-pronounced coastline and sometimes sediment deposits forming barrier islands or lagoons

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Researcher_1129 t1_j3xcrms wrote

Rocky coasts (high and low relief) result from resistant geology (to the erosive forces of sea, rain and wind), often in a high-energy environment, whereas coastal plain landscapes (sandy and estuarine coasts) are found near areas of low relief and result from supply of sediment from direct terrestrial and offshore.

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scrappyisachamp t1_j3xeerc wrote

Replying to Bob_Skywalker (#1,333,828)

Is the weathering “heavier” in the tropics or have the coastlines just had longer to smooth out? Glacial periods near the poles are somewhat frequent on geologic timescales, so the water there hasn’t had as much time to erode the coastlines compared to the tropics, but the erosion “rate” is probably similar between the two regions, no?

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newaccountscreen t1_j3xh30b wrote

Replying to scrappyisachamp (#1,334,478)

Frost wedging has a large impact in the poles, water melts gets into cracks, freezes and then expands causing more fractures to be infilled and cracked. The erosion rate between the tropics and the poles is probably close by I imagine the tropics have more erosion present. I'll be back to update this comment when I can read up more. Source, also a b.s in geology

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steeplebob t1_j3xhfv7 wrote

I believe in his book “Scale” Geoffrey West explains that the “crinkliness” of coastlines is primarily a function of the slope of the land, such that the steeper the slope the longer the actual coastline. I think he includes a mathematical expression for the relationship but I can’t recall it off-hand.

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geek66 t1_j3xjosz wrote

In addition to the others issue mentioned here - A lot of tropical areas have coral sand, so a different source than stone weathering ( quartz sand - common on the East coast of the us) -

Coral sand is also very light and more likely to be shifted in current.

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cthulhubert t1_j3xjtgd wrote

Replying to Bob_Skywalker (#1,333,828)

Oh man. That's something that's bugged me for as long as I've been on the Internet. It is absolutely amazing how often you can describe a trend, and somebody will mention one of the exceptions to that trend, and sit back smugly as if they've somehow proven your understanding of a statistical distribution is wrong.

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Past-Willingness-207 t1_j3xs563 wrote

Replying to scrappyisachamp (#1,334,478)

Weathering definitely happens at different speeds, depending on the type and intensity of weathering. There mainly are 3 types:

  • physical (water and wind move sand and small rocks around, they become smaller through friction; material reacts to temperature differences by extending/shrinking, making way for water to intrude through cracks; also water then potentially freezing, extending and breaking rocks)

  • chemical (water reacts with minerals, resulting in the emergence of more or less potent acids)

  • biochemical (mainly driven by organic acids, created by decaying organisms, root secretions, huminic acids)

These three interact with each other. Weathering intensity is highest near the equator, because of basically all the life that's happening there. Great biodiversity + heavy precipitation year round means lots of fuel for weathering.

Interestingly, there's also the concept of relief energy - weathering in great heights happens quicker simply because of the greater potential/gravitational energy.

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darth__fluffy t1_j3xtyio wrote

Replying to Bob_Skywalker (#1,333,828)

I’m working on this constructed world where there’s this series of tropical islands but the entire world was frozen until very recently in the past so they have like glacial erosion in a tropical environment

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Tuungsten t1_j3yu2rx wrote

Replying to Bob_Skywalker (#1,333,828)

There's one thing I wanna add to this, the Mercator projection zooms in on things closer to the poles. So you can get higher definition the further noth you are because of the distortion.

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ScoobyDone t1_j3yvcm8 wrote

Replying to Bob_Skywalker (#1,333,828)

You can really see this effect on the North American West coast. The ice sheets from the last ice age covered Canada, but not much of the US. The fjords (we just call them inlets) stop abruptly at Washington State, so the coast is rocky and jagged above Seattle, and smooth with a lot more beaches below Seattle.

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im_dead_sirius t1_j3yw9he wrote

Replying to Bob_Skywalker (#1,333,828)

I think you are remembering that part of geology class somewhat incompletely. That's fine, geology is a huge subject.

All photos by me.

Ice gouges U shaped channels (especially on a huge scale), water erosion creates V shaped gouges. Here is a good example of the U shape of glaciation erosion, and the river in front of it, carving a V.

https://i.imgur.com/oddQgRL.png

At the base of a formerly glaciated mountain, you get steep smooth talus/scree piles of sharp rock, as sharp stones have a steeper angle of repose, and rain water runs down through glacial till, then out at the base. How much of a talus pile is there has a lot to do with how friable the rock is, after major glaciation retreated.

Example of glacial talus in an area with low rainfall.

https://i.imgur.com/C9FvYJV.jpg

All the rock in that talus fell after major glaciation carved the valley and lake.

Here you can see the material washed down by water erosion fans out at a lower angle, and is also more subject to erosion, as the scree is smaller and mixed with organics.

https://i.imgur.com/jylENUG.jpg

When it comes to coasts (and beaches), they are built and shaped by tidal action. Beaches, whether gravel or sand, are defined as being relatively low in slope, close to water, generally homogeneous in material size, even when (and if!) they are actually talus piles.

You can see this in Australia, where I'm hoping to go some day, to take my own photos. You won't notice these beaches on a map, nor in Northern regions that look all crinkly.

https://i1.wp.com/beautifulplacestovisit.com/wp-content/uploads/2011/09/Apostles_3_GOR.jpg

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hogey74 t1_j3yzy1p wrote

This was a great question thank you. I've learned a heap from this thread. Hey maybe also look at the basic air flows and resulting ocean behaviour for another peice of the puzzle.

https://www.youtube.com/watch?v=Ye45DGkqUkE

NZ has awesome examples the glaciers vs erosion. Some of the coast is dominated by old glacial action and other parts formed by eroded volanoes.

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2011StlCards t1_j3yzz4f wrote

Replying to Alexis_J_M (#1,333,855)

This is also part of a theory why Sub-Saharan Africa never developed into large, world-busting empires like you saw in Europe/near east/Asia.

Jagged coastlines make for great Deepwater ports, which are necessary for bulk trading and information exchange. Lots of groups in Africa generally stayed fractured, which is why there are so many cultures and languages

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nichogenius t1_j3zee7b wrote

This might not be the actual cause of what YOU are seeing, but one big difference is caused by many maps using the Mercator projection. The mercator projection (the most common flat map of the world) causes the poles to become very stretched. Smaller features are visible at the poles than at the equator. It's as if you are looking at the poles through a higher magnification than the equator, so your viewpoint is closer to the ground.

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Rosevkiet t1_j3zeh3k wrote

This is a great observation and a good thing to keep in mind for worldle (would have been helpful for the South Atlantic Island the other day).

One of the weirder concepts to wrap your head around in sedimentology is relative sea level. Sea level at any location can change due to local effects, like heavy glaciers loading the crust, actually bowing it down, or for global effects, like changes in global climate or rate of sea floor spreading. It sounds pretty straightforward, but trying to sort out local vs global effects was really hard and took decades.

On a stable coastline, one where relative sea level has been more or less constant for a long time, sediments fill the basin in the water, and the coastline starts to advance out into where the water was before. This is what deltas do. When you look really closely, they are complicated and jagged too, but not on a regional scale.

On a coastline where there is rapidly changing sea level, particularly rapid rises in sea level, all the smoothed coastline will be underwater, and the new coastline will be jagged.

In Earth’s current state, the poles have experienced rapid, recent (50,000 yrs) changes in sea level due to glacial cycles, AND, the erosional pattern of alpine glacier leads to deep, steep valleys (glacier=sediment bulldozer, river = central conveyer belt), so the squiggliness of the coastline will be even more.

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Bob_Skywalker t1_j3zo0tx wrote

Replying to im_dead_sirius (#1,336,933)

>I think you are remembering that part of geology class somewhat incompletely. That's fine, geology is a huge subject.

I was just making a quick response as noted in my original comment. Hours before you replied I even said it was explained in more depth by comments further down. I was at work, I didn't have time to go into detail.

As far as remembering that part of geology class incompletely, which class are you talking about? Stratification and Sedimentation, Hydrogeology, structural geology, Geophysics, Sedimentology...? Because as I said in my original comment, I have a degree in geology.

Was your entire objective to post your photos, toot your own horn, and act smugly smarter than someone with a degree in the topic at hand? Because it's not a good look.

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aobtree123 t1_j3zwjed wrote

It’s due to centrifugal forces with earth as the rotating frame of reference. During island formation there is more centrifugal force at the equator which smooths the edges of an island. At the poles the centrifugal forces is less.

Think of it like making a pizza out of dough. The faster the dough spins the smoother the edges.

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craigiest t1_j4004ua wrote

Replying to StellarNeonJellyfish (#1,336,322)

As I understand it, coastlines are basically fractal in nature over many orders of magnitude, so no, it shouldn't. Shrinking or enlarging details beyond the resolution could also result in smoothing. So it really matters what the data and display resolutions are and how they interact. A Mercator projection could be enlarging polar regions beyond the data resolution or compressing equatorial regions beyond the display resolution or both resulting in different domains of smoothing.

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The_Frostweaver t1_j400oit wrote

Replying to austarter (#1,338,772)

It has more to do with things like total precipitation being way lower in the arctic so less sediment transportation from land to sea and coastlines being locked behind ice protecting them from wave action.

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jacobdrj t1_j40g2g9 wrote

2 ideas:

1: Recent receding of the glaciers from the most recent ice age. This the coast hasn't had enough time to be exposed to erosion from water

2: Lack of parrot fish chewing up non existent coral reefs to make sand that fills in the gaps.mm

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Archerfish23 t1_j40gfyl wrote

In the north, carving of deep valleys on the coast by glaciers and subsequent flooding after ancient global warming and melting. And in the south, in many cases, eroded volcanoes and coral reef development in the warmer tropical waters and lack of glaciation which crenulate the edges…

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Baxters_Keepy_Ups t1_j40l6vb wrote

Replying to CorrectCoyote926 (#1,339,502)

There’s a lot written about this. There’s a book called “Prisoners of Geography” which was a bestseller and touches on some of this.

In order to have advanced ‘quickly’ as a country/culture/people - it’s helpful to have load bearing animals (horses, donkeys, camels, llamas etc), deep straight rivers, and fertile land. Access to the sea is also hugely useful.

Much of the African continent simply doesn’t have the tools that European or Asian powers had, so that has made harder what was easier elsewhere.

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The_Ivliad t1_j40tenn wrote

Further to what others have said, it's interesting to note that the east coast of Russia isn't as crinkly as other arctic regions. This is because the prevailing winds didn't dump enough snow and ice to make a big enough ice pack for glaciation in that area.

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2011StlCards t1_j411p3z wrote

Replying to CorrectCoyote926 (#1,339,502)

u/Baxters_keepy_ups already basically stated exactly what I was going to write.

Up until about the 16th to 17th century, Europe was not the world superpower that it would become.

The idea goes that Europe was able to advance in technology and power pretty quickly because of the interconnectiveness of the people on the continent

Large, navigable rivers. Arable land. Beasts of burden (i.e., horses, donkeys, camels). Deepwater ports (for trade). All of these aspects help to advance trade and, thus the connections between groups.

When large groups of people have connections like that, ideas and technology flow from one group to the next.

When you have that interplay between people (i.e. trade), you theoretically would advance quicker since someone in Portugal may have a ship design that is more efficient that the French may get to see and create for themselves or trade for.

It also helps that most of the large European powers had languages of either romance or Germanic origins, which makes talking to one another even easier.

This doesn't preclude conflict, of course, as we see countless European wars between neighbors throughout history. But between those conflicts, trade reigns Supreme.

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2011StlCards t1_j412mtv wrote

Replying to coob (#1,340,520)

North American natives were effectively isolated from the rest of the world until the 15th century.

Europe, Africa, the Middle East, China, India, and southeast Asia had basically all been interconnected for millenia.

You had the Roman empire, the hellenestic kingdoms, Mongolian empire, trade routes of the Indian Ocean, trade routes like the Silk Road, etc... that had people, goods, and ideas transferring from one group to another for centuries.

That means technology, religion, science, and more from China can make it to Europe and have influence.

The Americas had some civilizations that were pretty damn advanced. The Inca somehow made a huge empire in the mountains with only humans and llamas. The Aztec basically had a capital that rivaled any city in europe at the time of its destruction.

If given time, these groups may have become more powerful, but the odds were against then when it came to trade and new ideas. There just weren't as many people involved there as in the Eurasian trade networks

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UncagedBeast t1_j416bhq wrote

Replying to 2011StlCards (#1,337,105)

I wholeheartedly disagree with this. As an anthropologist, it is undeniable determinism is a long outdated idea in anthropology and fundamentally stems from racist ideas of objective evolutionary stages of « civilisation ». Further, many large compex and imperial structures and polities existed in Africa, at all periods of its history.

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wombatlegs t1_j418p58 wrote

Replying to 2011StlCards (#1,337,105)

Deepwater ports!? Civilisation began in the fertile crescent, where there were not a lot of glaciers, and spread largely by conquest. Ancient empires barely had seagoing vessels, let alone any use for deepwater ports, which is a very recent development.

More recently, these empires spread to the New World and Australia , while Africa remained the "Dark Continent". The reasons Arabs and Europeans failed to make headway into sub-Saharan Africa are documented in the history books. On the science side, one big breakthrough was anti-malarial drugs.

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Busterwasmycat t1_j41cu0g wrote

Replying to Bob_Skywalker (#1,333,828)

Our planet saw a serious period of glaciation so many irregular features were revealed only recently. Ice isn't really very fluid so it creates irregularities, water and air are fluid and get rid of irregularities.

It is mostly a matter of time that the coast has been the coast and eroded only by interaction with nearby ocean which is the cause of irregular coastlines. the north was glaciated, and it created a new landscape (troughs, ridges, large sublinear scratch marks, and so one). It isn't just the coastlines that have obviously different features (lakes abound in recently glaciated terrains, for example). Only a few short thousands of years since the ice went away, so the rounding that happens when ocean meets land (eroding promontories like capes and points and filling in indents like coves and bays with sediment), and the work of long-shore drift, haven't had time to do the work well.

Basically, energy minimization is at work and energy is minimized by elimination of points and dents (just like a rock rolling in a stream gets rounded). The land/sea contact zone is always in the process of linearization and smoothing/rounding, equalization of forces of the ocean against the land. The presence of points acts to turn the waves toward the points and concentrate the energy of the ocean on those outstanding features, eroding them faster. The stuff broken off migrates to open spaces and fills them in.

Really, the same sort of process is happening everywhere that erosion by wind and water is occurring, leading to the smoothing of contact zones (elimination of zones of unequal exposure). Juvenile terrains and renewed terrains are marked by numerous irregular features. Those features do disappear with time. Basins fill in and high points wear away.

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BaldBear_13 t1_j41dc35 wrote

Replying to 2011StlCards (#1,340,529)

wars, or rather threat of them, can also contribute to development, as they encourage technological progress. E.g. gunpowder lead to cannons, and that lead to better metallurgy, which had all sorts of useful peaceful uses.

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2011StlCards t1_j41fgx8 wrote

Replying to wombatlegs (#1,340,927)

I never said civilizations required deepwater ports to begin. I am stating that larger empires or interconnected civilizations benefit from them.

Obviously, no early civ started by trading across the seas and oceans. Rivers, especially navigable ones, were key to the early governments that we see in Mesopotamia, Egypt, indus river valley, etc...

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2011StlCards t1_j41fo6y wrote

Replying to BaldBear_13 (#1,341,249)

Yep, nothing better for weapons technology than requiring better cannons to stop your enemy

And you need Good taxation structure to be able to obtain the wealth necessary to pay for those cannons

And good taxation structure leads to stronger, more centralized governments

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quatch t1_j41i040 wrote

Replying to WesternOne9990 (#1,337,219)

it's a meaningful distinction. Think of it like a potato peeled by a peeler or by a knife. Both are peeled(eroded), but when observing the result you wouldn't miss that the process was different.

Glaciers erode by chunks, water by bits? But by being more specific and less poetic you start to introduce errors.

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Bob_Skywalker t1_j41kly2 wrote

Replying to Muslim_Wookie (#1,339,319)

Lol. Here I am worried I came off as too defensive, but now that I've slept on it, I feel it was reasonable. They strike me as the person that thinks they are an expert on every subject because they read an article after taking some photos. Probably cans his farts to smell them later.

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WhalesVirginia t1_j41o223 wrote

Replying to Boatsnbuds (#1,334,980)

IIRC the last like year or two, has seen a reduction in average global temperatures(i haven't taken the time to verify this so grain of salt). As opposed to the predicted models that used exponential growth rates to pollution.

Not a denier. I just think a lot of the internet has never really looked that close into whatever they spew about science, and have a limited view of statistics. For me it's been a while since I've dived into this particular topic, hence my big qualifier on my claim, but there's this strange faith like behavior everyone has about science, like you aren't allowed to question it, despite unlike in religion, science has all of their data and methods public record.

Idk it just irks me the wrong way when I try to have a real discussion about something, like expressing the flaws I see in the methods, and the discussion is taken over by imbalanced expletives(meaningless statements) and politics.

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bitofrock t1_j42etdr wrote

Replying to WhalesVirginia (#1,341,801)

Uhm, the thing about science is that the only way to improve on the science is more science.

An opinion or pointing out a possible flaw doesn't advance science, but may be a part of future science that advances things further.

But if you don't have a solid grasp on the science done so far, then you're just having opinions that are unlikely to make much of an impact or be considered unless you have substantial credentials in the field.

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WhalesVirginia t1_j42o906 wrote

Replying to bitofrock (#1,343,174)

> Uhm, the thing about science is that the only way to improve on the science is more science.

The way to improve is not just more science. It's better science. Something like 80% of papers are never read after publishing, a surprisingly large number of papers are retracted. We have a quality problem, not a quantity.

>An opinion or pointing out a possible flaw doesn't advance science, but may be a part of future science that advances things further.

Being critical of models advances science. It shows where a model fails. My issue is with the politicization rising up science because it fits a narrative. There's plenty of great climate science, but there is plenty of kind iffy stuff that hits front page reddit on the daily.

> But if you don't have a solid grasp on the science done so far, then you're just having opinions that are unlikely to make much of an impact or be considered unless you have substantial credentials in the field.

Well obviously.

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UnarmedSnail t1_j44xnwd wrote

Replying to BaldBear_13 (#1,341,249)

There's a fair amount of randomness to it as well. You need all these important pieces, but you need them to come together in the right way, the right time, and in the right place. The ancient Greeks had all they needed to jumpstart the industrial revolution 3,000 years ago, but the pieces were locked away as religious displays and secret knowledge in mystery cults.

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UnarmedSnail t1_j4527gk wrote

Replying to BaldBear_13 (#1,347,735)

They had chemical batteries that would be connected to statues of Zeus that would shock when touched. They had primitive steam engines that would spin up when boiling water was heated inside them. They had the archimedes screw. Complex machines for milling,stamping, grinding. If someone had known of all these pieces and thought to combine these technologies to actually do work then you have an industrial revolution.

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UnarmedSnail t1_j4533uv wrote

Replying to coob (#1,340,520)

They did not have animals in north America suitable for domestication to magnify their work potential. They did not have farming technology on an industrial scale, and they didn't have the social structure suitable for long term growth in most cultures. There were a few exceptions in prehistory but they did not survive to contact with Europeans. The Aztecs being the only empire that did exist then. There were a long string of prehistoric empires in the Americas but for the most part they were separated by time and distance from each other.

Edit: the Incas also had contact with Europeans.

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UnarmedSnail t1_j459wed wrote

Replying to 2011StlCards (#1,340,529)

Europe was a boiling cauldron of death, plague, and blood since the fall of the Roman Empire.

Then it met China again.

Then Europe was a boiling cauldron of death, plague, blood and innovation as they used the cross pollination of ideas to find more efficient ways of death, plague, blood. WWII ended this...

for now.

Edit: Russia has unpaused the game.

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