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I really don't know anything about how mountains form and I'm not sure if my idea could actually work. My world is basically the same as earth (but with different continents ect) and my city is on a small-ish island (a bit bigger than Bali) and is in a valley surrounded by a U shape of mountains with a river running through it. Would this actually be possible? Also how close could it be to the ocean, since I'd like it to open straight onto the beach.

enter image description here

(edit: I added a map but i'm a terrible artist so i just used the free inkarnate, i'm not really sure how tall the mountains would be, but i was picturing a mountain range, like a shorter himalayas. Also it's in a temperate climate so no volcanos)

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    $\begingroup$ Perhaps you could edit your question to includea scale map showing what you mean, with rough heights for your mountains. However, at a first guess this is not impossible for volcanic islands. $\endgroup$
    – Monty Wild
    Mar 2 at 23:05
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    $\begingroup$ Climate doesn't usually have anything to do with vulcanism. $\endgroup$
    – Monty Wild
    Mar 3 at 1:09
  • $\begingroup$ If all you want is a U-shape, consider how the Alps wrap the upper Po. For a full ring I think you need an impact from above, rather than from the side. $\endgroup$ Mar 4 at 1:00
  • $\begingroup$ You mean like the city of Pasar Ulakan on Bali? $\endgroup$ Mar 4 at 18:09
  • $\begingroup$ Most obviously, it's up to you alone how your world is built, specifically including whether mountains might form in rings. Perhaps less obviously, what could be problematic about the ringish mountains in your illustration? Broadly, if you want mountain rings, why not simply write mountain rings? $\endgroup$ Mar 5 at 23:26

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Absolutely, mountains can form in rings.

Valles Caldera has a ring of mountains defining the caldera. https://upload.wikimedia.org/wikipedia/commons/thumb/0/04/Valle_Caldera%2C_New_Mexico.jpg/220px-Valle_Caldera%2C_New_Mexico.jpg Another caldera is the Naples caldera (Campi Flegrei) which is larger and contains a harbor. https://en.wikipedia.org/wiki/Phlegraean_Fields

Both of those are volcanic calderas which can be dangerous.

Another mountain forming that produces rings is Metamorphic core complex. In this event, a bulge is formed which then erodes. When the edge rocks are harder than the rocks pushed up, erosion takes away those in the center and creates a rink of mountains. Big Bend National Park has a great example of that. (I've been trying to find a picture.)

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  • $\begingroup$ The one in Big Bend NP is the Chisos Mountains (or the Chisos Basin within them). $\endgroup$ Mar 4 at 3:10
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Yes and no

If you think about it, the Pacific "Ring of fire" is what you're looking for on a scale so grand that you don't find mountains at every inch along the tectonic plates. But it demonstrates the points that are important.

  • Mountains are (simplistically) built due to two plates colliding or volcanism, usually (but not always) along plate edges.

At first guess, that would suggest that the scale of what you're looking for wouldn't produce the ring of mountains you want. But is that true? One way to get what you want is for the right size meteor to hit your world at the right spot to create the result.

enter image description here
Image courtesy How a ring of mountains forms inside a crater.

All you need is the right kind of erosion over time and you get what you want. Another solution would be for a crumple zone to form in a convergent plate boundary. Like any good goldilocks moment, it needs to form with mountains just the right height to form a large inland lake near the continental side of the zone. Over geologic time, the lake drains to the ocean, eventually forming the wide(ish) plain area you show in your map.

How realistic are these? They're not impossible, but they're pretty improbable (from my non-geologist perspective). But, does that matter?

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    $\begingroup$ While I'm always grateful to receive the coveted green checkmark, we recommend waiting 24-48 hours before awarding it. We have amazing users all over the planet and human nature is to not bother with questions where the award has been given. The result is you could get a better, more definitive answer... but won't. $\endgroup$
    – JBH
    Mar 3 at 0:37
  • $\begingroup$ Yes, the crater theory is a good one! $\endgroup$
    – Galaxy
    Mar 3 at 0:50
  • $\begingroup$ They can also do it geologically without impact? en.wikipedia.org/wiki/Salt_dome $\endgroup$
    – Pica
    Mar 3 at 21:23
  • $\begingroup$ Looking at Islands near Bali, I also see a few where 2-4 mountains just sort of happen to be in the right positions, and build up a flat silt bed between them. $\endgroup$ Mar 4 at 18:12
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    $\begingroup$ @MooingDuck I'm taking the OP at their word. The posted map doesn't show 2-3 mountains, but 2-3 mountain ranges. Pica's salt dome idea is a good one. So, too, a volcanic caldera that's worn into a valley of some sort... but both are too small to include a ring the size the OP is looking for. A small solution is available, quite realistically, using a number of solutions. $\endgroup$
    – JBH
    Mar 4 at 19:57
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There are a few geomorphological methods that could produce similar effects. You even mentioned a range that is produced by one of them. There are different physical scales for each, but most of them are produced by one main process: Tectonic activity. There's one other that I can also think of that can be caused by either tectonic activity or another process.

Generally what will happen is if you have hills surrounding a valley, you will get watercourses running from the hills to the valley, and these will meet to produce a larger water-course. Temperate climates can be very wet - the Pacific North West of the USA gets about 200 inches (16 feet/5 metres) of rainfall each year. Mountains tend to get more precipitation if they disrupt wet airflow from seas and taller mountains are cooler, so cause more precipitation. However, too tall and it'll be locked up as ice/snow.

The Himalaya and associated mountain ranges (Hindu Kush, Karakoram, Pamir) are produced by the Indian plate crashing into the Eurasian plate. These produce, what is effectively a ring of mountains that separate the subcontinent (India, Pakistan, Bangladesh etc) from Eurasia. These sorts of tectonic events generally operate on continental scale, producing very extended mountain ranges in both height and depth (i.e. not a single line of mountains; many many lines of mountains covering huge area). One of the world's greatest rivers is the result of this event - the Ganges.

I don't see why it couldn't happen on a smaller scale, with much of the plate(s) underwater and only small portions poking up and producing your island with mountains on the up-lifted parts, but the depth of the ranges might preclude this.

On a similar scale, tectonics can also produce separation events. The classic case here is the East Africa Rift, where the Somali Plate is separating from the African Plate. This produces a depression in the land, which is surrounded by the sides of the valley - now you might be thinking narrow here. the Rift is about 4000 miles (6400 km) long and about 30 miles (50 km) across, with a drop of 600-900 metres (~2000-3000 feet) in depth from the plateaus on either side.

On a smaller scale, plate tectonics produces earthquake zones due to land masses sliding around. These go by various names:

  • Horst-Graben (German for Range-Valley) structures produce valleys with hills around and can end at ranges. The East Africa Rift is a very large-scale horst-graben structure; but they come much smaller too. These are found in many areas of the world.

  • Strike-slip faults can result in a valley between two ranges, particularly where faults don't lie parallel to each-other. The Wellington and surrounding areas in New Zealand sits on a system like this. Check out a satellite view or topographical overlay on a map website/app to see.

  • Volcanism. Climate has nothing to do with volcanism - Iceland is famous for its volcanoes, and Antarctica has a couple of massive ones - Mts Erebus, Terror are both well over 3000 m (10,000 feet) tall. Ring structures from calderas are common around the world, as another answer pointed out.

The final mechanism I can think of is sea-level change. In many parts of the world you can find high-level plateaus that have sea-shells etc embedded in the rocks. For example, the central part of the USA, was, between the Cretaceous to the Paleocene, occupied by a sea known as the Western Interior Seaway. Obviously this is no longer the case because of sea-level change. The changes that can cause the sea-level to change are either through tectonic uplift, but also through climate change. During the ice-ages, vast amounts of water were solidified in the form of ice, lowering the sea-level, which could leave a former bay out of the water.

Another change in the opposite direction is that with warming of the climate, an valley system formed by glacial action could be left without ice as the glacier(s) melted. Fjords are examples of formerly glaciated valleys, though since submerged in water. The McMurdo Dry Valleys in Antarctica are also examples of formerly fully glaciated valleys, though most of them still have some glacier left at the head. I am sure there are plenty of other places around the world with similar systems as a result of glaciation and climate change.

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Consider the San Luis valley in Colorado: a wide, very flat valley between the San Juan mountains on the west, and the Sangre de Cristo mountains on the east, capped by an extension of the San Juans on the north. No river, but that's due to climate, not geology

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On an even smaller scale, a large dormant volcano can erode into a chain of mountains surrounding a valley, and a drop in sea level can turn an atoll into a ring of mountains (though the river you describe is hard to plausibly create on a former atoll.)

Parts of Pinnacles National Park in California are very similar to what you describe, ringed by jagged peaks that are the remnants of long-extinct volcanoes.

In the question you said "Also it's in a temperate climate so no volcanos" but there are volcanoes in all climates from tropical Hawai'i to Alaska and Antarctica.

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