Is there a scientifically valid explanation for a circle of islands?

Question

A couple friends and I have started a world called Lothaucan. On this world there is a continent called Novara with a circle of islands as its key feature.

(seen in the mid-left of the image)

Is this feature scientifically and geologically viable? If so, how so? If not, how can we change it for it to be viable?

The world is a moon roughly the size of Earth, orbiting a large Jovian, and has more volcanic and seismic activity than Earth. That is all that has been decided. Please note that I am not asking about the viability of the orbital mechanics, but simply included it so as to give more information. If more information is needed, let me know and I will do my best to get it.

Scale: Novara is a bit bigger than Australia (a little over 3,000,000 square miles/7,770,000 square kilometers).

Answer 1

An half submerged crater can give you a circular set of islands. You can choose among various craters:

• Volcanic craters, like Santorini
• Mountains subsided into the oceans or eroded by changing sea level, like all the coral atolls (the picture refers to French frigate shoals)
• Impact craters, resembling the Tycho crater

all of the above can also provide central peaks aswell.

Answer 2

For an alternative (to vulcanism) geological process, check out what was discussed in this thread at Earthscience.SE.

Link to Google Map of the affected area.

Basically, a very long ago tectonic collision caused part of the Earth's crust to fold in on itself and 'roll up.' Harder base rock gets wrapped around other softer layers of rock. Imagine a jelly roll.

Now, hundreds of millions of years later, other tectonic forces have tipped this region onto its side. The swirling jelly roll now faces the surface. Eventually, this part of the crust is raised to the surface and weathered by erosion (in the case of the linked map of Lake of the Woods, also by glaciation.)

Now you have a swirl of harder rock exposed and, if the elevation is just right, the softer 'jelly' part of the formation eroded away into a lake or the ocean.

Answer 3

Volcanic activity

If that moon has volcanic activity just like earth, then you can call it Ring of ..., just analogous to our Earth's Ring of Fire.

Just make sure the size of the ring is big enough (I'm thinking of 3000-8000 km in diameter). I don't have the science to back this up why, but I'm basing on the smallest ring I can find on earth.

As you can see above, the lower volcano line is stretched almost 5000 km from northwest Sumatera (Aceh) to Maluku on the east.

The islands in the center

They might be remains of a self-destructed volcano. Here is the image of Samosir island in Toba Lake in North Sumatera, with an island within the center of the lake, which is the remains of a dead volcano's caldera.

There is also an example of self-destructed volcano island becoming several pieces of islands: Krakatoa volcano.

Island in a lake in an island in a lake

Yo dawg, I heard you like islands, so I googled you this island in a lake in an island in a lake – Mazura

Answer 4

As L.Dutch pointed out, submerged volcano craters can create the necessary shape.

In the case of Santorini, there is even evidence to suggest that it is the original source for the Atlantis myth. Link

Also it's worth looking at the shape of the island pre-Minoan eruption, which gives you a much more interesting shape:

Hope this helps!

Answer 5

This is actually pretty simple. Rather than any sort of ring, your overall landmass appears to me a single line of land with decreasing width that spirals at the end. As long as that also vaguely corresponds to height, this formation could well be generated by just a single massive hotspot, undergoing intraplate deformation (because e.g. it's near a three plate boundary, or is much weaker than the other plate). As the plate with the hotspot under it subducts its neighbor (as e.g. continental plates are wont to do to oceanic ones), that neighbor continues strong shear motion, leading to the fragmenting of the weaker subducting plate and the formation of that sort of spirally shape via accretion. This means lots of earthquakes in that region, at least during the formative process, but most/all the volcanic activity is on the other end of the continent.

Your island also has a distinct bend in the chain a little while after the spiral. With a hotspot-generated landmass, that change would indicate a change in tectonic motion. This is reemphasized by the small gap in the landmass shortly afterwards: this gap is the location of the former shear/subduction fault that then became a regular subduction fault and is now recently divergent. While the western part of the continent used to be part of the same plate as the eastern part, it's now broken off and is its own tiny plate.

The implications of this system are that the rocks in the tail end of this chain will be highly metamorphic, both in terms of the islands and the underlying seafloor. Expect a lot of exposed and submerged blueschist, with an unusually large amount of Eclogite in the seabed. The Eastern Island will be much more tame, with more typical basalt formations transitioning to blueschist as one travels west.

Answer 6

Consider building a Weald

The Weald is a region in the south-east of England, situated between two ridges of chalk hills, the North and South Downs. The geology of the whole region is dominated by these hills, and the lower hills that run parallel to them as you move towards the core. This is analogous to the multiple rings of concentric islands in your map.

The region formed originally as an anticline, a place where the crust was folded and lifted, forming a single large ridge or dome. The upper layers then eroded away, leaving the deeper core exposed, with harder rocks resisting the weathering better and leaving behind hills.

For your purposes, of a very large, circular or near-circular formation, you would probably want the rock to be uplifted by a magma intrusion, rather than being folded.

Magma rose beneath the rocks, perhaps as a result of a mantle plume, hot spot, or similar, but rather than reaching the surface as a volcano, it found a planar weakness in the crust, and spread out, forming a sheet intrusion. Essentially, it was easier to spread out and lift the crust above the magma than to break through the crust to the surface. The entire region was thus uplifted, forming a geological dome. Erosion wears most of that dome away, leaving behind the harder igneous and metamorphic rocks, which form your islands.

Answer 7

First off - most of these suggestions assume a very Earth-like environment, where you can find plate tectonics & seismic activity, air & water/ice erosion, and a nice, mature atmosphere, as well as the usual Earth-like gravity & geological makeup.

Some of these have good SF/extraterrestrial substitutes - methane, for instance, is a good substitute for water under the right temp & pressure conditions. But variations in the others carry a fair amount of consequences/prerequisites - less dense atmosphere can give you the planet-sweeping erosive dust storms of Mars, more volcanic/seismic activity can give you the nightmare landscape of Venus.

With that in mind: Erosion - whether caused by wind/gas, water/liquid, or glaciers/solid - of a structure of non-uniform geology will tend to look a bit like a resistance sculpture. Harder materials resist longer, so that's the part that juts out, whether you're talking about islands, cliffs (in the reverse case, you get caves & sinkholes).

Now, several folks above have offered suggestions on how you get your concentric circles, and a decent geology textbook/website can give you loads of examples of such in the real world. Basically you either have:

• (A) Flat layers get distorted (tilted and/or warped), usually by local seismic activity, incl. intense heat & pressure. Think earthquakes, and active zones like the Pacific "Ring of Fire", but also lots of minor-level shifting occurs all the time. And all those layers get eroded and re-distorted, which is how we get all that lovely non-flat ground, aka topography.

• (B) Liquid magma gets extruded like toothpaste underground/underwater or above ground, basically volcanoes. In your case, think Hawaii, Indonesia, really almost every Pacific island. Volcanoes don't all erupt the same way*, aren't shaped the same way, and can also have "generations", where new eruptions happen and distort the solid remnants of old eruptions.

• (C) Catastrophic, extraterrestrial impact - meteorite impacts are really common, especially if the atmosphere isn't thick enough to protect the surface. Most of Earth's own major impacts just get "smoothed over" by the effects of erosion and the bulk-recycling of plate tectonics, but at times we have seen a lot of impact activity. There's a theory too that having the Moon kind of helps block some of it as well. If your world orbits a larger planet, perhaps that giant gravity well helps protect it, or maybe it just gets walloped all the more.

One last point - You mention that this circle of islands is a key feature, but if there's any takeaway, it's that the processes that can create such a structure are really common. If this island archipelago is meant to be mystical or revered because of its uniqueness, you might have to maybe come up with some kind of way to justify that assumption (maybe the natives just haven't traveled far enough to see it elsewhere, maybe A Wizard Did It, etc.).

Answer 8

If you don't require that the primary cause of the formation be seismic or volcanic forces, another possibility is that it's the product of some sort of organism.

A non-mobile plant, animal, or microorganism that grows in a specific area will consume nutrients in the area, and deposit some sort of solid byproduct. As the needed resources are consumed, the organism will tend to expand outward in the direction where more resources are available.

Over a long time, this gives rise to a ring or circle of active organisms with a central area that contains few nutrients. Most growth happens where the most nutrients are, around the edges, and the nutrient-poor middle area has little or no growth.

The ring continues to expand, eventually producing a circular shape.

On earth, an example of such is a coral reef atoll.

Over time, as the coral builds up, it reaches the surface, and since it cant grow in the air, it must grow by expanding outward. The coral grows in the direction which has the best conditions for survival for the individual organisms(temperature, water conditions, food availability, etc.)

With continued growth, combined with erosion, this eventually creates a ring-shaped island, generally with a central lagoon. Over time, it forms a ring of separate islands. In extreme cases it can form a spectacular ring of rings, each containing additional rings, as found in the Maldives.

Atolls can take on various shapes, from circles and ovals to spiral-like shapes, even squares.

Atolls are commonplace in some parts of our world, but rare in others, so it could be that some unusual special combination of features on your world would give rise to an atoll-like structure which doesn't appear anywhere else.

Coral atolls are the most striking examples on Earth, but the basic concept is that an organism starts in a central location, consumes resources in that area, and then begins expanding outward to obtain more resources, leaving a central area which no longer grows.

There are other organisms which produce ring-shaped structures here on Earth.

A smaller scale example of the same phenomenon here is the fairy ring, which is a circle of mushrooms which appear around the edges of an underground fungus. As the fungus grows and expands, the outer edges are where the most nutrients are, so the fruiting bodies tend to appear there.

Ringworm fungal infection is another even smaller-scale example.

Since we're not talking about Earth in this case, you can use your imagination to come up with an organism which produces such a formation.

It's easy to imagine some sort of seaweed that accumulates silicates or carbonates and leads to the formation of stony atoll-like structures. Plankton, or even large creatures like some sort of big crustaceans which only move as juveniles, then collect carbonates on their shells and become immobile in adulthood.

Finally, some sort of intelligent or semi-intelligent life could produce such effects. Humans have produced oyster shell middens large enough to change the local landscape within a few centuries, so it's not hard to imagine some other creature over tens of thousands of years creating entire islands this way.

Answer 9

Such picture of concentric wavelets could be created by some source of oceanic currents or winds directing or blowing out of the center, more or less constantly. I think, you have seen the picture of dunes or small sand wavelets on the bottom of the sea. They appeared in similar way. What could be that source of movement? I think that can be a very original detail of the world. Some anti-Maelstrom. The magic pole of the world. Notice, that while the distance between circles is about tens of kilometers, the length of waves should be about the same. Thus, the speed of the in/outward current/wind should be really great. On the other hand, that wind could appear once in thousand years...

Other variants don't give that picture.

A crystallization of mix of hard and soft minerals when there remains a source of heat (or frost) in the center could be concentric as a result, too, but it would have the size of meters, not hundreds of kilometers.

Glacier could create concentric structure, too, the islands on the lake on greater island on the greater lake... happen in Finland or Canada. But the concentric layers are very much thicker there.

A large caldera drowning down in warm waters will be doubled by corals. If the caldera is only partly drowned, we have three circles: outer reef, the crest of caldera circle, inner reef. If the structure of caldera plus central mountain is partly drown, there will be four circles around the central island. But the outer reef would never become the continuation of the continent mountain ridge. That ridge would go towards the central island instead.

The chalk or karst structures have much smaller size.

An asteroid could have concentric structure, and of any size but it should loose its half and fall accurately by this flat cut up. I can't imagine such accident.

The roulette structure is a funny theory, but roulette of 500 km means that before falling on the side, the core in that place was 500 km thick. Then there would be NO geological movements at all. But the layer of that roulette should be about 100 km. How that could be swiveled around? Five turns? When even much more thin Earth oceanic core (7km) never makes three turns? When it goes too low, it simply melts.

The structure hardly can appear as a result of spiral tectonic surges because tectonic movements do not go in spirals on that size level (for half a planet - you are welcome!). Because of the high viscosity of magma. Magma is much less viscous when it is more hot, but it were so hot, there would not be any normal continents, only maybe a few very hot islands in magma ocean. With normal temperature that structure could possible if the physic details on that planet are very different from the Earth. When magma behaves almost as water and the planet rotates much more quickly, but continents still have some plasticity as the Earthly ones. Volcanoes on such planet would behave as Hawaiian ones - those have low lava viscosity, too. There sure will be some lava lakes on such planet, too.