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.