how did San José Island (and similarly shaped islands) naturally form?

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They look so thin and weirdly smooth, and almost form the outline of a perfect circle

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There are a few islands by that name, it seems like – are you referring to the barrier islands off the coast of Texas in the Gulf of Mexico (which don’t seem like they form a circle at all to me?), or to some other island?

If you’re referring to barrier islands, like the San Jose Island on the coast of Texas, they’re a type of dune system but we don’t actually know how they form. There are three main theories so far, but no single theory can explain the development of all barriers, which are distributed extensively along the world’s coastlines. Scientists accept the idea that barrier islands, including other barrier types, can form by a number of different mechanisms.

* Offshore bar theory. Waves moving into shallow water churned up sand, which was deposited in the form of a submarine bar when the waves broke and lost much of their energy. As the bars developed vertically, they gradually rose above sea level, forming barrier islands.
* Spit accretion theory. Sediment moving in the breaker zone through agitation by waves would construct spits extending from headlands parallel to the coast. The subsequent breaching of spits by storm waves would form barrier islands.
* Submergence theory. Settling/sinking of coastal regions, with ridges on land becoming separated from the mainland. Later shown to be incorrect (at least for the particular set of islands the theory was trying to explain) when the ages of the coastal rock and sediment were more accurately determined.

https://en.wikipedia.org/wiki/Barrier_island

https://en.wikipedia.org/wiki/Barrier_island#Formation_theories