I am envisioning a scenario in which, probably for reasons of persecution or overpopulation, a people would begin to construct islands in the sea. Each island would be fairly close to the others and the society would want to be able to support agriculture on the islands. I had two questions about this idea:
Would the only way in which an island could come about in the ocean be by volcanic (and non-magical) means (eg like Surtsey) - or would it be possible for them to construct non-volcanic islands simply by transporting the necessary materials to a favourable location in the sea and beginning construction? I know that China is currently constructing artificial islands by non-volcanic means, but the society's technology would be roughly similar to that of Europe during the Age of Exploration, so this may affect the techniques that they could use. Would they be limited to flat islands or could they construct rock features such as mountains etc?
If this was possible what materials would they need to use for the various layers supporting the island? - going on the premise that each island would be roughly the size of a small city or town.
Thanks for any help