The full map's landmasses should look like 2300 A.D., not the current 1999 A.D. Although Porre probably did still exist before Lavos erupted, even though the continent seems to have sunk, since there's a skyscraper in front of the Sun Palace entrance. Although to scale, that skyscraper may represent an entire city or something, so maybe southern Zenan did disappear and leave behind only an island. Hard to say.