My understanding is seamount (underwater volcano) on which coral builds further toward the surface, from depths of &lt; 200&#x27; (~60m), topped with sand which is almost certainly formed from coral.<p>The sand topping will blow way in wind, but with slow sea rise, coral can build up. Coral <i>cannot</i> grow <i>above</i> sea level, so atolls are of necessity <i>close</i> to sea level. With rapid rise, particularly where coral growth is retarded (carbonate levels, other forms of coral blight), you&#x27;ll lose that ability entirely, and the island will flood.<p>More: <a href="https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Atoll" rel="nofollow">https:&#x2F;&#x2F;en.wikipedia.org&#x2F;wiki&#x2F;Atoll</a>