 Its land area is 17.467 km2 (6.7439 sq mi, or 4,316.1 acres), and there was no permanent resident population at the 2000 census.Fire Island is underlain by sedimentary rocks, atop which lie deep sand and gravel deposits from the surrounding tidal estuary.The island is ringed by steep bluffs that average about high, and the land elevation ranges from 25–90 above sea level.