The sand and pebble sediments form the beaches and spits around the island in some cays in the Spratly Islands. Under the influence of the dominant wind direction, which changes seasonally, these sediments move around the island to change the shape and size of the island. For example, Spratly Island is larger during the northeast monsoon, (about 700 × 300 meters), and smaller during the southwest monsoon (approximately 650 × 320 meters).
About Truong Sa information, some islands may contain fresh groundwater fed by rain. Groundwater levels fluctuate during the day with the rhythm of the tides.
Phosphates from bird faeces (guano) are mainly concentrated in the beach rocks by the way of exchange-endosmosis. The principal minerals bearing phosphate are podolite, lewistonite and dehonite.
Coral reefs are the predominant structures of these islands; the Spratly group contains over 600 coral reefs in total. In April 2015 the New York Times reported that China were using “scores of dredgers” to convert Fiery Cross Reef and several other reefs into military facilities (runways, etc.).
Little vegetation grows on these islands, which are subject to intense monsoons. Larger islands are capable of supporting tropical forest, scrub forest, coastal scrub and grasses. It is difficult to determine which species have been introduced or cultivated by humans. Taiping Island (Itu Aba) was reportedly covered with shrubs, coconut, and mangroves in 1938; pineapple was also cultivated there when it was profitable. Other accounts mention papaya, banana, palm, and even white peach trees growing on one island. A few islands that have been developed as small tourist resorts had soil and trees brought in and planted where there was none.