Hainan Island is located in the marginal, relatively shallow, tectonically stable part of the South China Sea, which is bounded to the south by the ndonesian plate collision zone and to the west by the subaqueous extension of the Red River strike-slip fault zone. The island is composed of various rocks, primarily Paleozoic to Mesozoic granite and metamorphic rocks to the centre and south, Mesozoic to Cenozoic basaltic rocks to the north, and thin cover of Quaternary sediments mainly to the northeast and east. It is dissected by several morphotectonic lineaments (major faults), some very old, others recent directed NE-SW and E-W. During the last two millions years or so, an interplay between neotectonic forces and climatic changes (glaciations) was responsible for the change in relative sea level of about 120 -150 m.

The southern area of Hainan Island is characterized by a shallow offshore area, a promontory-and-embayment coastline, and relatively narrow sandy coastal plains bounded by terraced bedrock hills and backed by mostly granite mountains. The steep slopes of the mountains are associated with a major east-wet trending fault. The sandy costal plains have evolved as response to the various Pleistocene transgressions and regressions. Their component sand was derived from the intense weathering of the granite under subtropical to tropical climate, transported to sea by seasonally flooding streams. It was reworked by monsoon (mainly south and southeast) waves in a generally micro-tidal setting, and it was redistributed by local cellular marine currents into a series of coastal bars separated by sandy lagoons. In general, there is very little clay preserved in the coastal plains. Remnants of pre-Pleistocene, highly indented shorelines occur as terraces on the hills at different elevations ranging from 80 m to 40 m. Lower, more recent terraces at 20, 10 and 5 m asl record the development of the coast during Late Pleistocene and Holocene. In the coastal plains, where best exposed in the Sanya region, there are up to eight to nine costal bars, from small residual inland uplifted ones around promontories of hills to the recent extensive (approximately 10-15 Km long and 5-600 m wide) more recent bars that occur along the present shore. The most recent bar has wells sorted, loose medium light gray quartzose sand, generally showing well developed structures, such as plane bedding and cross bedding. The sand of the older bars becomes progressively more weathered (reddish in colour) with development of some interstitial fines and light cement, and the internal structure becomes apparently massive. The internal architecture of the more recent bars can be readily defined using Ground Penetrating Radar (GPR). This shows that the bars have developed over bedrock, reefal platforms or beachrock. Their deeper horizons have characteristic lexeme-like GPR facies of shoreface to coastal sand, and their top parts are characterized by locally well developed, stacked, seaward prograding 3-4 m thick cross beds. Where not stripped by human activities, aeolian dunes generally cap them. The total sand thickness of the bars reaches up to 8-10 m.