Pacific islands are composed of more than 1, islands, which can be divided into mainland islands and sea island according to their causes, and sea island can be divided into volcanic islands and coral islands. Continental islands are similar to or associated with the mainland in geological structure, generally located near the mainland. They were originally a part of the mainland, but they were separated from the mainland because of crustal subsidence or sea level rise, such as Irian Island, South Island and North Island of New Zealand. This kind of island is generally large in area and high in terrain. Volcanic islands are formed by the accumulation of volcanic eruption materials, mainly lava, on the seabed, such as the Hawaiian Islands in Polynesia, where volcanoes are still active, characterized by high altitude and steep terrain. Coral island is an island formed by coral reefs exposed from the sea or a sand island formed on coral reefs. Most of the three islands in the Pacific Ocean belong to coral islands, which are characterized by small area, low topography and ring shape. There is a lagoon in the middle, which is connected with the ocean with gaps, and the lake is calm, making it an excellent haven for ships. Coral reefs are formed by the accumulation of coral remains. Coral is a coelenterate, which lives in tropical shallow sea with an average water temperature of 25 ~ 3℃ and high salinity and transparency. After death, calcareous bones form limestone. At first, coral grew around the island to form a shore reef. With the sinking of the island, the coral reefs continued to grow outward and upward to form a barrier reef. When the island continued to sink without entering the sea, the coral reefs around the island formed an atoll shape. Originally, the top of the island was submerged by seawater and became a lagoon.