● crustal activity

Due to crustal movement, land subsidence or sea level rise, part of the land is separated from the mainland into islands. As a result, islands can be divided into mainland islands and islands. The mainland island is the "home" of the mainland. Most of them are distributed in color chains around the continental margin. Geologically, it is connected with the nearby mainland, but due to crustal changes or rising sea water, some land is surrounded by water to form islands. China Taiwan Province Island is the most typical mainland island.

● Volcanic eruption or coral secretion

It is a coral reef formed by the accumulation of eruption (mainly lava) produced by submarine volcanism, or by the accumulation of coral secretions and remains. The island formed by submarine volcanic eruption and volcanic eruption accumulation is called volcanic island. Hawaii Island in the Pacific Ocean is a typical volcanic island. Coral polyps are the main force in shaping coral islands. The island formed by the accumulation of coral remains is called coral island. Coral islands are mainly distributed in the tropical shallow sea area between 20 north and south latitude, and the shallow sea in the Pacific Ocean is relatively concentrated, such as the Great Barrier Reef in northeastern Australia. Most of the South China Sea Islands in China are coral islands.

● Sediment accumulation

It is formed by the accumulation of sediments in rivers and lakes. An alluvial island is an island formed by alluvial rivers or waves. Chongming Island, located at the mouth of the Yangtze River in China, is the largest alluvial island in China.