Ocean chain pool

In fact, the bottom of the trench is the junction of the continental and oceanic crust, but they do not live in harmony here, just like two "Hercules" colliding with each other. Because of the high density and low position of the oceanic crust, and the heavy sea water on its back, it can't lift its head, so it has to dive down to the lower part of the continental crust and arch up the land to make the continental crust rise and bend into an island. This is why the trench is mostly associated with island arc. When the island arc is pushed by the ocean bottom, it will keep rising, and the slope on the continental side will inevitably become steep, which naturally becomes what it is now

Island arc a long string of islands on the continental margin. A long curved oceanic island chain associated with strong volcanic activity, seismicity and orogeny. Such geological features as Aleutian Alaska arc and Thousand Island Kamchatka arc
the island is mainly mountainous and adjacent to deep trench. The Western Pacific island arc is the most typical one, which is divided into two parts: the north part is composed of thousand island islands, Japan islands, Ryukyu Islands, Taiwan Island and Philippines islands, facing the Pacific Ocean, which is the East Asia Pacific island arc! The southern section is composed of Andaman Islands, Nicobar Islands, Sumatra island, Java island and Nusa Tenggara islands, which protrudes to the Indian Ocean and is called the Sunda island arc of the Indian Ocean. The two arcs connect in Sulawesi island. The Western Pacific island arc is located in the chimeric zone of Pacific plate, Eurasian plate and Indian Ocean plate, with unstable crust and volcanic earthquakes. According to statistics, there are more than 500 active volcanoes in the world, more than half of which are concentrated in the island arc zone; 95% of the global earthquake energy is also released here. Frequent volcanic activity caused by magma eruption makes the island arc zone become the most mineral rich area in the world
island arc is divided into:
Inner island arc. The continental side is the contact zone between oceanic plate and continental plate, where volcanoes and earthquakes are concentrated< (2) outer island arc. Near the ocean side, there is no volcanic seismic zone

According to the crustal structure of island arc, there are two different genetic types. A typical continental crust underlying the island arc has a granitic basement. They are usually composed of older islands, which are distributed close to the mainland. It is generally believed that they are fragments separated from the mainland e to back arc expansion, such as the Japanese archipelago; The other is the transitional crust between ocean and continent underlying island arc, which is generally composed of young islands, far away from the continent and distributed in the ocean, such as Mariana Islands. When one oceanic plate subcted under another oceanic plate, island arcs developed and grew on the basis of oceanic crust. Due to the accumulation of magma and sediment, the crust of such intra oceanic island arcs graally thickened and transformed into continental crust