The okinawa and amami great island are the biggest among the ryukyu islands. how were these bigger islands created?

The Okinawa ans Amami Great Island were created by volcanic activity from the sea bed, where the Eurasian and Pacific continental plates separating line is. The pressure from the mantle layer managed to get enormous amounts of magma up to the surface, thus lifting the sea floor to the surface and creating an island. Since there were multiple volcanic activities in very close proximity, the newly created little islands were merging into one and were creating bigger islands. The best example to observe nowadays is the Japanese island of Nishimoshima which is in the beginning faze of this process.

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