Difference between olcanic island and coralisland

The volcanic islands of Oceania are the peaks of dormant or active underwater volcanoes. Among them there are islands of the area from ten square kilometers to several thousand and they are the main type of islands of Oceania.  

The most famous of the volcanic islands are the Hawaiian Islands, Easter Island, Tahiti and Samoa. Coral Islands of Oceania (Biogenic) in the oceanic shallow water usually settled entire colonies of small marine animals-corals. For centuries, with the death of corals, their skeletons are the bottom of the ocean, pressed and form a mountain breed. Over time, coral reefs and whole islands appear above the surface of the water, and if the coral deposits occurred on the contour of the vent of the underwater volcano, there are atolls-coral islands with a lagoon in the center.  

The Coral Islands (atolls) in Oceania are hundreds, both single and forming whole archipelalloes. These are the Carolinskie, Mariana, Marshall Islands and Gilbert and Tuammotu islands. The largest atoll of Oceania is Quadjalane. Its area is 2.3 thousand square kilometers (including the lagoon area) and belongs to the archipelago of the Marshall Islands.

aa

explanation:

literally the answer is aa. not kidding. aa is a group.