These are the actuate chains of volcanic islands which mostly occur adjacent to continental margins. These are the sites of greatest instability found anywhere on the earth and are the most active of the various segments of the crust.
Here, processes involving the crust are in action even at the present time. It is now recognized that these are the places where the present organic (period of mountain building) and igneous (crystallized magma) activities are taking place.
Most of the island arcs are located in the Pacific Ocean where they almost form a rim around the entire northern, western and southern margins. Others are located in the Atlantic, especially in the Caribbean Sea. The followings are considered to be the modern island-arcs-
(i) New Zealand & Tonga
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(ii) Melanesia
(iii) Indonesia.
(iv) Phillipines.
(v) Formosa and West-Japan.
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(vi) Marianas and East-Japan.
(vii) Kurile and Kamchataka.
(viii) Aleutian and Alaska.
(ix) Central America.
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(x) West-Indies.
(xi) South America.
(xii) Western Antarctica.