Because of the volcanoes formed above the island. Those volcanoes released magma which made the volcanic island.
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The explanation of @BANANAMANXD is correct but it made of volcanic island arc not volcano island / volcanic island. kapag volcanic island arc maraming volcano na mag ka kahelera. kapag volcano island naman isla lng sya na may bulkan.kagaya ng taal volcano. tandaan: volcanic island arc maraming bulkan na mag kahelera. volcano island isang bulkan. kapag volcanic island lng maraming bulkan pero kalat kalat yung bulkan.