An island is formed when magma builds up and breaks the ocean’s surface. In some cases, like the island of Hawaii, land masses merge together. Each volcano is a bit different, and so are the rates at which they form. For many volcanoes, formation can take thousands of years, though some volcanic islands can sometimes appear quite suddenly.

When the tops of the volcanoes appear above the water, an island is formed. While the volcano is still beneath the ocean surface, it is called a seamount. Oceanic islands can form from different types of volcanoes. One type forms in subduction zone s, where one tectonic plate is shifting under another.