Here's why:

* Volcanoes: Volcanic islands are formed by eruptions from underwater volcanoes. Over time, the lava builds up and eventually rises above the surface of the water.

* Mountain Formation: The volcanic mountain itself is formed by the accumulation of lava flows, ash, and other volcanic materials.

Some famous volcanic islands include:

* Hawaii: Formed by a chain of volcanoes.

* Iceland: A country almost entirely made up of volcanic islands.

* Mount Fuji: A famous volcano and island in Japan.