nother type of mountain in the sea is an island. It rises from the sea floor, but reaches above the ocean’s surface, sometimes just barely. An island is a solitary mountain formed by volcanic activity. Lava erupting from the sea floor builds up on the sea bed over thousands, or even millions of years. The lava cools rapidly as it reaches the ocean water and forms solid rock. Each eruption of the sea floor volcano builds up the volcano a little bit higher. Until one day, the eruptions have built the top of the undersea mountain so high, that it rises above the ocean’s surface.