Volcanic mountains form when molten rock erupts onto the Earth's surface. They can either form on land or in the ocean. The Cascade Range in Washington, Oregon and northern California is made of volcanoes. Some of the largest volcanoes are on divergent boundaries, which form the mid-ocean ridges. The mid-ocean ridges have big volcanic mountain chains that run through the Atlantic, Pacific and Indian Oceans. The mountains in the mid-ocean ridges can grow tall enough to create islands such as Iceland or the Azores.