Volcanic eruptions do not occur just anywhere. Sixty percent of all active volcanoes are found at plate boundaries such as the Pacific plate, often called the Ring of Fire because of the active volcanoes on its perimeter. Where the plates under the earth are moving apart or colliding with one another, volcanoes may form. Many volcanoes form oceanic islands in the Pacific Ocean or Mediterranean Sea. These volcanoes form over “hot spots” in the crust and mantle. The Northern Hemisphere has approximately two-thirds of the world's land-based volcanoes.