Earth's crust is fractured into 13 major and approximately 20 total lithospheric plates. Most of the Earth's volcanoes are located at lithospheric plate boundaries. The belt of volcanoes along the boundary of the Pacific Ocean is called the Ring Of Fire because it contains about 75% of the world's active volcanoes. Why does a volcano erupt? When magma rises towards the surface, gas bubbles expand and increase in pressure. The force of the gas pushes the magma out of the vent and causes an eruption.