The Hawaiian Islands were formed over a hotspot. Places where hot magma currents rises from deep in the earth are called hotspots. Rising magma from the center of the earth rises to the crust of the earth. When it encounters solid rock and cannot force it aside, the magma collects and pressure builds. Once in a while some pressure is let go, magma escapes, and eruptions happen. After many underwater eruptions, the volcano builds an underwater mountain. The mountain grows with each eruption and when it rises above sea level, it becomes an island.

Most of the earth's volcanoes do not occur over a hotspot. The earth's crust is divided into sections called plates. These plates are constantly moving (about an inch per year). Some plates move apart, and some come together and collide. Most earthquakes and volcanoes happen where plates come together. The Ring of Fire is located at the boundary of the Pacific Ocean Plate (the fastest moving plate). Over half of the world's volcanoes are located on the Ring of Fire. Underwater volcanoes are formed when two plates move apart.