Most volcanoes are mountains, particularly cone-shaped ones, which were built up around the opening by lava and other materials thrown out during eruptions. Eruptions of volcanic mountains are spectacular sights. In some eruptions, huge fiery clouds rise over the mountain, and glowing rivers of lava flow down its sides. In other eruptions, red-hot ash and cinders shoot out the mountaintop, and large chunks of hot rock are blasted high into the air. A few eruptions are so violent they blow the mountain apart.
Some eruptions occur on volcanic islands.
Such islands are the tops of volcanic mountains that have been built up from the ocean floor by repeated eruptions. Other eruptions occur along narrow cracks in the ocean floor. In such eruptions, lava flows away from the cracks, building up the sea bottom. People have always been both fascinated by the spectacle of volcanic eruptions and terrified of their power. Eruptions have caused some of the worst disasters in history, wiping out entire towns and killing thousands of people.