According to Wikipedia, a volcano is a rupture in the crust of a planetary mass, such as earth. They allow hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. Earth's volcanoes occur because its crust is broken up into 17 major tectonic plates. The plates float on a softer layer in its mantle. Therefore, on Earth, volcanoes are generally found where tectonic plates are diverging or converging. The diverging plates pull apart, convergent plates come together. Erupting volcanoes can pose many hazards, volcanic ash threatens aircraft, particularly those with jet engines. Eruptions can also affect temperature blocking the sun with volcanic ash and droplets of sulfuric acid. There are also volcanoes of the moons of Jupiter, Saturn, and Neptune Called cryovolcanoes, or ice volcanoes. Underwater volcanoes can also cause tsunamis to occur. Mt. St. Helens also caused a land slide.
Top 10 Deadliest Volcanic Eruptions in History