Volcanic eruptions come in many different forms. Shield volcanoes usually only spew lava and hot gases. These lavas flow slowly down the mountain with speeds of 15 miles per hour or slower. Composite volcanoes can put forth lava accompanied by clouds of ash, bombs, lava fragments, crystallized, glassy material, as well as hot gases. In some eruptions, ash and lava are buoyied by hot vapors and pour down the slopes of a volcano very rapidly, with speeds up to 100 miles per hour. This special type of eruption destroyed the city of St. Pierre in 1902. In other cases hot material from the volcano can melt snow and ice at the volcano summit and the whole mass of mud and lava can sweep rapidly down the mountain, destroying everything in its path. This type of flow is called lahar.(Lahars are mudslides caused by the mixing of volcanic ash and debris with water. They can occur when the heat from a volcano melts snow and ice on the volcano’s summit, or if an eruption disturbs a crater lake.Lahars can cause great environmental and economic damage. They can cover fertile fields and topple buildings. Lahars are very dangerous, and anyone caught in the path of one is in great danger of death from severe crushing injuries.)
There have been some spectacular eruptions in Earth history. These include Mt. Pelee, Krakatao (see also : Indonesian Volcanoes), Crater Lake (formerly Mt. Mazama), Mt. Vesuvius, Mt. St. Helens, Mt. Pinatubo.