This is an image of the eruption of Mt. Pinatubo.
Click on image for full size
Image from: U.S. Geological Survey
Volcanic eruptions come in many different forms. Shield volcanoes typically spew lava accompanied only by 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 is buoyied by hot vapors and pours down the slopes of a volcano very rapidly, with speeds up to 100 miles per hour. This is called pyroclastic flow. 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 a lahar.
Because eruptions release significant amounts of gas into the atmosphere from the deep interior of the earth, volcanic eruptions are considered to add to the world's inventory of atmospheric gases. Moreover, if an eruptive column of gases and ash reaches the stratosphere, it can have a significant affect on the weather.
There have been some spectacular eruptions in
Earth history. These include Mt. Pelee, Krakatoa, Crater Lake (formerly Mt. Mazama), Mt. Vesuvius, Mt. St. Helens, Mt. Pinatubo.
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