why do some volcanoes erupt explosively whereas others erupt quietly

Some of the factors are the amount of gas in the lava (lava compositions can be quite different) and another factor may be the cooling causing rapid fracturing. See the link for better information. Below are some excerpts. Phreatic eruptions (or steam-blast eruptions) are driven by explosive expanding steam resulting from cold ground or surface water coming into contact with hot rock or magma. Fuel-coolant reactions may be more critical to the explosive nature than thermal contraction (Starostin et al 2004). Fuel coolant reactions fragment the material in contact with a coolant by propagating stress waves widening cracks and increasing surface area leading to rapid cooling rates and explosive thermal contraction (Heiken Wohletz 1985).


Plinian eruptions are usually the most powerful, and involve the explosive ejection of relatively viscous lava. Large plinian eruptions Б such as during 18 May 1980 at Mount St. Helens or, more recently, during 15 June 1991 at Pinatubo in the Philippines Б can send ash and volcanic gas tens of kilometres into the atmosphere. The resulting ash fallout can affect large areas hundreds of miles downwind. Fast-moving pyroclastic surges and pyroclastic flows together with Бnuцes ardentes,Б are often associated with plinian eruptions.
Some of the factors are the amount of gas in the lava (lava compositions can be quite different) and another factor may be the cooling causing rapid fracturing.


See the link for better information. Below are some excerpts. Phreatic eruptions (or steam-blast eruptions) are driven by explosive expanding steam resulting from cold ground or surface water coming into contact with hot rock or magma. Fuel-coolant reactions may be more critical to the explosive nature than thermal contraction (Starostin et al 2004). Fuel coolant reactions fragment the material in contact with a coolant by propagating stress waves widening cracks and increasing surface area leading to rapid cooling rates and explosive thermal contraction (Heiken Wohletz 1985).


Plinian eruptions are usually the most powerful, and involve the explosive ejection of relatively viscous lava. Large plinian eruptions Б such as during 18 May 1980 at Mount St. Helens or, more recently, during 15 June 1991 at Pinatubo in the Philippines Б can send ash and volcanic gas tens of kilometres into the atmosphere. The resulting ash fallout can affect large areas hundreds of miles downwind. Fast-moving pyroclastic surges and pyroclastic flows together with Бnuцes ardentes,Б are often associated with plinian eruptions.

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