Vulcanian eruption
From Wikipedia, the free encyclopedia
Giuseppe Mercalli witnessed the 1888-1890 eruptions on the island of Vulcano. His description of the eruption style is now used all over the world. Mercalli described vulcanian eruptions as "... Being like cannon fire at long intervals... ." Their explosive nature is due to increased silica content of the magma. Increasing silica levels increase the viscosity of the magma which means increased explosiveness.
Almost all types of magma can be involved, but magma with about 55% - or more silca - basalt-andesite, is most common.
Vulcanian eruptions usually commence with phreatomagmatic eruptions which can be extremely noisy due the rising magma heating water in the ground. This is usually followed by the explosive throat clearing of the vent and the eruption column is dirty grey to black as the old weathered rocks are blasted out of the vent. As the vent clears further ash clouds become grey-white and creamy in colour with convulations of the ash similar to those of plinian eruptions. This phase is followed with production of viscous lava containing high amounts of gas and producing glassy volcanic ash. Pyroclastic flows are known to occur. - Stromboli 1930, Montserrat 1995 to present, Mount Unzen 1991-1995 and others.
The tephra is dispersed over a wider areas than that from effusive - Hawaiian or Strombolian eruptions. The pyroclastic rock and the base surge deposits form an ash volcanic cone while the ash covers a large surrounding area. The eruption ends with a flow of viscous lava.
They can throw large metre size blocks several hundred metres, blocks have been hurled a few kilomteres too. The 11th September 1930 eruption of Stromboli was a vulcanian eruption. It started at 08:10hrs (local) when ash was vented for about 10 minutes. Then at 09:52hrs (local) two incredibly powerful explosions occurred which shook the whole island. Blocks were hurled about 2km. These fell out of the sky smashing through buildings etc. A tsunami 2 to 2.5m high was generated. By 10:40hrs (local) the explosive phase of the eruption was over. Expulsion of lava followed and this flowed down the Sciara del Fuoco lasting into the night. At the same time incandescent scoria flowed down the Vallonazzo Valley and entered the sea near Piscità.
When it was over 6 people had died, four fishermen at sea when the avalanches of hot scoria caused the sea to become very disturbed. One person was killed in Stromboli village by falling blocks., whilst the 6th was killed by the tsunami. It is believed that water entered the conduit due to a partial collapse of the conduit. The water flashed into steam a took the easiest "escape route," via the open conduit. As it expanded in the molten magma it generated the two very large explosions. The ultimate cause of the collapse of the conduit is believed to have been due to an influx of fresh magma.
There are several sources which have details of the 1930 explosion, including Italian Volcanoes, auth. Chris Kilburn and Bill McGuire, Terra Publishing, ISBN 1-903544-04-1 paperback and Volcanoes of Southern Italy, John Guest, Paul Cole, Angus Duncan and David Chester., published by The Geological Society (London) ISBN 1-86239-138-6
A good publication to all volcanic activity is the Encyclopedia of Volcanoes, Haraldur Sigurdsson, Bruce Houghton, Stephen R McNutt, Hazel rymer and John Stix (eds) Academic Press, ISBN 0-12-643140-X and is considered to be the volcanologists "bible."
The eruption of Soufrière Hills Volcano, Montserrat, from 1995 to 1999, T. H. Druitt and B. P. Kokelaar (eds), Geological Society Memoir No. 21. Pub. Geological Society (London), ISBN 1-86239-098-3, contains a lot of information regarding the several vulcanian eruptions that have, and continue to, occur at the Soufrière Hills Volcano.
