Vulcanian eruption

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 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, for example the eruptions of Stromboli 1930, Soufrière Hills 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. Vulcanian eruptions may throw large metre-size blocks several hundred metres, occasionally up to several kilometres.

The term Vulcanian was first used by Giuseppe Mercalli, witnessing 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 "..."Explosions like cannon fire at irregular intervals"... ." Their explosive nature is due to increased silica content of the magma. Almost all types of magma can be involved, but magma with about 55% - or more silca - basalt-andesite, is most common. Increasing silica levels increase the viscosity of the magma which means increased explosiveness.

Vulcanian eruptions are dangerous to persons within several hundred metres of the vent. One feature of this type of eruption is the Breadcrust bomb. These can be blocks often 2 to 3 m in dimensions. At Galeras a vulcanian eruption ejected bombs which impacted with several volcanologists who were in the crater and many died or suffered terrible injuries.

1930 eruption of Stromboli

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 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 the explosive phase of the eruption was over. Expulsion of lava followed, 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.

By the end, 6 people had died. Four fishermen died 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, and 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.

References

*"Italian Volcanoes", Chris Kilburn and Bill McGuire, Terra Publishing, ISBN 1-903544-04-1
*"Volcanoes of Southern Italy", John Guest, Paul Cole, Angus Duncan and David Chester, The Geological Society (London) ISBN 1-86239-138-6
*"Encyclopedia of Volcanoes", Haraldur Sigurdsson, Bruce Houghton, Stephen R McNutt, Hazel rymer and John Stix (eds) Academic Press, ISBN 0-12-643140-X
*"The eruption of Soufrière Hills Volcano, Montserrat, from 1995 to 1999", T. H. Druitt and B. P. Kokelaar (eds), Geological Society Memoir No. 21. Geological Society (London), ISBN 1-86239-098-3,
* [http://volcano.und.nodak.edu/vwdocs/vwlessons/kinds/kinds.html Kinds of volcanic eruptions]