Vulcanian eruption?

1 ANSWERS

1. GiGi
   

   Vulcanian eruptions are a type of volcanic eruption characterised by a dense cloud of ash-laden gas exploding from the crater and rising high above the peak. They 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 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.

   Report (0) (0)  |   12 years, 9 month(s) ago