1792 Eruption of the Unzen Volcano

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  • Topic: Volcano, Pyroclastic flow, Lava
  • Pages : 2 (668 words )
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  • Published : April 14, 2013
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Mike Lillis
GEO 100
Unzen, Japan 1792
In 1792 Mount Unzen erupted which devastated Japan. Mount Unzen is a group of active overlapping stratovolcanoes. They are located on the island of Kyushu which is Japan’s most southern main island. Not only was the Eruption deadly but the Mayu-Yama Lava domes collapsed which triggered a Mega-Tsunami that killed nearly 15,000 people. This was one of Japans worst volcanic disasters. The Volcano has been dormant for 198 years until the early 1990’s when the volcano became active and erupted again in November 1991 creating a pyroclastic flow which killed 43 people. A Pyroclastic flow is a fast moving current of hot gas and rock known as tephra. The flow all depends on the density and gradient of the slope. The pyroclastic flow consists of two parts. The Basal flow, flows on the ground and contains coarse boulders and an extremely hot ash plume. Another good example of a pyroclastic flow was seen in the Pompeii.

The Unzen eruption in 1792 started with a dacitic lava flow coming from the Fugen-Dake lava tube. Suddenly the Mayu-Yama dome collapsed unexpectedly following a post eruption earthquake which started a deadly landslide and mega-tsunami which reached over 300 feet tall. The lava that flowed from the Unzen eruption was very Basaltic which meant it was Mafic. The lava was very hot and had a low viscosity and silica content. This lava can flow rapidly and for long distances. Unzen contained several lava tubes which acted as a conduit for the Basaltic lava. A cooled crust formed on top of the flow which prevented cooling and allowed flow under the cooled crust. The Volcano showed signs of Pahoehoe after the eruption which is basalt with a glassy ropy texture. The Unzen eruption also created landslide debris and Lahars which acted as transports for water rich slurries. The 1792 eruption was Effusive compared to the 1990s eruption which was explosive.

The Unzen Volcano range is part of the Pacific Ring...
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