Case Study Summary_James



Mt. Unzen Volcano, Japan on June 3, 1991 (Sadly enough,  the people shooting this video were killed as the volcano erupted due to a misjudgement in location.)

Japan is the source for subduction zone volcanos which results from two oceanic crust that form into and island arc. The Unzen volcano complex makes up most of the Shimabara Peninsula east of Nagasaki. The Unzen volcano complex makes up most of the Shimabara Peninsula east of Nagasaki. Two oceanic plates that are on either side of the convergent boundary causes subduction to occur. As the result of one plate subducting under the other it causes a trench to form. Once this happens, deep, shallow earthquakes occur. As this is happening, an arc of volcano forms. Magma is lava and pressurized gases. Subduction zone volcanoes also create felsic magma. Felsic magma is thick and sticky or very viscous.The Unzen volcano had trapped gas which then caused an explosive eruption that was composed of layers of ash and lava.There was an increase in seismic activity that was measured and seen about a year prior to the eruption and immediately following the eruption. The volcano began to undergo small and minor eruptions starting on November 17, 1990. Thirteen domes were formed and before the 1991 eruption almost all of the domes were active. A forced evacuation was enacted.

Because of the immense mixing of ash and lava the eruption resembled a large black cloud that carried toxic gases that were heavier than the air.

Deaths were caused by heat and asphyxiation from the toxic gases. The heat of the flow caused hair and clothes to be burned off of the bodies of victims that were found later and brought in for further examination by medical teams. 43 people were killed. Several bodies were unable to be recovered.

The years immediately following 1991, forced many people to leave their homes from the ongoing after effects and threats of future volcanic occurrence that were being proposed.




In 1792 was the last time Unzen erupted killing 15,000 people.I don’t think that the population (1991) was fully informed and aware of the grave danger they were in. Their guards were down because the volcano had not erupted during their life time. The most pre-planning it seemed was the evacuation that was put in to affect.43 people were killed. There were 2,593 buildings damaged. 1,270 were residential homes. The cost of damages of agricultural and fish facilities due to the volcano were 17.4 billion yen.



3 thoughts on “Case Study Summary_James

  1. It seems Mt. Unzen is a perfect example of a subduction zone volcano. The felsic magma, comprised on lava and pressurized gases, built up over such a long period of time and eventually erupted.

    I really liked that video because it showed the pyroclastic flow moving down the valley at an extremely high velocity. It left no time for the reporters to escape the pyroclastic flow. As the video says, the reporters set up their equipment inside the evacuation zone out of indifference. They were aware of the risks, but chose to ignore them because the have worked so closely with volcanoes.

    I thought it was really interesting how seemingly low the death toll was. What was the population at the time of the eruption? Even though Mt. Unzen hadn’t had an eruption in hundreds of years, it seems like the population listened to the evacuation order.

  2. You wrote a great summary. It’s very thorough and you give lots of good facts and details. I like that you mentioned the eruption in 1792, and made the connection that people weren’t prepared because they hadn’t seen it erupt. Its interesting that the volcano complex was somewhat active for a while before the real eruption.
    This was very interesting to read, especially after doing my case study on Japan too. In my research, I learned about the meeting of several plates gives Japan frequent tectonic activity.
    I liked watching the video, I felt like I got a good account of what pyroclastic flow is like. It moves so fast! Isn’t it frustrating to see that these people knew there was danger but ignored it? It’s good that the rest of the population heeded the evacuation order, it’s better to be safe then sorry.

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