3 - 1 Overview of the eruption

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The volcanic activity of Mt. Fugen (Fugendake) started with an earthquake swarm lasted for about one year. This was followed by a phreatic eruption on 17 November 1990, after 198 years of dormancy. Phreatomagmatic eruption thereafter continued for about half a year, and then a large amount of lava was successively extruded over a period of 3 years and 9 months to form a huge lava dome. Since this lava dome has grown on the east flank just below the summit of Mt. Fugen, it was very unstable and frequently collapsed to bring about many times of pyroclastic flows, whose number amounted to several thousands. Thereby 44 people were killed and 820 houses were burned.
The extrusion of lava ceased finally in February, 1995. The total amount of lava extrusion reached about 200,000,000 m3 (estimated by the Geographical Survey Institute), and almost one half of it has collapsed down. In addition, a great volume of volcanic ash caused debris flows, by which about 1,300 houses were destroyed.
The volcanic activity of 1990 - 1995 can be roughly classified into the following four periods .


Map of Mt. Fugen and surrounding area

Top The 1990 - 1995 eruption

Period of precursory earthquakes (Nov. 1989 - 16 Nov. 1990)
The earthquake swarm began beneath Tachibana Bay (Chijiwa Caldera) (Nov. 1989)
The hypocenters moved to the area of Mt. Fugen cone and volcanic tremors occurred. (Jul. 1990)
Period of phreatomagmatic eruption i17 Nov. 1990 - 19 May 1991j
1) Kujukushima and Jigokuato craters were active. (17 Nov. 1990 - 11 Feb. 1991)
Phreatic eruption began from these craters. (17 Nov. 1990)
2) Byobuiwa crater was active. (12 Feb. 1991 - 28 Mar. 1991)
A new Byobuiwa crater began phreatic eruption. (2 Feb.)
3) The above three craters were simultaneously active. (29 Mar. 1991 - 11 May 1991): Phreatomagmatic eruption
4) Period of heavy change of the crust (12 May 1991 - 19 May 1991)
Inflation of the edifice of Mt. Fugen, thermal demagnetization, strong activation of earthquakes right under the crater, appearance of fissures
Period of formation of lava dome (20 May 1991 - Mar. 1995)
1) The first period (20 May 1991 - Dec. 1992): Formation of lobes, mainly
Start of lava extrusion (20 May), Beginning of pyroclastic flow (24 May)
The large-scale pyroclastic flow (3 June, 8 June and 15 Sep., flowed down to the Annaka areaj
Lava extrusion sharply declined. (at the end of 1992)
2) The second period (Jan. 1993 - Oct. 1993): Formation of lobes, mainly
Lava extrusion started again (2 Feb.)
The large-scale pyroclastic flow (23 - 24 June: flowed down to the Senbongi area)
The large-scale pyroclastic flow (26 Jun. and 19 Jul.: flowed down to the Annaka area)
3) The third period (Nov. 1993 - Feb. 1995) Formation of clastic lava domes, mainly
Small and middle scale of pyroclastic flows run down to both the south and north sides of Mt. Fugen. (Jan. - Sep. 1994)
Cease of lava extrusion (Feb. 1995)
Period of succeeding variation (Mar. 1995 - present)
Cooling and shrinking of the lava dome and the magma conduit, sinking down of the dome


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