Asama-Yama

The Asama-Yama is a Stratovolcan being in the center of the island of Honshu, to approximately 140 km of Tokyo. It is one of the most active volcanos of the Japan.

It dominates of 1.300 m the surrounding plains. This stratovolcano presents two craters which were installed on the remainders of an old stratovolcano. The western crater, called Kurofu-Yama is to 2405 m of altitude. The other crater in the center of Asama is most recent and most active. It is a crater in another crater, oldest called Maekake-Yama, built during the fourth and last stage of evolution of the solid mass which began approximately 10.000 years ago. Its lip which culminates to 2521 m, surrounds an active cone Kama-yama.

This building presented more than one ten eruptions pliniennes, the last eruption of the plinienne type being that of 1783. The Japanese volcanologists indexed approximately 121 known historical major eruptions; 66 eruptions were indexed from 685 to 1900 (of which 12 which caused human losses) and some 2.000 explosive phases were recorded between 1900 and 1960. The last eruptions go back to 1990 and on November 14th, 2004, where a strong explosion occurred. evaluated. But it is the eruption of 1783 which was most outstanding.

The eruption of 1783

The historical eruption most important was that of 1783. It began on May 9th and finished on August 5th of the same year, by an eruption cataclysmale. A volcanic cloud and lahars caused the death of more than 1.500 people. This eruption was responsible, by the cloud of dust and ashes which went up in stratosphere, of serious famines in the north of Japan (300 000 to 1 million victims), and would be responsible with the eruption for Laki in Iceland, at the time of the same year, of a cooling of the climate in the Northern hemisphere.

The volcanic activity of Asama began on May 9th in Maekake-Yama, with a small explosive phase with the sporadic emission of pounces and ashes and the installation of a scoriaceous casting. A new eruptive phase began on July 26th. The activity became continuous then and its intensity nothing but did increase until the first days of August. The essence of the volume of pounces (0,17 km3) was emitted during the two days which preceded the cataclysmic event of August 5th. In the afternoon of August 4th, 0,1 km3 of pyroclastic material was emitted in the form of a volcanic cloud, which descended the northern side of the crater, recovering a surface of 19 km ² of which the major part was occupied by the forest.

August 5th in the morning, 10 local times, a gigantic explosion occurred on the level of the summit crater. She was heard with more than 300 km. In the village of Karaizawa, located at 11 km of the volcano, the roofs of the houses were perforated by incandescent pumiceous blocks and 52 dwellings were completely burnt. Enormous blocks of incandescent lava were ejected, forming the remains avalanche of Kambara (" Kambara volcanic cloud "). Four villages, of which that of Kambara, localized on the northern side of the volcano, were immediately absorbed by the volcanic cloud (477 of the 560 inhabitants perished buried under this avalanche). This one, after having reached the throats of the Agatsuma river, caused the installation of a lahar (a muddy casting) which descended the side of the volcano, destroying more than 1.200 houses and killing more than 1.300 people.

Immediately after this paroxysmal phase, a long lava flow 5,5 km is emitted. This casting called Oni-Oshidashi (" Oni-Oshidashi washed flow "), a volume of 0,17 km3 presents. Then the eruption ceased very quickly. The lava emitted at the time of this eruption is an andesite with augite-hypersthène and olivine. The total volume of emitted material was estimated at 0,45 - 0,50 km3, of which approximately the third in the form of pumiceous repercussions. This value is however lower of two hundred times than that allotted to the eruption of Tambora (Indonesia) in 1815. Part of projections would have been injected into troposphere (between 6 and 17 km of altitude) and into stratosphere (between 17 and 50 km of altitude).

Be-X-old: Асама

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