The 1813 eruption of Suwanose-jima

There occurred an gigantic eruption in Suwanose-jima in 1813 AD, 10th year of Bunka era. This is called "Bunka eruption." Suwanose-jima island is thought to have been a peace island before the eruption. In "Jittou joukyou-roku", one of a few old literature that refer to this island, says, 'Slopes were covered with many pine trees from the top to the foot of Mt. O-take,' indicating that this volcano had been fairly inactive, or less active than today, before "Bunka eruption". I studied about this eruption and constructed the sequence of it. It is as follows.

One day in 1813 AD, there occurred an phreatic eruption which emitted white volcanic ash moderately followed by the alternation of magmatic and phreatomagmatic eruptions. As ejecta by these eruptions are deposited around "Tongama" at the southern edge of the ridge of Mt. O-take, eruptions of this phase are thought to have occurred at "Tongama crater" at the southern end of the 1813 crater row. The magmatic eruptions are considered to be those of strombolian for its small area (R = 2 km) of distributuion.

After that, the greatest event in Bunka eruption occurrd. This is classified to sub-plinian type. The eruption column might have reached as high as about 20 km, and brought about huge amount of pyroclasts, thick air fall deposit of scoriae (Fig. 2), all over the island.This pyroclastic fall deposit can be devided into 9 units by comparing morphologic characters of ejecta and named unit A to unit I in acsending order. You can easily indentify these units in the south part of the island (residential area). Out standing layers are unit F and G. The former is an alternation of sub-unit layers consisted of black poorly-vesicular scoria (Mean diameter is about 1 mm). The latter is a thick layer (about 20 cm at the residential area) of black to brown highly vesicular scoria (Mean diameter is about 3-5 cm).

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