Zao Volcano
1: Introduction - 2: Overview of geology for Zao Volcano and surrounding area
3: Topography of Zao Volcano
4: Eruptive history of Zao Volcano
5: Eruptions during historic times
6: Petrological characteristics of rocks of Zao Volcano - 7: Recent conditions - 8: Observation system for volcanic activities
9: Notes on volcanic hazard in the future
Acknowledgements / Reference
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9: Notes on volcanic hazard in the future
Based on the recent researches, the most likely craterd of the future eruption is Okama, and the eruption style is expected to be similar to that of past Okama eruptions. According to recorded eruptions data, the eruptive activities involving the Okama crater will be progressed in the following order: 1) changing in the color of Okama lake, increasing lake water temperature, floating sulfur on the lake surface, steam/gas rising from the lake (e.g., during 1918–1928 and 1939–1943); 2) phreatic or phreatomagmatic eruptions and ash fall, pyroclastic surges, overflow of crater and lahars reaching far distances (e.g., the 1894–1897 eruption).
The eruptions tended to occur repeatedly with dormant periods over several months to several years. They began with phreatic eruptions and sometimes changed to phreatomagmatic eruptions. According to the recorded cases, the scale of individual eruptions has been relatively small, with VEI of 1 to 2. However, pyroclastic surges have occurred with ash fall over a wide range of area and volcanic blocks around the crater (Miura et al., 2012); therefore, the area of crater is highly dangerous during high activity periods.
In distant locations, in addition to damages caused by ash falls, landslides and floods have occurred owing to lahars from the crater lake. Therefore, even a small eruption can cause damages to areas near rivers on the foot of the volcano. In the case of the eruptions on similar volcanoes, the lake surface rising by the change of the hydrological environment, resulted in collapse of the crater edges and the flooding. Therefore, areas along the downstream of Okama should be aware of such disasters.
The most recent eruption did not cause a large amount of ash fall on the foot of the mountain; however, lapilli fell on the foot of the mountain during the 1230 eruption. Furthermore, lava flows have not recorded on historical eruptions, although lava flows are exactly found in the last activity in Stage VI. Therefore, occurrence of lava flow should also be considered. The maximum activity during Stage VI was an eruption of VEI 4 that released Za–To 2 tephra. We should consider not only the most recently VEI ~2 but also VEI 4 level eruptions.