鹿児島湾北岸におけるマグマ水蒸気噴火とこれに影響を与えた縄文海進

Abstract

In the volcanic areas to the north of Aira caldera, southern Kyushu, two maars named Sumiyoshi-ike and Yonemaru were formed by phreatomagmatic eruptions in the early stage of Holocene. These maars were located in a lowland composed of coastal deposits. In this paper, we discuss first the characteristics of these eruptions on the basis of distribution, structure of pyroclastic deposits called Yonemaru and Sumiyoshi-ike tephra formations which were produced by these eruptions. And then the paleo-environment of the vents is discussed with special reference to coastal changes associated with Holocene sea level rise, because the nature of such eruptions was possibly affected by water condition around eruption plumes.Yonemaru tephra formation, 0.04km3 in volume, is characterised by many thin consolidated beds containing abundant scoriaceous ash with accidental lapilli and block. These thin beds show plane bedding forms, with intercalating wavy or lenticular beds. Such structural features and distribution highly affected by topography indicate that this eruption was phreatomagmatic and dominantly generated a base surge flowing towards the southeast to a distance of about 3 or 4km from the vent, with minor amount of tephra falls. Sumiyoshi-ike tephra formation, 0.005km3 in volume, also emblaces alternating thin ash and scoriaceous lapilli beds with accidental blocks, generally coarser than Yonemaru tephra. Structural features and distribution of this formation indicate that this eruption was also phreatomagmatic, and produced mainly tephra falls dispersed north of the vent.A chronological study of these two tephra formations and of Holocene coastal terraces and deposits around the maars strongly suggests that changes in water condition around the maars associated with the Holocene sea level rise were responsible for these maar-forming eruptions of phreatomagmatic type. Sumiyoshi-ike and Yonemaru maars were respectively formed around 7, 0006, 500 yBP, when sea level was rapidly rising and the sea invaded inland to the vicinity of the maars. As a results, the condition that rising magma most effectively contacted with water was produced in this area at these ages.This region has been so affected by active uplifts that paleoshores at several stages are found considerably higher than the present sea level as follows : c. 6m at c. 7, 000 yBP, c. 10m at c. 6, 500 yBP and c. 15m at 6, 0006, 300 yBP.