Base-Surge Deposits of Mukaiyama Volcano, Nii-jima, Izu Islands

Abstract

Base surges from rhyolitic magma occurred repeatedly in the 1st stage of continuous eruptions which formed Mukaiyama volcano in A.D. 886. The Mukaiyama base-surge deposits constitute a low-lying and flat-topped hill, overlain by 2nd-stage pyroclastic cone and 3rd-stage lava domes. The Mukaiyama base-surge deposits are divided into two major groups; the lower coarse-grained Mukaiyama-1, and the upper fine-grained Mukaiyama-2. Both are further subdivided into two. The Mukaiyama-1L, which constitutes the lowermost part of the Mukaiyama-1, is characterized by the pile of plane beds. In the Mukaiyama-1U, which constitutes the remaining upper part of the Mukaiyama-1, the lateral transition of sedimentary structures can be seen. This is best observed along the continuous sea cliff in the eastern coast. Here, plane beds, wavy (undulating) beds large-sized “antidune” structures up to several tens of meters in wave length, and middle-sized “antidune” structures appear one after another from south to north, namely with distance from source, presumably reflecting the gradual decline of the energy of base surges. The Mukaiyama-2 is subdivided into the lower (Mukaiyama-2L) and the upper (Mukaiyama-2U) by a pyroclastic fall layer at a lower horizon. Both are characterized, all over, by the superposition of small-sized “antidune” structures and cross beddings. However, the Mukaiyama-2L is slightly bigger in size of "antidune" structures and in grain size. The upward change of type and size of sedimentary structures within the Mukaiyama base-surge deposits may indicate the general decline of the power of base surges generated with the lapse of activity, suggesting the gradual decrease of the contact of magma with sea water with the growth of the volcano from submarine to subaerial. The size of large-sized “antidune” structures in Mukaiyama-1U far exceeds any other reported ones. One possible explanation for this may be given by the best exposure along eastern sea cliff, which is large enough to expose sedimentary structures of any size and, in addition, is nearly parallel to the direction of the spread of base surges. Exposures with other directions and/or of small size inhibit the accurate measurement of large-scale sedimentary structures if any. Another more probable explanation is attribution to the rhyolitic magma of Mukaiyama volcano. The more acidic the magma is, the more powerful the base surge (and consequently, the larger the bed form) will be. In contrast to the rhyolitic magma of Mukaiyama volcano, most of the hitherto-reported base-surge deposits are from basaltic magma.