抄録
The AD1663 ejecta of Usu volcano, western Hokkaido, northern Japan, have a thickness of approximately 200, cm and are divisible into 7 units on the eastern foot of Usu volcano. The basal deposit, Unit-A, is composed of fine-grained, poorly sorted pumice and ash with well-developed stratification. It contains base surge beds near the sources, and is interpreted as phreatomagmatic in origin. Unit-B is typical in many respects of the product of plinian activity. It is characterized by coarse grain size and wide dispersal, combined with low lithic contents and poor development of internal stratification. This fallout deposit (Usu-b tephra) can be seen in the eastern regions of Usu volcano, and has a thickness of about 4 cm at 200 km from the vent. Unit-C, Unit-E, Unit-F, and Unit-G have characteristics that are satisfactorily interpreted as a consequence of phreatomagmatic activity : fine grain size near source, poor sorting, well-developed stratification and base surge bedforms. Unit-D is typical of phreatic deposits, being composed largely of non-juvenile lithic fragments.
According to historical records, the eruption was preceeded by earthquake swarms for 3 days from August 13, 1663. The first eruption (Unit-A) occurred on August 16, followed by an eruption associated with a large amount of pumice and ash (Unit-B) on August 17. The explosive eruptions lasted approximately 20 days.
The petrographic properties of the plinian product differ considerably from those of the phreatomagmatic products. The pumice clasts of Unit-B contain abundant glass and little phenocryst (plagioclase, orthopyroxene, and hornblende). The glass has a relatively high refractive index (n = 1.490-1.492) and a low SiO2 content. Orthopyroxene in Unit-B has a relatively high refractive index (γ = 1.735-1.740) and low Mg#. The phreatomagmatic products are rich in massive glass. This glass has a relatively low refractive index (n = 1.475-1.490) and a high SiO2 content, with values differing among horizons. The compositional diversity of the 1663 products indicates the existence of some magma. Presumably, their mixing triggered the 1663 eruption of Usu volcano.
