ソレアイト質無斑晶安山岩溶岩は前期更新世の魚沼層群中部累層中に挾在している。無斑晶安山岩はオージャイト,ハイパーシン,斜長石,かんらん石の微斑晶と針状斜長石,ピジオン輝石およびサブカルシックフェロオージャイトの石基結晶を含み,岩石学的にアイスランダイトに似ている。
無斑晶デイサイトは火山礫凝灰岩中の小岩片または泥流堆積物中の大ぎな岩塊としてみいだされるが,フェロオージャイト,フェロハイパーシン,斜長石の微斑晶と針状斜長石とフェロピジオン輝石の石基結晶を含んでいる。一部のデイサイトは粒状のフェロピジナン輝石にとりかこまれたフェロフォートノライトを含んでいる。
全岩化学成分と構成鉱物の性質から,無斑晶安山岩およびデイサイトはソレァイト質マグマから分化したものと考えられる。

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Geological study of the Ulreung Island has been carried out to establish the evolutional history of the island. The island is pentagonal (12km × 10km) and situated off the eastern coast of the Korean peninsula (37°30'N, 130°50'E).
The evolutional history of the island can be divided into five stages on the basis of the mode of activity and the nature of the volcanic products.
Stage I. A large amount of pyroclastics of trachybasalt has been erupted with some lava flows and dikes and they formed the basement of the island.
Stage II. Successive eruptions of trachytic pyroclastic flows and lava flows covered the basement rocks of stage I, and several dikes intruded into the basement. A minor amount of trachyandesite has been erupted as well.
Stage III. After a long quiescence, trachytic to phonolitic lavas have been extruded and completed a large stratovolcano of the island. The phonolite is characterized by the presence of groundmass nepheline.
Stage IV. The mode of the volcanic activity changed after the completion. A voluminous pumice and ash of trachytic composition were ejected from the central vent and this resulted in the formation of caldera on the northern flank of the volcano.
Stage V. A small central cone was formed by the eruption of trachytic pyroclastics and trachyandesite lava in the caldera. The trachyandesite contains leucite in groundmass.

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Petrography and geochemistry of the volcanic rocks of Moriyoshi volcano, northeastern Japan, reveals the mechanism which had produced the different chemical trends of the rocks between the earlier stage (pre-caldera activity) and the later one (post-caldera activity).
In the later stage, phenocryst assemblage of basalt and andesite (SiO₂=53-66%) is complex (olivine+orthopyroxene+hornblende+clinopyroxene+Fe-Ti oxide+ quartz+ plagioclase). The compositions of phenocrystic minerals, clinopyroxene (Wo45 En40), orthopyroxene (Wo2 En60) and plagioclase (An 35-55), are nearly constant in spite of the wide variation of whole-rock chemistry. Magnesian olivine phenocrysts (Fo 78-71) are in disequilibrium with quartz, iron-rich orthopyroxene and clinopyroxene. Compositional zonations of phenocrystic minerals are normal in olivine and reverse in clinopyroxene, orthopyroxene and plagioclase. These features cannot be explained in terms of simple crystallization differentiation model. The linear trends on variation diagrams indicate that two magmas, with different temperature and compositions, were mixed. Low temperature magma (SiO₂=66%) would contain plagioclase, quartz, orthopyroxene, clinopyroxene and hornblende phenocrysts. This dacitic magma was mixed with basaltic magma, containing olivine phenocryst, to produce the calc-alkaline trend (low iron-enrichment on the SiO₂-FeO_t/ MgO diagram) in the later stage.
In the earlier stage, the rocks consist of augite-hypersthene andesite to dacite, which are accompanied by a small amount of olivine-bearing augite-hypersthene andesite. The compositions of phenocrystic minerals gradually change according to whole-rock chemistry. The distinct disequilibrium petrographic features are not found in olivine free rocks. On the contrary, in olivine-bearing rocks, normally zoned olivine phenocryst (Fo87-73) is in disequilibrium with reversely zoned orthopyroxene (mg value 68-62) and clinopyroxene (72-67). The olivine-bearing rocks would be produced by magma mixing process, between basaltic magma and andesitic magma. But magma mixing process at the earlier stage is negligible, because the proportion of mixed magma to eruptives of the earlier stage is estimated to be less than 10%. Therefore, the tholeiitic trend (strong iron-enrichment on the SiO₂-FeO^t/MgO diagram) of these rocks can be explained in terms of fractional crystallization mechanism. Basaltic magma would not have crystallized calcic plagioclase with olivine phenocryst (Fo87-71). This delay of crystallization of plagioclase does not depend on H20 content judging from F content. Crystallization of plagioclase is supressed by crystallization of basaltic magma at high pressure. So basaltic magma of Moriyoshi volcano had stopped at the deeper level (⟩25km) and crystallized olivine phenocryst. This fractionated basaltic magma had rised rapidly to shallower level magma chamber to mix with andesitic or dacitic magma.
Mixed magma effused mainly after the formation of caldera. In the earlier stage, which is the stage of constructing a stratovolcano, if magma mixing process had occurred, mixed magma could not rise easily under the regional compressional stress field on the northeastern Japan. Hence evidence of magma mixing would not be clear. In the later stage, mixed magma would have rised easily through the crack formed by the caldera collapse.

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