A microstructural analyses of groundmass of dacite lavas, and their flowage and solidification processes.

对岛根半岛桂岛的云仙-富根湖、大森-米森、崖竹湖和中新世水下穹窿等英安岩熔岩穹窿进行了研究。重点研究了它们的显微结构和空腔的出现。得到了以下四个结果：1。位于陡坡上的熔岩包含由机械过程形成的空洞。孔洞从边缘粗糙到边缘锋利不等，边缘粗糙的孔洞被解释为韧性结晶浆液岩浆的破坏。边缘锋利的空洞被解释为岩石达到均匀强度时形成的脆性裂缝。在熔岩流的上部、下部、近端和远端都观察到这些膨胀结构。所观察到的剪胀和脆性流动结构是位于陡坡上的英安岩的特征。日本云仙火山、八竹火山和大仙火山英安岩熔岩的孔隙具有不同于囊泡的特征。大多数孔壁凹凸不平，有突出的孔壁。这些孔隙的形状从不规则到平面不等。许多孔隙出现在全晶附近的“压力阴影”中。有些孔呈裂纹状，绕过斑晶，末端钝。大多数孔隙被解释为岩浆在韧性-脆性转变过程中被流动破坏的结果。这种现象被称为空化，在水力学和材料科学中得到了很好的证实。陡的就位坡度、带间细微的力学差异以及刚性晶体和岩屑周围的应变强化促进了本研究中熔岩的空化。熔岩中的空化可以从岩浆中提取挥发物，导致岩浆过早脆化和变弱。据推测，空化对岩浆前缘火山碎屑流的形成至关重要。水下侵位的英安岩岩浆呈现因水快速冷却而形成的带状结构。中新世岛根半岛的Katurajima熔岩穹丘，其带由基性暗隐带和长英质突出带组成，SiO2变化幅度为58 ~ 74wt.%。岩浆的快速冷却和岩浆在喷口的分异作用共同解释了如此大范围的变化以及大地质量结构。水下英安岩熔岩是由水侵入热岩体形成的透明碎屑岩的一个例子。这种侵入性透明质碎屑岩可以与以前描述得很好的水下自角化熔岩区分开来。在浅水区，透明碎屑岩既可由热岩的猝缩作用形成，也可由岩浆-水相互作用引起的小爆炸作用形成。少

Such dacite lava domes as, Unzen-Fugendake, Daisen-Misen, Yakedake, and Miocene subaqueous dome at Katsurajima, Shimane Peninsula were investigated. Especially their microstructures and occurrences of cavities are studied. Following four results are obtained.1. Lavas that emplaced on steep slopes contain voids formed by mechanical processes. The voids vary from having ragged margins to sharp margins, and ragged margin voids are interpreted as failures in the ductile crystal mush magma. Voids with sharp margins are interpreted as brittle cracks formed when the rock had attained homogenous strength. These dilatant structures have been observed in upper, lower, proximal and distal parts of the lava flows. The dilatantancy and brittle flow structures observed are characteristic of dacite emplaced on the steep slopes.2. Pores in dacite lavas of three volcanoes in Japan (Unzen, Yakedake and Daisen) have characteristics unlike vesicles. Most of the pores have ragged walls marked by the protru … More sion of groundmass crystals and phenocrysts. The shapes of these pores range from irregular to planar. Many pores occur in 'pressure shadows' adjacent to plenocrysts. Some pores are crack-like and pass around phenocrysts and have blunt terminations. Most of the pores are interpreted as the result of failure of the magma by flow during ductile-brittle transition. This phenomenon, known as cavitation, is well established in hydraulics and the material sciences. Cavitation of the lavas in this study was promoted by steep slope of emplacement, subtle mechanical contrasts between bands, and strain intensification around rigid crystals and lithic fragments. Cavitation in lava can extract volatiles from magma causing premature embrittlement and weakening. Cavitation is inferred to be critical to the initiation of lava-front pyroclastic flows.3. Dacite magma emplaced subaqueously show band structures caused by rapid cooling by water. In the case of Katurajima lava dome in Shimane Peninsula, Miocene in age, the bands consists of mafic recessive zone and felsic prominent zones, SiO2 variation being58-74wt.%. Such a wide range variation, along with the groundmass structures are interpreted by combination of rapid cooling by magma and magma differentiation at the vent.4. Subaqueous dacite lavas provide an example of hyaloclastite that were formed by invasion of water into the hot rock body. Such an invasive hyaloclastites can be distinguished from the formerly well described subaqueous autobrecciated lavas. In the shallow water, hyaloclastite can be formed not only by quench-contraction of hot rock but also by small explosion induced by magma-water interaction. Less