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

研究了岛根半岛Katsurajima英安岩熔岩穹隆、Unzen-Fugendake、Daisen-Misen、Yakedake和中新世水下穹隆。重点研究了它们的显微组织和空洞的产状。得到了以下四个结果：1.位于陡峭斜坡上的熔岩含有由机械过程形成的空洞。这些空洞从边缘参差不齐到边缘锐利不等，而参差不齐的边缘空洞被解释为韧性晶体混杂岩浆的失败。边缘锐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了这些扩张性结构。所观察到的扩容和脆性流动结构是侵位在陡坡上的英安岩的特征。日本的三座火山(云顶火山、矢可达克火山和代森火山)的英安岩熔岩中的孔隙具有不同于小泡的特征。大多数毛孔的壁上都有突出的…标记。更多的基质晶体和斑晶。这些孔的形状从不规则到平面不等。许多气孔出现在与全晶相邻的“压力阴影”中。有些毛孔呈裂隙状，在斑晶周围穿行，末端钝化。大多数孔隙被解释为韧性-脆性转变过程中岩浆流动破坏的结果。这种现象被称为空化，在水力学和材料科学中已经得到了很好的证实。本研究中熔岩的空化是由于侵位的陡峭坡度、条带之间细微的力学对比以及刚性晶体和岩屑周围的应变增强所致。熔岩中的空化可以从岩浆中提取挥发物，导致过早的脆化和弱化。推测空化是熔岩锋面火山碎屑流形成的关键。侵位于水下的英安岩岩浆表现为水快速冷却形成的条带结构。岛根半岛中新世Katurajima熔岩穹隆带由镁铁质隐伏带和长英质凸起带组成，SiO_2变化58-74wt.%。岩浆快速冷却和喷口岩浆分异共同解释了这种大范围的变化以及基团结构。水下英安岩熔岩提供了一个透明碎屑岩的例子，这种透明碎屑岩是由水侵入热的岩体而形成的。这种侵入的透明碎屑岩可以与以前所描述的水下自碎屑熔岩区分开来。在浅水中，透明碎屑岩既可以通过高温岩石的急冷收缩作用形成，也可以通过岩浆-水相互作用引起的小爆炸作用形成。较少

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