Geology and petrology of Akita-yakeyama volcano: 1. geology
Geology and petrology of Akita-yakeyama volcano: 1. geology
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秋田烧山火山的地质和岩石学：1.地质
DOI：
10.2465/ganko.86.305
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发表时间：
1991
期刊：
影响因子：
--
通讯作者：
T. Ohba
中科院分区：
文献类型：
--
作者：
T. Ohba
对日本秋田县东部的秋田-八山火山进行了地质调查，以确定该火山的演化历史。此外，对火山产物进行了详细的描述，以有助于目前正在进行的火山岩石学调查。根据火山活动方式或喷发产物的特征，火山活动可分为三个阶段，或两者兼而有之。大约30万年前，火山活动开始于一个火山口内，这个火山口是由100万年前大量的长英质火山碎屑喷发形成的。这是早期阶段的开始；新火山喷发了大量鸽质岩系的安山岩熔岩，而火山碎屑流和瀑布较少。在这一阶段形成的火山的形状和大小并不清楚，因为它被下面的火山产物所覆盖，严重的侵蚀使它发生了很大的变化。在中期，熔岩流活动与爆炸性喷发有关：火山碎屑从中央通风口反复落下和流动。熔岩和火山碎屑的相互作用形成了一座巨大的层状火山。这一阶段的火山岩均属于超深岩系。中期以后，活动暂时停止，热液蚀变扩展到几乎整个火山。因此，经过改变的熔岩和火山碎屑的崩塌，在山顶周围形成了五个马蹄形的火山口。休止期后，晚期活动以火山碎屑落下的中部和侧翼爆炸性喷发开始；熔岩流和浮石瀑布紧随其后。在这一阶段，形成了安山岩熔岩穹丘和英安岩穹丘。在山顶火山口的中心区域形成了一个英安岩圆顶，并喷发了英安岩火山灰；这是最新的活动。最后阶段的岩石也均为超深岩系。
Geological survey has been carried out on Akita-yakeyama volcano, eastern Akita Prefecture, Japan to establish the evolutional history of the volcano. Furthermore, the volcanic products have been described in detail to contribute to the petrological investigation of the volcano, that is currently in progress. The volcanic activities can be divided into three stages in terms of mode of volcanic activity or characteristics of the erupted products, or both. Approximately 300, 000 years ago, the volcanic activity started within a caldera formed by the eruptions of the voluminous felsic pyroclastics at 1 million years ago. This is the beginning of the early stage; the new volcano erupted number of andesitic lavas of the pigeonitic rock series while pyroclastic flows and falls were less abundant. The shape and size of the volcano built during this stage is not clear since it was covered with the following volcanic products, and severe erosion altered it much. In the middle stage the lava flow activities have been associated by explosive eruptions: pyroclastic falls and flows were repeated from central vent. The alternation of lavas and pyroclastics could have completed a large stratovolcano. All the volcanic rocks during this stage belong to hypersthenic rock series. After middle stage, the activity ceased for a while, and hydrothermal alteration expanded to nearly all over the volcano. Consequently, collapses of the altered piles of lavas and pyroclastics have led to five horseshoe-shaped calderas around the summit. After the repose period, the late stage activity started with explosive central and flank eruptions of pyroclastic fall; lava flows as well as pumice falls followed. During this stage, two andesite lava domes and two dacite domes were constructed. In the central area of the summit crater a dacite dome was built and dacitic ash was erupted; this is the latest activity. All the rocks of the last stage are of the hypersthenic rock series as well.