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许多活火山地区已发表的重力资料通常是反映了位于环形断裂的火山塌陷洼地内(如酸性火山的破火口)低密度的火山渣或浮岩堆积;或者反映了位于玄武岩盾和火山锥之下已凝固的高密度岩浆通道。本文报导了三个横穿复杂的安山岩火山的重力详查结果,揭示了上述两种类型的特征。重力场含有多种成分,中心为火山口的正异常(直径1—2km),外围为异常强度与之相近但范围更大的(5—10km)负异常带。我们认为前者反映了火山以下正常密度的岩浆管道(2.5—2.6Mgm~3[吨/米~(-3)译者注,下同]),外围则是一些依次对应于负异常带的具有低净密度(1.8—2.4Mgm~(-3))的火山碎屑质火山渣,火山灰、偶而有熔岩流,因为安山岩火山机构深部较固结的和较老的部分具有较正常的密度(2.3—2.6Mgm~(-3))。人们对那些低密度的物质特别感兴趣,因为他们充填地貌洼陷的深度达数百米之多,在那些破火口状的古构造区更是这样,这些构造是根据环形断层带地区显示的陡的重力梯度带推断而来的。本文提出这样一个模式;伴随有间歇性巨大高垃岩浆体侵位的破火口塌陷是在短期内形成的,两次火山塌陷之间则为具狭窄地下岩浆管道的正常火口喷发活动。从顶部火口以火山碎屑喷发形式产生的物质逐渐充填在早期形成的地貌洼陷内。本文结合地表地质资料,对重力勘查的资料加以论述,以搞清活火山顶部地区的演化过程并建立模式。
Gravity data published in many active volcanoes usually reflect low-density volcanic or pumic buildups in ring-shaped volcanic subsidence depressions (eg, acidic volcanic breaches) or reflect the presence of volcanic rocks located below basalt shields and volcanic cones Solidified high-density magmatic channels. This paper reports the results of gravimetric examination of three andesite volcanoes crossing the complex, revealing the above two types of features. The gravity field contains many kinds of components. The center is the positive anomaly of the crater (1-2km in diameter), and the periphery is the negative anomalous zone (5-10km) with similar intensity but similar scope. In our opinion, the former reflects magma pipes with normal density below volcano (2.5-2.6Mgm ~ 3 [t / m ~ (-3)], and the periphery is a series of magma pipes with lower negative anomalies Volcaniclastic volcanic slag with net density (1.8-2.4Mgm -3), volcanic ash and, occasionally, lava flows, because the deeper, consolidated, and older parts of the andesite volcanism have a more normal density (2.3- 2.6Mgm ~ (-3)). People are particularly interested in those low-density materials because they are filled with geomorphic depressions up to several hundred meters in depth, especially in those rupture-like paleo-tectonics that are based on the steep Gravity gradient with inferred. In this paper, a model is proposed. The collapse of the fracture with intermittent huge high-volume magma emplacement is formed in a short period of time, and the normal volcanic eruption with narrow underground magma pipeline occurs between the two collapsed volcanoes. Material from pyroclastic eruptions at the top of the crater gradually fills the geomorphic depressions formed early on. Based on the surface geological data, this paper discusses the data of the gravity exploration to clarify the evolution process and establish the model of the top of the active volcano.