Abstract:

Bingmajiao volcano is a coastal volcano, located in Eman Town, Leiqiong volcanic field, China. In this paper, based on satellite image and unmanned aerial vehicle(UAV)image data interpretation, as well as field investigation, typical cross sections at different locations of the coastal volcanic cone were analyzed to identify the volcanic eruption sequence and determine the physical mechanism of eruption. The origin of pyroclasts was analyzed under microscope and scanning electron microscope. There are three types of pyroclasts in Bingmajiao volcano. The first type is in the shape similar to ropes or tree root and experienced obvious plastic deformation. The micro-plastic lava droplets with different sizes and irregular shapes are agglomerated on the surface of clasts. The vesicular structure in the clasts is extremely developed. All lines of evidence support this type of pyroclasts derived from magmatic explosive eruption without significant water-involving. The second type of pyroclasts is featured by crusted and moss-like surface with superficial cracks. The rigid shell surface fragmented, forming a large number of sheet-pieces that were re-disordered cemented. Under the surface, fine-honeycomb-like vesicular structure appears. The surface cracking supports the quenching by water under high temperature, and the interior vesicular structure shows that the core part may not be affected. These features indicate moderate water-magma interaction in the pyroclasts. The third type of pyroclasts shows no distinction between the surface and the interior. Irregular vesicles account for the major volume in the pyroclasts. Thin film-like lava separates these vesicles. Some lava broke into a large number of sheet-like pieces and agglomerated, forming strongly brittle-ductile deformed pyroclasts. Abundant cracks appear on the surface of lava. These features support this type of pyroclasts formed in relatively strong water-magma interaction. The study shows that the Bingmajiao volcano erupted in littoral environment, with the characteristic of transition from submarine volcano to terrestrial volcano. In the early stage of volcanism, submarine “fire fountain” type eruption prevailed, and pyroclastic deposits dominated by the third type of pyrolcasts formed underwater. Most were composed of sharp-hornlike volcanic lapilli. The pyroclastic deposit is loose and has no bedding, and the particle size sorting is not obvious. There is a large number of black fluidal juvenile lava with highly vesicular structure. As the eruption continued, when the pyroclastic deposits rose above the water surface, the volcanism transformed into the phreatomagmatic eruption, resulting in surge current and tuff deposit, which has obvious parallel bedding and cross-bedding. The second type of pyroclasts formed in this stage. In the late period of volcanic activity, Strombolian and Hawaiian type eruption were the main types, which formed black and red welding aggregates. Finally, the eruption turned into an overflow of lava, forming a lava platform. According to the eruption physics of Bingmajiao volcano, it is speculated that the potential eruption hazards of littoral volcano in the future include underwater “fire fountains”, surging currents, ballistic falling volcanic bombs, lava fountains and lava flows. Among them, the surge current may move at a high speed close to the sea level, affecting a range of 10km around the crater, which is the most dangerous type of volcanic eruption hazard.

Key words: Leiqiong volcanic field, submarine volcano, littoral volcano, eruption physics

摘要：

文中以雷琼火山群海南岛儋州地区峨蔓镇的兵马角海岸火山为研究对象, 在卫星影像及无人机影像数据解译的基础上, 分析了海岸火山锥不同位置典型剖面的火山喷发序列, 并利用显微镜及扫描电镜分析碎屑物成因, 再现火山喷发物理过程。兵马角剖面存在3类玄武质碎屑。第1类碎屑整体呈绳状、 树根状, 经历明显的塑性变形, 碎屑内部气孔构造极发育, 代表纯岩浆爆破喷发成因。第2类碎屑表壳之下呈蜂窝状细密气孔构造, 表面龟裂与内部普遍的脆性破裂显示其经受过淬火作用; 内部仍保留的气孔构造说明其内部可能没有受到影响, 以上证据支持该类碎屑形成于中度的水-岩浆相互作用。第3类碎屑有强烈塑性变形特征, 表面出现大量龟裂纹; 内部碎裂为大量薄皮碎屑并重新熔结, 不规则气孔或空腔占碎屑的绝大部分, 说明该类碎屑形成于相对较强的水-岩浆相互作用。兵马角剖面显示, 兵马角火山喷发于滨海环境, 具有水下火山向陆上火山过渡的特点。火山活动早期发生水下“火喷泉”式喷发, 在水下构筑火山碎屑堆积, 主要由第3类碎屑构成。碎屑松散无层理, 粒度分选不明显, 其中出现大量有塑性变形的、 气孔构造极其发育的黑色熔岩浆屑。在火山活动的中期, 当碎屑堆积物高出水面时, 火山作用转变为射汽岩浆喷发, 产生具有明显平行层理与交错层理的涌流凝灰岩, 以第2类碎屑为主。火山活动晚期以斯通博利型-夏威夷型喷发为主, 形成不同熔结程度的黑色、 砖红色集块岩, 含有大量第1类碎屑。火山喷发最终过渡为熔岩溢流, 形成熔岩台地。根据兵马角火山喷发的物理机制, 推测未来潜在的滨海火山喷发灾害包括水下“火喷泉”、 射汽岩浆喷发成因的涌流、 弹道喷射坠落的火山弹、 熔岩喷泉以及熔岩流。其中涌流可能贴海平面高速运动, 影响火口周边约10km范围的区域, 是最有威胁的火山喷发灾害类型。

关键词: 雷琼火山, 水下火山, 滨海火山, 喷发物理