关键词: 雷琼火山, 水下火山, 滨海火山, 喷发物理

Abstract: This paper study on Bingmajiao volcano, a coastal volcano in Eman, Leiqiong volcanic field, China. Herein, based on satellite image and Unmanned Aerial Vehicle (UAV) image data interpretation, as well as field investigation, we analyzed typical cross sections at different locations of the coastal volcanic cone to identify the volcanic eruption sequence, and determined the physics of volcanic eruption. Microscope and scanning electron microscope were adopted to analyze the origin of pyroclasts. There are three types of pyroclasts in Bingmajiao. 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 agglomarated on the surface of clasts. The vesicular structure with different size 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 and form a large number of sheet-pieces that were re-disordered cemented. Under the surface, fine-honeycomb-like vesicular structure appears. The surface cracking support the quenching by water under high temperature, and the interior vesicular structure shows that the core part may not be affected. These features Indicates moderate water-magma interaction in the pyroclasts.The third type of pyroclasts shows no distinction between surface and interior. Irregular vesicles account for most of the volume in the pyroclasts. Thin film-like lava separate these vesicles. The thin lava broken into a large number of sheet-like pieces and were 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 Yingmajiao volcano erupts in littoral environment and has a transition from submarine volcano to terrestrial volcano. In the early stage of volcanism, submarine "fire fountain" type eruption occurred, and pyroclastic deposit 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, and its internal vesicular structure is extremely well developed. As the eruption continues, when the pyroclastic deposits rise above the water surface, the volcanism transforms 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 aggregates with different degree of welding. 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 10 km around the crater, which is the most dangerous type of volcanic eruption hazard.

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