The Mechanism Of Underwater Volcanic Eruption In Naozhou Island,Guangdong Province

Different from volcanic eruptions in inland environments,explosive underwater eruptions in shallow sea environments are considered to be the most complex underwater volcanic eruption mechanism.This paper took Zhanjiang Naozhou Island located in the Leiqiong Volcano Group as the research object,which is the largest volcanic island in China.And in this study,I focused on the evidence of shallow sea volcanic products and the mechanism of underwater volcanic eruption,provides basic research data for scientific defense against future shallow sea volcanic eruptions in the Greater Bay Area and other coastal areas.Naozhou Island is located 45 kilometers southeast of the urban area of Zhanjiang City,Guangdong Province.The exposed area of the island is about 56 km².The terrain of the whole island is not undulating,generally centered on the lighthouse crater in the southeastern part of the island,and it gradually decreases to all sides.The volcanic rocks of Naozhou Island are dominated by subalkaline series of basaltic andesite,and the isotope of Naozhou Island basalt has similar characteristics to Ocean Island Basalt（OIB）,which means has similar magma sources to the Late Cenozoic volcanic rocks in the Leiqiong area.The eruption age of the Naozhou Island basalt is 0.45 Ma,which is the same time as the short-term concentrated eruption in the North of Leizhou Peninsula.The series of Nayan Bay sections records a relatively complete explosive eruption accumulation sequence,including lava layers,littoral clastic layers,pyroclastic layers,underwater tuff layers and onshore surge tuff layers.The following types of lithofacies are developed:onshore overflow facies,underwater weak blasting facies,underwater blasting origin turbidite facies and onshore surge facies.According to the lithofacies analysis of the profile,there is a crater near the Nayan Bay.Among the tuffs in the Nayan Bay profile,there are mainly three types of primary debris with large particle size（2～5 mm）of volcanic origin:fluidal clasts,sideromelane and tachylite.According to the degree of water participation,the order is as follows:sideromelane>tachylite>fluidal clasts.It is found that the M_water/M_melt ratio is the key factor to control the explosion intensity and the size of clasts.When M_water/M_melt=0.1-0.3,the pyroclastic particle size is the smallest（<50μm）.The analysis of volcanic eruption mechanism shows that it can be divided into three stages in total.The early stage is an Icelandic Surtseyan eruption in a shallow water environment with vapor explosion and eruption,and in the middle period it turns into a fire-fountain eruption,and a phreatomagmatic eruption occurred above sea level.The volcanic activity eventually ended in a magmatic overflow eruption.In the whole process of volcanic eruption,with the gradual increase of pyroclastic accumulation on the seabed,the explosion intensity gradually decreased,and the M_water/M_melt of hydrovolcanism gradually decreased.Overflow eruption is the main eruption type of basaltic magma in the later stage of Naozhou Island.Indicative products for the contact reaction of lava flows with seawater—hyaloclastite and basal breccia were discovered.Hyaloclastite is a typical product of the reaction of basalt lava with seawater in an open environment.They are typically composed of sideromelane fragments,quenched lava fragments,and later formed palagonite（clay）.Basal breccia is a breccia formed by lava flow in contact with water in a closed environment,usually at the junction between the intertidal zone and the bottom layer of the lava flow,and the degree of quenching in contact with seawater is lower than that of hyaloclastite.Based on the distribution of vitreous clastic rock and basement breccia in Naozhou Island,this paper believes that there is a periodic contact process between lava flow and seawater,that is,the emplacement environment of lava flow is intertidal zone.Basal breccia is easily formed in the area where lava flow contacts wet beach during ebb tide.During the high tide phase,lava flows are in full contact with water to form hyaloclastite.The positional relationship of lava flow direction,period and water direction leads to differences in the proportions,orientations,and production patterns of pahoehoe lava,hyaloclastite,and basal breccia.Hyaloclastite and basal breccia,as the special reaction products of the intertidal lava and seawater,can be used to indicate the coastline range of ammoniac island in the period of volcanic eruption.The position of the ancient intertidal zone during the Naozhou Island volcanic eruption basically coincided with that of the present intertidal zone,indicating that the sea level of the ancient ocean has not changed significantly.The Naozhou Channel（the water area between Naozhou Island and Donghai Island）already existed during the lava overflow period,which proves that Naozhou Island was not an island formed by later transgression and separated from Donghai Island.The Leiqiong volcanic group is the only Quaternary volcanic group in China that has been confirmed to be located above the mantle plume.Combined with the fact that the Leiqiong volcanic group has Holocene eruptive volcanoes,it fully shows that it is in danger of erupting again.Volcanic eruptions in coastal environments should first be alert to vapor explosions caused by violent water-magma mixing reactions.The second is the surge formed by phreatomagmatic eruption,which may cause impact damage to the land within 10 km from the crater.Scoria and volcanic bombs formed by the volcanic explosion will affect the volcanic cone within 1 km.Lava flows have a much larger impact area,and if they enter the ocean,they could trigger vapor explosions from water-magma interactions,causing secondary disasters.