关键词: 琼北, 第四纪火山, 射气岩浆喷发, 力学模型

Abstract: Maars of various scales produced by phreatomagmatic eruption are widely distributed in the Quaternary volcanic region of the Northern Hainan Island. The Pleistocene volcanic activity in Northern Hainan Island was characterized by the alternating of effluent magma eruption and phreatomagmatic eruption. In fact,phreatomagmatic eruption was concentrated in Middle Pleistocene to Upper Pleistocene,resulting in a lot of maars of various sizes,such as Longfeng-Longji,Shuangchiling,and Yanghua maars etc. In this area,different maars have different scale. The radius of maar is variable from several ten meters to above 1km. When ascending basaltic magma meets with ground water,violent explosion would occur,i.e. phreatomagmatic eruption. This kind of volcanic eruption may result in maars exposed on the earth's surface,and gives rise to the formation of base-surge. As to the base-surge,it has long been treated as sedimentary tuff by mistake. Usually,base-surge is distributed around maar,different from the distribution of sedimentary tuff. The base-surge is characterized by large-scale and low-angle cross-bedding,slaty-bedding,current-bedding and distal facies accretionary lapilli. In order to explain the dynamic mechanism of phreatomagmatic eruption thoroughly,we propose a simple mathematic model in this paper in the light of the elasticity theory. In addition,according to several parameters,such as the radius of maars,thickness of overlying formation when explosion occurred,and the radius of contact plane between the magma and groundwater,we have simulated the relation between the explosive wallop and these parameters,as well as its variation rule. Furthermore,we calculate the stress state at any point in the overlying formation. Some conclusion can be drawn as follows: the larger the radius of maar,the larger the explosive wallop needed for the formation of maar is; provided that the radius of maar and depth of explosive point are limited,then the larger the area of contact surface between magma and groundwater,the stronger the explosive energy will be; if the explosive energy and area of explosive point are restricted,the larger the radius of maar,the greater the depth of explosive point can be inferred; when the explosive energy and radius of maar are qualified,the depth of explosive point decreases with increasing of the area of contact surface between magma and groundwater. As for the maximum stress,undoubtedly it should occur on the surface of the overlying formation.

Key words: Northern Hainan Island, Quaternary volcanoes, phreatomagmatic eruption, dynamic mechanism