火山活动状态及其机理的初步探讨

PRELIMINARY STUDY ON ACTIVE STATES OF VOLCANO AND THEIR MECHANISMS

根据火山喷发实例总结了火山喷发在不同阶段的活动状态,并探讨了可能的物理机理。火山活动从岩浆补给到岩浆喷发的物理过程可分为3个阶段:1)岩浆补给阶段,岩浆囊压力差或过剩压力的大小决定了火山活动是否休眠或扰动,岩浆补给速率对压力差起了决定性的作用;2)通道形成阶段,当过剩压力超过围岩破裂强度时,围岩开始破裂,之后水热活动起了重要的作用;3)岩浆运移与失稳喷发阶段,主要是岩浆运移与地壳盖层的相互作用与失稳的过程。文中还讨论了火山活动状态与火山喷发危险性等级之间的关系,7个危险性等级分别对应于火山活动的7种状态,即休眠、平静、扰动、动荡、临界、活动、灾变。

Based on the examples of active volcano, the active states of volcano in different stages are summarized, and the conceivable mechanisms are discussed in this paper. The physical situations of volcano activity, from magma supply to eruption, can be divided into 3 phases（stages）. Phase 1： magma supply： the overpressure of magma chamber is the dominant parameter for whether the volcano is in the stage of dormancy or disturbance, and the magma supply rate is essential. Phase 2 ： formation of conduit： the wall rock fails and a crack will be initiated when the overpressure exceeds the tensile strength of the wall rock, and then the hydrothermal effects play an important role. Phase 3 ： instability and eruption： magma moves in conduit, and it interacts with overlying crust, the dynamic of mag- ma flow in dykes is essential. The volcano threat in China can be rated into 7 levels as safety, attention, stand by, alarm,threat, hazard and disaster, and accordingly the active states, the inferred major mechanism and their primary physics are： （1） Dormancy represents the stage with magma supply; the pressure of magma chamber is less than the pressure of wall rock. （2） Late-dormancy represents the stage of pressurization of magma chamber, and the pressure of magma chamber is equivalent to the pressure of wall rock. （3） Disturbance represents the stage with updoming and extension of shallow crust, and the pressure of magma chamber is greater than the pressure of wall rock. （d） Unrest represents the stage of fracture of wall rock; the overpressure exceeds the tensile strength of wall rock. （5） Critical represents the stage of hydrothermal activity ; Microcracks are filled of fluids, the effective stress depresses, the hydrothermal circulation prevails, and long-period earthquakes appear in this stage. （6） Eruption represents the stage of initial irreversible eruption ; the content of volatiles, magma pressure and the viscosity of magma affect the outflow rate, the gas exsolution and fragmentation, and then determine the type of eruption. （7） Violent eruption represents the stage of unstabilized eruption, and if the overpressure of magma chamber is still greater than that needed to transfer the magma along the conduit, the eruption will continue. The activities of volcano are multifarious. This paper is only a preliminary discussion. The divi- sion of process is primary, and some volcanoes can develop very rapidly, and one of the states can be jumped over. On the other hand, some volcanoes can develop very tardily, and one of the states can be repeated, even back up to the original stage, e.g. as the magma supply intermits, the evolution will stop.