MECHANISM DIFFERENCES BETWEEN SEVERAL TYPICAL PYROCLASTIC ROCKS AND THEIR VOLCANISM SIGNIFICANCE
Pyroclastic rock is the most direct object of physical volcanology and the most important topic of identifying the volcanic explosive fragmentation processes.Some particular species of pyroclastic rocks and equivalents can indicate key characteristics of the volcanism process,which is the basis to estimate the eruptive risks.Volcanic hazard is potential risk related to volcanic eruption,and it is one of the most important types of disasters that human beings face in nature.Volcanic disasters are directly related to the types of volcanic eruptions,among which explosive volcanic eruptions can cause the deadly intensive volcanic risks.The direct product of explosive volcanic eruption is to form various pyroclastic rocks,which represent the different types and intensities of volcanic hazards caused by the eruption process.The primary pyroclasts and secondary fragments reflect the difference of volcanic surface processes during eruptive or intermittent periods,while the distinguish of magmatic,phreatomagmatic and phreatic eruptive deposits marks the systematic development of modern volcanology,which is the leading work in the study of volcanic hazards.1)Pyroclastic rocks are formed directly by transporting,accumulating and diagenesis of the expelled materials during the eruption.They usually consist of the primary materials such as broken magma,accidental fragments trapped by the volcanic conduit,as well as the epiclasts captured by the volcanic fluid flowing on the surface.Pyroclastic rock,as a direct product of explosive volcanism,has naturally becomes the most important research object in volcanology.The volcanic tephra laminae preserved by fine airfall volcanic ash in basins has been attracted attention because of their good isochron and environmental indication,and the associated rocks may need to be distinguished from different types of volcanic sedimentation such as bedded tuff,sedimentary tuff and tuffaceous mudstone.The autoclastic breccia produced by lava emplacement and the hyaloclatite formed by the quenching of lava under water represent fragmentation that is closely related to the lava flow,rather than those from explosive volcanism.2)Pyroclast is mainly the product of explosive volcanism,but it can contain a certain amount of normal sedimentation and a small amount of rock fragment near the volcanic channel and the magma chamber roof.Pyroclats are generally defined as the direct products of explosive eruption behavior,while volcaniclastics are formed by volcanic degradation such as slope displacement,avalanche,lahar,and the autoclast generated by lava flowage and quenching.This classification not only emphasizes the difference in the forming process of different volcanic products,but also helps to distinguish the different mechanism in volcanological research and hazard estimation.Different types of pyroclastic rocks are formed with different fragment mechanisms and diagenetic ways,and some specific pyroclastic rocks represent various special types and scales of volcanic hazards.Although they are usually classified as primary clastics,the hazard caused by autoclastic breccia is significantly different.Cryptoexplosive breccia,although we have employed a rock name from pyroclastic rocks,is actually more concerned with its resource economics.3)When we study the genetic types of pyroclastic rocks,the most important basis for identification is the forming mode of the materials,that is,the type of fragmentation,which include primary volcanism and secondary volcanism.Primary clasts are divided into pyroclast,which is formed by the direct action of volcanic eruption,and autoclast,which is produced by the flow process of lava flows,While secondary(exogenous)volcanism includes various kinds of exogenous clasts(epiclast)formed by volcanic surface processes.According to the proportion of magma and water content at eruptive environment,explosive eruption can be divided into three types:magmatic eruption,phreatomagmatic eruption and phreatic eruption,which represent the most basic process of explosive eruption,and are also the problems of genetic classification and identification often faced in the study of pyroclastic rocks.
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