关键词: 射汽岩浆喷发作用, 玛珥式火山, 基浪堆积物

Abstract: Phreatomagmatic eruption is a kind of special eruption, which usually occurs when hot magma rises and contacts with the ground water. The water/melt interaction produces explosive eruption and base-surge deposits, which resulted in maars. Monogenetic maar is a common volcano type on continents and islands. This kind of volcanoes is widely distributed in many countries. Researchers have studied eruption process and products of phreatomagmatic eruptions by means of petrological, sedimentological, volcanic physical and geochemical methods and techniques. Additionally, they have also explored influence factors over eruption process through experimental and computer simulations.
Phreatomagmatic eruptions can be considered as the natural equivalent of a class of physical processes termed fuel-coolant interactions(FCI)by investigators of large industrial explosions. Initially, a small volume of water is vaporized due to contact with the melt, with pulsating increasing in the high-pressure steam volume within the aquifer, the dominant effect of the vaporization energy is to fragment the melt and country rocks. Subsequently, the steam, melt, and country rock mix and vapor explosion occurs after vaporization energy increases beyond the confinement strength in chamber. Finally, maar and base-surge deposits form when the overlying layer is broken by the impact of the explosion. Two contrasting environments exist with respect to groundwater availability for the phreatomagmatic explosions. 1)In hard rock environment, the wall rocks are cut by joints and faults, many of which are hydraulically active. Under such groundwater conditions, a maar-diatreme volcano would form. 2)When the magma rises into soft-rock environment rich in water or with high permeability, it will lead to the formation of tuff-rings.
Maar consists of the crater at the surface(which is cut into the pre-eruption land surface), the tephra ring surrounding the crater and the cone-shaped diatreme, root zone and feeder underlies the maar crater. This tephra ring is easily eroded in the late evolution process, and it usually contains base-surge deposits with obvious dune-like bedding, fallout deposits and individual blocks and bombs that were emplaced ballistically, in which the base-surge deposits are dominant. Besides, the base-surge deposits and individual blocks and bombs are deposited near the crater. Maar lake usually forms in the center of the maar crater. It may form in many years after the phreatomagmatic eruptions. After the eruptions, the maar craters may be filled with groundwater and surface water. Maar lake is different from other crater lakes. For example, its surface of crater is often lower than the pre-eruption surface. In addition, the hydrology and sedimentary environment of maar lake are relatively simple. Archives from sediments of maar lakes, especially annually laminated sediments, will provide high-resolution dataset and are conducive to the study of paleoenvironment and paleoclimate. Compared with the maar-diatreme volcano, the tuff-rings volcano is formed in water-rich and shallower environments and has a wider crater which is not cut into the pre-eruptive land surface. The tuff-rings ejecta usually contain less than 5% of country rock clasts only.
Base surge is a kind of pyroclastic density currents with great velocity, and it carries debris further than ballistic fragments. Base surge transports lapilli, magma fragments, broken country rocks and ash formed by the explosion. When the base surge flows move, it generates shear force to the lower ground. The base surge can be subdivided into two parts by the interface where the shear stress is zero. The density of lower base surge currents is relatively large and the particles are coarser. In contrast, the upper currents have less density, and the particles accumulate slowly with the decrease of energy. Some indicative sedimentary structures, such as climbing bedding, dune-like bedding, and accretionary lapilli, would form in the base-surge deposits due to their special genetic mechanism. The climbing bedding helps to determine the location of the crater during field investigations. Accretionary lapilli indicate the distant source facies.
The entire eruption process of phreatomagmatic eruption is relatively complicated. This process may be influenced by several factors, such as the characteristics of the magma, the location and topography of the explosion, the country rocks, and the amount of water involved in the explosion. Foreign scientists have carried out many quantitative studies on the dynamic process of phreatomagmatic eruption through field geology and simulation experiments, while domestic scientists mainly focus on the analysis of the structure, particle size, composition and morphology of base-surge deposit, and the study of the dynamic process is relatively rare. Quantitative studies of the process of phreatomagmatic eruption will be the key in the future research.
Volcanic hazard is one of the major disasters in the world. Base surge generated by phreatomagmatic eruptions owns great energy and velocity. It would generate great damage to people’s life and the environment due to its special transportation process. Further, volcanoes formed by phreatomagmatic eruptions are common in China and relevant research is important. This paper introduces the research progress concerning phreatomagmatic eruptions and their products, aiming to advance our understanding of this special eruption so as to improve our strategies for preventing future volcanic hazards and protect people’s lives and properties.

Key words: phreatomagmatic eruption, maar, base surge