中国不同气候带盐风化作用的地貌特征

盐风化作用是地球表面普遍存在的一种物理风化作用,由于盐类的周期性结晶作用而造成地表岩石和建筑材料的破坏,形成诸如风化穴或蜂窝石构造等地貌景观。盐风化作用也是差异风化的主要表现形式之一。然而,到目前为止盐风化作用在中国地学界仍然被严重忽视,以至于盐风化作用造成的地貌景观常常被地学研究者和科普人士误读为海浪冲蚀、流水侵蚀、风蚀作用等。经过近十年的野外观察与探讨,笔者等对盐风化的形成机理和表现形式有了深入的理解。本文以中国境内东部海岸带、华北半干旱区、西北干旱区和东南湿热气候带基岩露头为例,系统地分析了盐风化作用的机理及其在不同气候带的表现形式。盐风化的必要条件是:适当的可溶性盐类(如Na_2SO_4、NaCl等)供应、周期性的干湿交替和温度变化。盐风化作用主要在发育可渗性孔隙的砂砾岩类和富含微裂隙的花岗岩类之露头表面表现明显,可以形成特征显著的盐风化穴。盐风化作用形成的地貌景观在东部海岸带和西北干旱区表现尤为明显,常常形成蜂窝石构造和大型风化穴,与风蚀作用的痕迹明显有别;而在华北半干旱区和南方湿热气候带虽然受到降雨等其他因素的影响而常常遭受改造、叠加甚或清除,但在某些露头区仍然保留有重要的识别标志,形成大型风化穴以及小型蜂窝石构造。笔者等强调:地表各种地貌景观形成过程中都有盐风化作用的贡献,而建筑物和景观保护也必须考虑到盐风化作用的影响。建议地学同仁重视盐风化作用的普遍性和重要性,在相关教材中补充更新盐风化的概念,并以科普的方式通过多种媒体纠正过去的错误认识。

Landform Features of Salt Weathering in Different Climatic Zones in China

Salt weathering is a type of physical weathering existing almost everywhere on the world land, damaging surface of rocks and building stones, forming geomorphological sceneries, such as tafoni or honeycomb structures because of the periodical salt crystal growth. Salt weathering is also one of the major forms of differential weathering. However, salt weathering process has been largely neglected by geologists and geographers and almost all the landscapes formed by salt weathering in China have been misinterpreted as the results of surf wash, running water erosion or eolian erosion etc. After almost a decade of field investigation, the authors of this paper have thoroughly understood the origin and mechanism of salt weathering. In this paper, several salt weathering scenarios have been selected from seacoast areas in Qingdao and Taiwan, arid areas in the northwest, semiarid areas in the north, humid areas in the south, of China. All these examples are discussed here to systematically demonstrate the mechanism of salt weathering and its behaviors in different climatic zones. The favorable conditions for salt weathering are the supply of soluble salts (such as Na2SO4, and NaCl) and the repeated change in temperature and humidity; and the suitable host rocks to provide the pore spaces and micro cracks for the saline solution to seep in and for the solid salts to crystallize are sandstones and conglomerates, granites and gneisses etc. Salt weathering commonly results in the formation of honeycomb structures on the surface of sandstone bedrocks and large tafoni on the vertical or near vertical sides of granite outcrops along the coast area above sea level, the formation of tafoni of different sizes on the sunny sides of sandstone or granite/gneiss outcrops where there is little rainfall attacking in the arid and semiarid inland areas, and the formation of large tafoni/caverns or honeycomb structures on the sandstone cliffs of humid areas in southern China where rainfall frequently impacts the outcrop surfaces. We can conclude that salt weathering plays an important role in making the natural landforms or corroding the building stones. We should pay attention to the universal nature of salt weathering, update salt weathering information in the related geological textbooks or treatise, popularize the geological science to clear up the misunderstanding of salt weathering via all kinds of media.