Changes in rock properties of some landform materials due to weathering

为了探讨岩石强度降低的机制，由于风化，多孔流纹岩的微观结构的时间变化进行了研究，选择Kozu-shima，在太平洋的一个小岛，作为研究地点。在1100、2600、20000和40, 000 yBP流纹岩喷发的同时，岛上先后形成了四个熔岩穹丘，这四个流纹岩主要由玻璃质基质组成，具有流动结构，这里称之为“玻璃柱”。每个熔岩穹丘的深度和均匀风化表明，可以假定日岩石特性代表每次喷发后经过的时间内的风化程度。通过对每个穹丘露头岩块的强度、孔隙度分布和显微结构的测试，得到以下主要结果：（1）单轴抗压强度和抗拉强度在初始阶段均迅速降低 关于我们 （2）大孔隙（直径约10 mm）体积也在此阶段迅速增加，（3）中孔隙（直径约1 mm）体积在此阶段略有增加，后期（2,000-40,000年）增加速度加快。扫描电镜观察表明，微裂纹的宽度为1毫米，这似乎对应于中等孔隙的PSD，开始在玻璃柱的表面上形成的初始阶段。这些证据表明，岩石强度在初始阶段的迅速降低是由于大孔隙体积的增加和微裂纹的形成，而大孔隙体积的增加则是由于玻璃柱的“溶解”使相邻的封闭孔隙组合成开放孔隙。另一方面，玻璃柱表面微裂纹的形成是由“水化”过程中产生的膨胀压力引起的。结果表明，化学风化引起的岩石微结构变化对岩石强度的降低有重要影响。少

To examine the mechanism of rock-strength reduction due to weathering, temporal changes in microstructure of porous rhyolite were investigated selecting Kozu-shima, a small island in the Pacific, as the study site. In this island four lava domes formed successively with eruption of rhyolite at the time of 1,100,2,600,20,000, and 40,000 y BP.These four rhyolites are mainly composed of glassy groundmass, with flow structure, which is called here "glass columns".Deep and uniform weathering in each lava dome indicates that the present-day rock properties can be assumed to represent the degree of weathering in the elapsed time from each eruption. Using rock block samples taken from the outcrop in each dome, strengths and pore size distribution (PSD) were measured, and microstructure of rocks were observed by the scanning electron microscope (SEM).Main results of the measurements are as follows : (1) Both uniaxial compressive strength and tensile strength reduce rapidly in the initial stage … More of weathering (for the first 20,000 years) ; (2) Rapid increasing in volume of large pores (about 10 mm in diameter) also occurs in this stage ; (3) A little increase in volume of medium pores (about 1 mm in diameter) occur in this stage, and the rate of the increase accelerated in the later stage (2,000-40,000 years). The SEM observations show that microcracks with a width of 1 mm, which seems to correspond to medium pores in PSD,begins to form on the surface of the glass columns in the initial stage. These evidences suggest that the rapid reduction of rock strength in the initial stage is resulted from increasing in volume of large pores and formation of microcracks.Increasing in large pores is explained to be caused by the combination of contiguous closed pores into open pores due to "dissolution" of the glassy columns. On the other hand, formation of the microcracks on the surface of the glass columns is explained to be caused by swelling pressure generated in "hydration" process. Above discussion leads to the conclusion that the changes in microstructure due to chemical weathering strongly influence on the reduction of rock strength. Less