玄武岩柱状节理六边形结构形成机制探讨

Discussion on the formation mechanism of basalt columnar joint hexagonal structure

玄武岩柱状节理往往呈现完美的六边形石柱,这种现象引起了人们的广泛关注和好奇,但是对于其形成机理尚无合理的解释。本文通过对冷却过程中的玄武岩进行受力分析,提出当岩石(浆)冷凝收缩的凝聚力达到其抗张强度时,岩石(浆)内部发生潜在破裂和微形变;当潜在破裂面形成后,因岩石的泊松效应,微小潜在破裂面处的应力状态发生重整,形成新的潜在破裂面;当新的潜在破裂面处的剪应力等于岩石抗张强度时,岩石(浆)发生剪切破裂,形成如今所见的柱状节理。根据前人相关岩石实验数据,推算得到玄武岩相应温度下的内摩擦角、黏聚力、抗张强度和泊松比,采用应力莫尔圆方法进行数值计算,获得玄武岩在冷却到800℃左右时发生破裂,六棱柱形柱状节理开始形成,内角约119.1°。进一步分析认为,岩石的黏聚力、石英含量等因素可能控制着柱状节理的发育和形状。

The perfect hexagon showed by columnar jointed basalts gets great public appeal. However, the convincing explanation remains unresolved. This contribution proposes a new explanation for its origin by stress analysis and rigid calculation. With the cooling of basalts, the potential rupture and micro-deformation generates while the normal stress equal to the tensile strength of basalts. Because the Poisson effect, the distribution of stress at the micro-deformation changes and new potential rupture appears. With the growing of tangential stress, the condition of the shear fracture happens when it equals to the shear strength of basalts, which shows the perfect hexagon structure of columnar jointed basalts. According to previous relevant experimental data, we get the internal friction angle, the cohesive force, the tensile strength and Poisson’s ratio of basalts at the corresponding temperature. By Stress Mohr circle method, the numerical calculation shows that cleavages appear at about 800 ℃ and the hexagon columnar begin to generate with ~119.1° angle among the cleavages. By the further analysis, it shows that the angle and process may depend on the cohesive force and the percentage composition of quartz mineral.