Columnar joint morphology and cooling rate: A starch‐water mixture experiment
Columnar joint morphology and cooling rate: A starch‐water mixture experiment
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DOI：
10.1029/2003jb002686
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发表时间：
2004-02
影响因子：
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通讯作者：
A. Toramaru;T. Matsumoto
中科院分区：
文献类型：
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作者：
A. Toramaru;T. Matsumoto
[1]为了了解冷却速率对玄武岩柱状节理形态特征的影响，进行了淀粉-水混合物模拟实验。如果材料的收缩对于柱状节理结构的形成是必不可少的，则实验中的脱水损失速率（以下称为脱水速率）类似于凝固玄武岩的冷却速率。在实验中，通过改变淀粉-水混合物与用作热源的灯之间的距离来控制干燥速率。我们发现，节理形成与干燥速率之间的关系有三个区域：（1）在干燥速率高于1.4 × 10 - 2（g cm-2 h-1）（正常柱状节理区域）时，柱状体的平均横截面积S与平均干燥速率成反比，即，S −δ，δ = 1）。(2)在该干燥速率和临界干燥速率0.8 × 10−2（g/cm 2 h）之间，S接近于无穷大，因为S减小到接近临界干燥速率（即，指数δ从1单调增加到无穷大）（临界状态）。(3)低于临界干燥速率，没有柱状结构的形式（没有柱状关节制度的形式）。将本实验结果应用于玄武岩柱的形成，玄武岩柱的横截面积与冷却速率成反比，其因素包括弹性、裂纹扩展系数、热膨胀、玻璃化转变温度和应力最大时的裂纹密度比。此外，可以预测，存在一个临界冷却速率，低于该临界冷却速率，柱状接头不会形成;也可以预测在一定的冷却速率范围内，正常柱状接头和无柱状接头之间存在一个临界状态。我们发现，在较高的冷却速率，首选的柱形状是一个五边形，而在较低的冷却速率，它是一个六边形。
[1] An analogue experiment using a starch-water mixture has been carried out in order to understand the effect of cooling rate on the morphological characteristics of a basalt columnar joint. If the contraction of material is essential for the formation of columnar joint structure, the water loss rate by desiccation (hereafter referred to as desiccation rate) in the experiment is analogous to the cooling rate in solidifying basalt. In the experiment the desiccation rate is controlled by varying the distance between the starch-water mixture and a lamp used as the heat source. We find that there are three regimes in the relation between joint formation and desiccation rate: (1) At desiccation rates higher than ∼1.4 × 10−2 (g cm−2 h−1) (normal columnar joint regime), the average cross-sectional area S of a column is inversely proportional to the average desiccation rate, 〈〉 (i.e., S ∝ 〈〉−δ, with δ = 1). (2) Between that desiccation rate and a critical desiccation rate, 0.8 × 10−2 (g/cm2h), S approaches infinity as 〈〉 decreases close to a critical desiccation rate (i.e., exponent δ monotonically increases from unity to infinity) (critical regime). (3) Below the critical desiccation rate, no columnar structure forms (no columnar joint regime forms). Applying the present experimental result to the formation of basalt column, the basalt columnar cross-sectional area is inversely proportional to the cooling rate with factors including elasticity, crack growth coefficient, thermal expansion, glass transition temperature, and crack density ratio at stress maximum. Also, it can be predicted that there exists a critical cooling rate below which the columnar joint does not form; the presence of a critical regime between the normal columnar jointing and no columnar jointing during a certain cooling rate range can also be predicted. We find that at higher cooling rate the preferred column shape is a pentagon, whereas at lower cooling rate it is a hexagon.