Columnar joint morphology and cooling rate: A starch-water mixture experiment

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.