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

Abstract

[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 Â 10 À2 (g cm À2 h À1 ) (normal columnar joint regime), the average crosssectional area S of a column is inversely proportional to the average desiccation rate, h. (2) Between that desiccation rate and a critical desiccation rate, 0.8 Â 10 À2 (g/cm 2 h), S approaches infinity as h _ M i decreases close to a critical desiccation rate (i.e., exponent d 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.

Morphometric analysis of polygonal cracking patterns in desiccated starch slurries

“…Our sample preparation method was essentially similar to those reported in Refs. [18,33,35], as explained below.…”

“…It should be noted that for columnar joints, the dominant polygon order N depends on the cooling rate 18 . More precisely, it was proposed that a pentagon be preferred as the columnar shape at a higher cooling rate, whereas a hexagon is preferred at a lower cooling rate.…”

“…Earlier studies have elucidated the mechanism of desiccation crack formation for various grain-liquid mixtures [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28] . The mixtures typically consist of a skeleton of solid grains and pore spaces that are filled with either liquid or air bubbles.…”

Nonequilibrium scale selection mechanism for columnar jointing

“…1 A and D), that have been known since at least Victorian times (9), although they have only recently been investigated quantitatively (17)(18)(19)(20)(21).…”

“…1, which only occur in relatively thick layers, the similarity between crack patterns induced by drying (16)(17)(18)(19)(20)(21) and cooling (16,(22)(23)(24)(25) can be traced to the fact that the transport of water in a drying slurry and the extraction of heat from a hot solid are mathematically analogous (1)(2)(3). In a poroelastic medium, fluid flow is coupled to the elastic deformation of a porous solid, whereas in a thermoelastic medium, heat conduction is coupled to elastic deformation of a conducting solid; pressure in one case is analogous to temperature in the other.…”