April 13, 2018 - 17:45 AMT
PanARMENIAN.Net - In ancient times strange rock formations, displaying a regular pattern of almost hexagonal sections, were sometimes explained as the work of a supernatural stonecutter, maybe the devil himself, trying to build a wall or a palace. To the Sioux people Devils Tower in Wyoming, with its characteristic polygonal columns, was sacred and some of their stories tell how this mountain formed. A long time ago a giant bear chased a group of children onto the mountain. Out of reach of the animal, the bear started to scratch the rocks with its claws, forming the joints separating the single columns from bottom to the flat top. Reportedly Devil Towers got its name from this legend. Instead of "bear", the interpreter said "bad god", which later became "Devil", Forbes says.
Swiss physician and naturalist Conrad Gessner explained the columns in his 1565 book “De omni rerum fossilium genere”, a compendium on fossils, gems and metals, as gigantic crystals, noting the similarities of the rocks to the regular shape of some minerals. In 1693, an expedition of the Philosophical Society of London to the famous Giant's Causeway in Ireland confirmed the natural origin of the columns. Slowly cooling lava will contract in volume and a pattern of regularly spaced cracks and fissures will form.
However, as this is a really slow mechanism and the complete cooling of a large volume of lava can last over a century and longer, no direct observations of their formation exists. A research team of the University of Liverpool now replicated in the laboratory the mechanisms how such rocks crack during the cooling process on a small (and quicker) scale. The experiments used basaltic rock samples recovered from a borehole at Eyjafjallajökull volcano in Iceland. The samples were heated in special cylinders to more than 1,000°C. Basalt melts at temperatures of approximately 1,000 to 1,200°C. The samples were fixed at each end in a mechanical grip and cooled to test at what point the cylindrical core cracked. The basaltic rocks fractured at between 840 to 890°C. So the joints of the regular polygon pattern of basalt columns form at a relatively high temperature, when the rock is already solid, but still at an early stage of the cooling process. As the joints form, fluids can start to circulate inside the rock formation, speeding up the cooling process. The experiments showed also that faster cooling produces also smaller columns.
The observations could also explain why the columns at Devils Tower are so large. Devils Tower is an ancient volcanic plug, once covered by sediments. As the volcanic rocks cooled slowly, insulated by the sediments, large columns could form. Only later erosion exposed the unusal shape of Devils Tower. Still some mysteries surround the formation of the columns. The cooling experiments were done on small samples, just eight inches in lenght. What exactly controls the formation and pattern of columns in large bodies of volcanic rocks still remains unclear.