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" ... 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 unusual 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 length. Geologists are still puzzled about the details controlling the formation and pattern of columns in large bodies of volcanic rocks. In nature, the joints can form only if the rock is brittle enough and grow over time from the cooling outside into the inner, still hot and soft, inside of the intrusion. The joints of the columns develop at approximately right angles to cooling surfaces, which are usually the tops or bottoms of lava flows, the walls of a magma chamber or a volcanic conduit. In case of the Devils Tower, the columns are tilted to the base suggesting also that the cooling surface, the walls of the volcanic plug, were tilted towards the outside. But why the columns become almost vertical, and so a challenge to rock-climbers, in the reconstructed core of the Devils Tower intrusion is a bit of a mystery. Geologists assume that tectonic forces and chemical composition of the volcanic rocks influence the distribution of heat inside the intrusion. As the joints follow the isotherms (lines that connect points of the same temperature), maybe the modern geometry of Devils Tower reflects the still poorly studied mechanisms of magma emplacement in earth's upper crust. ... "