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Field Division of Education The Geology of Devils Tower National Monument |
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COLUMNAR STRUCTURE
One of the most notable features of Devils Tower is its excellently developed columnar structure. Therefore it may be interesting to note briefly how these structures may be formed. (Pirsson, L.V. and Knopf, A. 1926, p. 166).
As a body of magma cools and crystalizes into rock it is subject to extensive contraction, which causes great fissures and cracks to form, dividing the mass into variously shaped blocks. Sometimes these cracks form rudely cubic or rhomboidal blocks, commonly shown in granites; or they may form platy partings, making the rocks look much like bedded sediments; sometimes the planes of cracking are curved, forming concentric or spheroidal masses; or in other cases columnar structures are formed.
In columnar structure the whole mass is made up of columns, regularly fitted together and from a few inches to several feet in diameter and from one foot to many feet in length. The columns always form perpendicularly to the greatest extension of the main cooling surface of the igneous mass; hence in a lava flow, intrusive sheet or laccolith, the columns would be nearly vertical - when in a dike they tend to be more nearly horizontal. It is believed that when a homogeneous mass is cooling slowly and regularly, centers of cracking seem to occur on the cooling surfaces at equally spaced intervals. From each central interspace these cracks radiate outward at angles of 120 degrees from each other. These intersecting cracks produce regular hexagons, and the cracks penetrating downward make columns. This regular arrangement produces the greatest amount of contraction with the least amount of cracking, providing the centers of cooling are equally spaced. But as the contractional centers are not always equally spaced, 3, 4, 5, 7, or 8 sided columns occur. The columns contracting lengthwise, break into sections as they form. This same principle is also shown in drying mud flats where the clay cracks into polygonal shapes, or again in the prisms of drying and contracting starch. Such columns, no matter how regular their appearance, are not crystals but pieces of rock, and should not be confused with prisms produced by the crystalization of certain minerals such as quartz, beryl, etc., which are formed by an entirely different process.
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Last Modified: Sat, Feb 9 2002 10:00:00 am PDT |