
Columnar jointing is a geological structure where sets of intersecting closely spaced fractures, referred to as joints, result in the formation of a regular array of polygonal prisms, or columns. Columnar jointing occurs in many types of volcanic rocks and forms as the rock cools and contracts. Columnar jointing can occur in cooling lava flows and...
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http://en.wikipedia.org/wiki/Columnar_jointing

(from the article `igneous rock`) ...rupture directly associated with the formation of a rock or later superimposed upon it. Primary fractures generally can be related to emplacement ...
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http://www.britannica.com/eb/a-z/c/113

A system of fractures that splits a rock body into long prisms, or columns. It is characteristic of lava flows and shallow intrusive igneous flows.
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http://www.evcforum.net/WebPages/Glossary_Geology.html

The type of jointing that breaks rock, typically basalt, into columnar prisms. Usually the joints form a more or less distinct hexagonal pattern.
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https://www.encyclo.co.uk/local/22327

(in basaltic igneous rocks) a series of generally hexagonal columns formed by vertical joints as a result of contraction during cooling.
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https://www.infoplease.com/dictionary/columnar-jointing
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