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Columnar jointing
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 ashflow tuffs (ignimbrites), as well as in some shallow intrusions.〔
The columns can vary from 3 meters to a few centimeters in diameter, and can be as much as 30 meters tall. They are typically parallel and straight, but can also be curved and vary in diameter. Whereas an array of regular and straight columns is called a colonnade, an irregular array is termed an entablature. The number of sides of the individual columns can vary from 3 to 8, with 6 sides being the most common.〔(Oregon State University > Volcano World > ... > Columnar Jointing ). Accessed 29 December 2013.〕
==Places==
(詳細はDevils Tower in Wyoming, the Devils Postpile in California and the Columbia River flood basalts in Oregon, Washington and Idaho. Other famous places include the Giant's Causeway in Northern Ireland and Fingal's Cave on the island of Staffa, Scotland.〔(southernhebrides.com > Staffa - A Geological Marvel ). Accessed 29 December 2013.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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