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 igneous 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.
Some famous locations in the United States where columnar jointing can be found are the 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.
The Devils Tower in Wyoming in the United States is about 40 million years old and 382 meters (1,253 feet) high. Geologists agree that the rock forming the Devils Tower solidified from an intrusion but it has not been established whether the magma from this intrusion ever reached the surface. Most columns are 6-sided, but 4, 5, and 7-sided ones can also be found.
The Giant's Causeway (Irish: Clochán An Aifir) on the north Antrim coast of Northern Ireland was created by volcanic activity 60 million years ago, and consists of over 40,000 columns. According to a legend, the giant Finn McCool created the Giant's Causeway, as a causeway to Scotland.
S?unky? Gorge, a part of the town of Kamikawa, Hokkaido, Japan, features a 24-kilometer stretch of columnar jointing, which is the result of an eruption of the Daisetsuzan Volcanic Group 30,000 years ago.
Several exposures of columnar jointing have been discovered on the planet Mars by the High Resolution Imaging Science Experiment (HiRISE) camera, which is carried by the Mars Reconnaissance Orbiter (MRO).