Columnar Basalt

A natural volcanic formation, columnar basalt has a seemingly man-made appearance. The (mostly) hexagonal columns formed naturally as thick lava cooled rapidly, contracting and creating cracks in the surface of the new rock. Basalt headliner in the world located in the Giant’s Causeway in Northern Ireland.

Large eruptions of basalt lava may create deep flows of molten rock. As the rock slowly cools it shrinks slightly. The stresses cause jointing in several different planes, and columns of rock form with a generally hexagonal shape, like pencils. The flow shown here is at Sheepeaters Cliff, in Yellowstone National Park. Note that there is a strongly developed horizontal jointing here, too.

The piece of basalt below displays the six-sided cross section of a column. Some columns may have five or seven sides instead.

The shape, formation and texture of a basalt is usually determined by the way it erupted and also where it erupted in terms of if it erupted in the sea, in an volatile cinder eruption or as creeping pahoehoe lava flows, the standard image of Hawaiian basalt eruptions.

The Columnar Basalt is produced during the period of the cooling of the thick lava flow, which forms contractional joints or fractures. The flow usually shrinks in the vertical measurement without fracturing, and cannot manage to sink in horizontal direction until the cracks are formed. This exclusive fracture networks results into the columnar formation. Some of the famous columnar basalts formed are Giant's Causeway, Devil's Postpile, Narooma Basalt (Narooma, New South Wales, Australia), Samson's ribs and Isle of Staffa, Inner Hebrides