Columnar jointing

Columnar jointing is a pattern of polygonal rock columns formed when cooling rock contracts and fractures. Its joints commonly meet in three-way junctions, producing mostly five- or six-sided columns.

Thermal contraction: The reduction in a material’s dimensions as its temperature falls. Cooling lava or shallow intrusions contracts and builds stresses that open column-bounding fractures.
Giant's Causeway: A coastal exposure of polygonal basalt columns in County Antrim, Northern Ireland. Its exceptionally accessible basalt columns make it a classic example of the pattern.
Basalt: A dark, fine-grained volcanic rock formed when basaltic lava cools at or near the surface. Basalt is the rock type in many of the best-known columnar jointing exposures.
Cooling rate: The change in temperature of a material per unit time. Column diameter and fracture spacing can provide clues to how rapidly rock cooled.
Mudcrack: A polygonal crack pattern formed when wet sediment dries and shrinks. Both patterns reflect contraction, but mudcracks form at a surface as sediment loses water.
Joint (geology): A fracture in rock along which there has been little or no measurable displacement. Column boundaries are joints that divide cooling rock without substantially offsetting it.
Devils Postpile National Monument: A protected exposure of columnar basalt near Mammoth Lakes, California. Its tall, regular columns show the geometry produced by cooling basalt.
Lava flow: A body of molten rock that moves across Earth’s surface during an eruption. Cooling lava flows commonly develop columnar joints as their interiors solidify.
Paleothermal reconstruction: The inference of past temperature conditions from geological evidence. Joint geometry and propagation patterns can help constrain ancient cooling conditions.
Columnar basalt: Basalt divided into elongated polygonal columns by cooling joints. This is the most familiar rock-specific expression of columnar jointing.

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