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Columnar jointing
Columnar jointing
Columnar jointing
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Columnar jointing in Giant's Causeway in Northern Ireland
Columnar jointing in the Alcantara Gorge, Sicily

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 (e.g. basalt, andesite, rhyolite, tuff), 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.[1] Columnar jointing also occurs rarely in sedimentary rocks, due to a combination of dissolution and reprecipitation of interstitial minerals (often quartz or cryptocrystalline silica) by hot, hydrothermal fluids and the expansion and contraction of the rock unit, both resulting from the presence of a nearby magmatic intrusion.[2]

The columns can vary from 3 meters to a few centimeters in diameter, and can be as much as 30 meters tall.[1] They are typically parallel and straight, but can also be curved and vary in diameter.[1] An array of regular, straight, and larger-diameter columns is called a colonnade; an irregular, less-straight, and smaller-diameter array is termed an entablature.[3] The number of sides of the individual columns can vary from 3 to 8, with 6 sides being the most common.[1]

Physics

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When magma or lava cools into solid igneous rock, for example basalt, much heat still remains. As it cools down further, the basalt contracts, and it forms cracks to release the tensile energy. It then is cooled down further by groundwater boiling and reflux.

When the cracks first form at the surface, the cracks are dominated by T-junctions, like mudcracks, because they were formed individually. One crack would form and move across the surface, until it hits upon a previous crack, forming a T-junction.

Then, these cracks extend downwards in a moving front that is roughly planar and parallel to the surface. As it moves, the crack pattern anneals to become lower in energy. The speed v at which the front moves is determined by the groundwater flow rate. After the front moves a few meters deep, it would evolve into a hexagonal grid with roughly equal width L. The width L is determined by the basalt's material properties, and the speed v at which the front moves.[4]

Scaling

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Define Péclet number where is the thermal diffusivity of the material. For all columnar jointing, the value of Pe is around 0.2, and thus the shape and speed of all columnar joints are similar after scaling. A scaled model can be made by drying cornstarch a centimeter thick, which creates columns about 1 mm wide.

For basalt, . For cornstarch, .[4]

Places

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Main article: List of places with columnar jointed volcanics

Some famous locations in the United States where columnar jointing can be found are Devils Tower in Wyoming, 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, Fingal's Cave on the island of Staffa, Scotland and the Stuðlagil Canyon, Iceland.[5]

Devils Tower

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Main article: Devils Tower
Devils Tower is an eroded laccolith in the Black Hills National Forest, Wyoming.

Devils Tower in Wyoming in the United States is about 40 million years old and 382 meters (1,253 feet) high.[1] Geologists agree that the rock forming 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.[6]

Giant's Causeway

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Main article: Giant's Causeway

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.[1][7] According to a legend, the giant Finn McCool created the Giant's Causeway, as a causeway to Scotland.[8]

Sōunkyō Gorge

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Main article: Sōunkyō

Sōunkyō Gorge, a part of the town of Kamikawa, Hokkaido, Japan, features a 24-kilometre (15 mi) stretch of columnar jointing, which is the result of an eruption of the Daisetsuzan Volcanic Group 30,000 years ago.

Deccan Traps

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Main article: Deccan Traps

The late Cretaceous Deccan Traps of India constitute one of the largest volcanic provinces of Earth, and examples of columnar jointing can be found in St. Mary's Island in the state of Karnataka.[9]

High Island Reservoir

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Formed in Cretaceous, the columnar rocks are found around the reservoir and the islands nearby in Sai Kung, Hong Kong. It is special that the rocks are not mafic, but felsic tuff instead.

Hexagonal volcanic tuffs at East Dam of High Island Reservoir



Makhtesh Ramon

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The columnar jointed sandstone of the HaMinsara (Carpentry Shop) in the makhtesh (erosion cirque) of Makhtesh Ramon, Negev desert, Israel.

Columnar jointing in sandstone, Cerro Koi, Paraguay

Cerro Kõi

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There are several examples of columnar jointed sandstones in the greater Asunción region of Paraguay. The best known is Cerro Kõi in Areguá, but there are also several quarries in Luque.

Mars

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Main article: Marte Vallis

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).[10][11]

Columnar jointed rocks in unnamed crater wall, Marte Vallis region, Mars. Image courtesy of the High Resolution Imaging Science Experiment, University of Arizona.

Sawn Rocks

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Columnar jointing at Sawn Rocks

Sawn Rocks, in Mount Kaputar National Park close to Narrabri, New South Wales, Australia, features 40 meters of columnar jointing above the creek and 30 meters below the surface.[12]

Basaltic Prisms of Santa María Regla

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Main article: Basaltic Prisms of Santa María Regla

Alexander von Humboldt documented the prisms located in Huasca de Ocampo, in the Mexican state of Hidalgo.

Columnar basalt of Tawau (Batu Bersusun)

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At Kampung Balung Cocos, Tawau, Malaysia, the river flows through the area of columnar basalt. One section is seen vertically high on river bank. The rest lies on river bank. The water flows from the lowest area forming waterfall.

Garni gorge

Gorge of Garni, Armenia

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The Garni Gorge is situated 23 km east of Yerevan, Armenia, just below the village of the same name. This portion of the Garni Gorge is typically referred to as the "Symphony of the Stones." On a promontory above the gorge the first-century AD Temple of Garni may be seen.

Stuðlagil Canyon, Iceland

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Main article: Stuðlagil

The Stuðlagil Canyon, situated 45 miles west of Egilsstaðir, showing a view of columnar joint basalts rock formations and the blue-green water that runs through it.

Black Point, Australia

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Black Point, situated in the D'Entrecasteaux National Park, in Western Australia is an example of black columnar basalt.

Jusangjeolli, South Korea

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The columnar jointing (Jusangjeolli in Korean) on the southern coast near Seogwipo on the island of Jeju is a popular tourist destination.

See also

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References

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Revisions and contributorsEdit on WikipediaRead on Wikipedia

Columnar jointing

View on Grokipedia
from Grokipedia
Columnar jointing is a geological characterized by the development of parallel, prismatic fractures in igneous rocks, typically forming polygonal columns with five to seven sides, most commonly hexagonal, as a result of thermal contraction during the cooling of lava flows or shallow intrusions. These fractures propagate to the cooling surfaces, creating elongated columns that can reach heights of up to 30 meters and diameters ranging from a few centimeters to 3 meters. The formation process begins as hot or lava cools and contracts, generating tensile stresses that initiate cracks at the rock's periphery and propagate inward toward the center. The hexagonal pattern emerges because it provides the most efficient geometric arrangement for stress relief, with three fractures meeting at 120-degree angles to minimize . Cooling rates significantly influence the structure: slow, steady cooling from the base or sides produces regular, straight-sided colonnades, while rapid cooling from the top—often enhanced by exposure to air or water—results in irregular, more fractured entablatures with thinner columns. Columnar jointing occurs in various igneous settings, including basalt lava flows, sills, dikes, ignimbrites, and shallow intrusions of all compositions, and is particularly prominent in volcanic regions where uniform magma composition and slow cooling allow for well-developed columns. Notable examples include the in , featuring approximately 40,000 interlocking basalt columns from an ancient lava flow; in , with 100,000-year-old columns up to 60 feet long where about 55% are hexagonal; and Devils Tower National Monument in , an approximately 50-million-year-old volcanic neck exhibiting irregular columns 6 to 8 feet in diameter at the base. These features not only reveal insights into past cooling histories but also facilitate hydrothermal fluid circulation through the cracks, influencing subsequent rock alteration.

Definition and Characteristics

Geological Description

Columnar jointing consists of sets of regularly spaced, parallel fractures that intersect to form prismatic columns, primarily in igneous rocks such as basaltic lava flows, sills, dikes, and shallow intrusions. These fractures develop as the rock cools and contracts, creating a network of tension cracks that divide the material into elongated, prism-like units. The columns are typically oriented perpendicular to the cooling surface, resulting in a structured pattern distinct from other fracture types in geology. In exposed outcrops, such as cliffs or faces within volcanic flows, columnar jointing appears as vertical or near-vertical columns extending from meters to tens of meters in length. For instance, at sites like Devils Postpile in , columns reach up to 18 meters tall with diameters of about 0.75 meters, standing in orderly arrays that highlight the jointing's geometric regularity. This visibility in cross-section or plan view reveals polygonal outlines, often forming a mosaic-like pattern across the rock face. Unlike systematic joints, which feature parallel fractures with consistent spacing and orientation across broader scales, or irregular joints that exhibit curved, haphazard patterns without uniformity, columnar jointing is characterized by its intersecting, prismatic confined to cooling-induced contexts in igneous materials. The cross-sections of these columns commonly display polygonal shapes, with hexagonal forms dominating due to their efficient packing.

Geometric Features

Columnar jointing typically manifests as prismatic structures with polygonal cross-sections, predominantly hexagonal in mature patterns. Statistical analyses of over 3,000 columns from 50 global sites reveal that approximately 50% are hexagonal, 33% pentagonal, and the remainder include occasional heptagons, quadrilaterals, or other polygons, with an average polygon order of 5.71. This distribution arises from the of contraction cracks, though pentagons are more prevalent in regions of faster cooling. The joints forming these columns are generally oriented perpendicular to the primary cooling surface of the igneous body, resulting in elongated prisms that extend inward from the exterior. This perpendicular alignment produces orthogonal or sub-orthogonal sets of fractures, with column axes often vertical in horizontal flows or radial in intrusive bodies like sills and dikes. Deviations from perfect orthogonality can occur due to irregular cooling gradients, but the overall geometry maintains a systematic pattern. In layered structures, geometric variations distinguish and zones. zones feature straight, well-organized columns with consistent diameters and parallel sides, reflecting uniform cooling. In contrast, zones exhibit curved, irregular columns that are narrower and more disorganized, often with branching or twisting forms due to heterogeneous thermal stresses. Column diameters and joint spacings vary but commonly range from 0.5 to 2 meters in basaltic formations, with side lengths averaging around 0.5 to 1 meter in many exposures. For instance, measurements in the show averages near 0.95 meters, while thicker flows can produce columns up to 3 meters across. These dimensions scale with the extent of tensile stress propagation during cooling, influencing the overall robustness of the pattern.

Formation Mechanisms

Cooling and Contraction Process

Columnar jointing initiates during the cooling of molten , such as lava flows or intrusions, where the material undergoes volumetric contraction as its temperature decreases. This process begins with relatively uniform cooling at the exposed surfaces, including the top, bottom, and sides of the flow, leading to a reduction in volume that generates internal stresses. As the rock solidifies, the contraction is most pronounced at the cooler exterior, while the hotter interior remains more fluid, creating differential shrinkage. Tensile stresses develop as the solidification front advances inward from the exterior, with the outer layers contracting against the less-contracted core. These stresses accumulate parallel to the —surfaces of equal —resulting in fractures that propagate to these isotherms to relieve the tension. The cracks typically start at the surfaces and extend toward the center, forming polygonal patterns as multiple fractures intersect. This outward-to-inward progression ensures that the joints are oriented normal to the direction of heat loss. The cooling process often produces distinct zonation within the rock body, characterized by a colonnade of regular, elongated columns near the base or cooler margins and an entablature of irregular, more chaotic fracturing in the upper or interior zones. The colonnade forms under steady, slower cooling conditions, yielding straight, prismatic columns, whereas the entablature arises from rapid, variable cooling—often influenced by interaction with water or air—leading to haphazard crack orientations. The duration of this cooling and contraction process varies with the thickness of the rock body, ranging from days for thin flows to years for thicker ones. For instance, a lava flow several meters thick may develop joints over weeks to months, while deeper bodies, like the approximately 400-foot-deep (120 m) Iki lava lake, required approximately 35 years to fully solidify and complete joint formation.

Thermal Stress and Fracturing

As a lava flow cools from its margins inward, the outer layers solidify and contract more rapidly than the still-hot interior, generating differential thermal stresses that accumulate as tensile forces perpendicular to the cooling front. This contraction arises from the thermal expansion of the rock, with a coefficient typically on the order of 5–10 × 10⁻⁶ °C⁻¹ for basalts, leading to circumferential tensile stresses that increase with the temperature gradient across the solidifying layer. When these thermal stresses exceed the tensile strength of the , which ranges from 10 to 30 MPa for intact material, tensile occurs, initiating fractures normal to the principal stress direction. In basaltic lavas, this typically happens between 840 and 890 °C, where the rock has transitioned to a sufficiently solid state to support elastic stresses of 12–18 MPa without significant viscous relaxation. The fractures serve to relieve the accumulated strain, partitioning the cooling body into discrete prisms that further facilitate internal cooling. Crack in columnar jointing proceeds incrementally toward the hotter interior, advancing along isotherms where stresses remain high enough to drive extension. Each increment of growth is marked by small steps on the surface, reflecting episodic stress buildup and release as the cooling front progresses; these steps are often 1–10 cm in height and align with the direction of maximum tensile stress. Branching occurs when secondary cracks deviate slightly from the primary plane due to local perturbations in the stress field, ultimately forming polygonal cross-sections that optimize perimeter-to-area ratios for efficient stress distribution. This is arrested deeper in the flow where gradients diminish and stresses fall below the tensile threshold. During the semi-solid state above the temperature (around 700 °C), the lava's high —on the order of 10¹⁰–10¹² Pa·s—allows partial relaxation of thermal stresses through ductile flow, delaying initiation. Below this transition, the material behaves more elastically, with a increasing to 50–80 GPa, enabling brittle failure as stresses accumulate without viscous dissipation. This shift from viscous to elastic dominance is critical, as it marks the onset of irreversible fracturing that defines the columnar .

Physics and Scaling

Column Dimensions and Influencing Factors

The dimensions of columns in columnar jointing, typically measured by their diameter or spacing between joints, are primarily governed by the cooling rate of the rock body, with faster cooling producing narrower columns and slower cooling yielding wider ones. This relationship arises because rapid cooling generates higher thermal gradients, leading to more frequent fracturing to relieve contraction stresses. Empirical observations from basaltic lavas show that column widths are inversely proportional to the cooling rate, with widths ranging from 17 to 55 cm in flows cooled at rates of 200–2000 W/m². Latent heat release during crystallization further influences this process by buffering temperature drops, effectively slowing the cooling front and allowing for larger column formation in rocks with higher crystallization enthalpies, such as those in mafic compositions. Flow thickness also plays a key role, as thicker bodies experience more gradual internal cooling, resulting in larger average column diameters. Studies of igneous rocks indicate that column diameters increase with flow thickness, with typical ratios of 0.01 to 0.1. For instance, in 10 m thick basaltic flows, column diameters are often around 0.3–0.5 m, though this varies with local conditions. This scaling relates to the (Pe ≈ 0.3 ± 0.2), a dimensionless measure of versus that links column size to the thermal boundary layer thickness. Joint spacing tends to increase with distance from the cooling surface, being smallest near the exterior and enlarging inward, which reflects the diminishing thermal gradient deeper within thicker flows. Rock composition affects column size through differences in thermal diffusivity, viscosity, and mechanical properties; felsic rocks generally form larger columns than mafic ones due to slower cooling rates and higher fracture toughness. Environmental cooling conditions exacerbate these effects: subaerial exposure to air promotes slower cooling and broader columns, while interaction with accelerates heat loss, producing finer jointing with diameters reduced to centimeters. Variations are evident across rock types, with smaller columns (often <0.1 m) in rapidly cooled volcanic tuffs compared to the larger prisms (up to 1–2 m) in thick basaltic flows.

Mathematical Models and Patterns

Mathematical models of columnar jointing primarily rely on linear elastic fracture mechanics (LEFM) to describe crack propagation driven by thermal stresses during cooling. In LEFM frameworks, cracks advance when the at the crack tip reaches a critical value, governed by Griffith's criterion, where the release of elastic energy balances the creation of new surfaces. Specifically, the is expressed as K=σπaK = \sigma \sqrt{\pi a}
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