Publication details
Origin of columnar jointing and direction of cooling
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| Year of publication | 2015 |
| Type | Conference abstract |
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| Description | Columnar jointing has been studied since the first report of Giant´s Couseway in North Ireland. This phenomenon was studied for centuries and many theories tried to explain its origin. The formation of igneous columnar joints is now reasonably understood. These joints, which result from thermal contraction during cooling of magma or lava flow, reflect the thermal history of a rock mass. The symmetric natural fracture network is formed during the cooling of volcanic bodies. Usually, we may observe a variety of columnar shapes in the nature such as quadratic, pentagonal or hexagonal. However, the most common geometry is hexagonal. The reason is that hexagonal honeycomb-like arrangement of contraction cracks would maximize the area-to-fracture-length ratio and minimize the total crack area at the same time relive as much of the thermal stress as possible. The columns always grow perpendicularly to the cooling surface, and the fracture pattern tend to be propagated from the surface towards the center of the volcanic body as it cools. The fractures have usually developed tensile cracks known as growth bands on their surface. The bands are the most significant features on the surface of the column; prolongation of the bands is oriented perpendicularly to the axis of the columns. Initial point showing where the tectonic band began to grow can be found thanks to the geometry of plumose structure. Relationship between two bands on the same column side allows us to determine the direction of gradual cooling. Special type of little ridges within the plumose structures has an echelon shape and it is known as twist-hackle. |
