Columnar Joints and Landscapes
Columnar jointing and morphology
Rock that experienced columnar jointing has a straight columnar part and a distorted columnar part including irregular polygonal blocks. The former part is called “colonnade” and the latter “entablature”. The entablature might be lost by erosion in some exposures. Polygonal columns have three to eight sides, but five to seven sided columns are common. As mentioned later, when hot lava cools under an ideal condition, the cross section of column is hexagonal. Platy joints may develop perpendicular to the elongated direction of column.
Figure 1: Columnar jointing
Columnar jointing occurs in cooling process of lava and sill. Hot lava contracts as it cools. If physically uniform lava with the flat surface is static and cools evenly, the lava contracts equally with many centers of contraction across the surface. The each contraction area represents a circle due to physical uniformity. As it contracts, cracks, ideally hexagonal, are formed among the circles. These cracks extend into the lava, forming parallel hexagonal columns. Actually, since the lava does not cool evenly, columns including five-sided and seven-sided columns are created as well as six-sided columns.
When lava has a horizontal cooled surface, the columnar jointing creates vertical columns. The cracks propagate normal to isotherms, not gravitational direction. However, joints do not always grow perpendicular to isotherms. Accordingly, patterns of columnar joints represent not only vertical lines but also various forms such as Chevron, Rosette, Fan, and Basin (Figure 2), depending on cooling conditions including topography under lava and the environment around the cooling surface. The width of a column is depending on cooling rate; the faster lava is cooled down, the narrower columns are formed. Entablature is produced in a part without temperature gradient, off the cooling surface. Columnar joints well develop in the body of aa lava and massive lava with more than several meters in thickness.
Some patterns of columnar joints
The chevron pattern is formed when the lava has more than one cooling surface and joints propagate from each surface.
The rosette pattern is formed when the lava spouted from a point has a domed cooling surface.
The fan pattern is formed when the lava flowed into a valley.
The basin pattern is formed when isotherms curved attributed to the topography under the lava.
Photo 1: Wave cut bench and sea cliff
Columnar joints developed within rock are exposed by which the rock is weathered and eroded by wind, wave, stream current, and so on. Therefore, good outcrops of columnar joints are often found in coasts and valleys. Rock developing columnar joints is apt to be exfoliated or collapsed along the joints, forming cliffs. In some rocky coasts, wave cut benches (shore platforms) are seen in front of sea cliffs. A wave cut bench is produced in the intertidal zone by wave eroding and removing the rock of cliff. As a result, the cliff shifts landward. When the rock has columnar joints, a landscape characterized by columns of the cliff and hexagonal patterns on the bench is created. In a young valley formed in an area of igneous rocks or welded tuff developed columnar joints, a narrow gauge with vertical walls and falls are produced.