Overview
In volcanic rocks, such as lava flowing from basaltic lavas or ignimbrites formed from condensed volcanic ash, shrinkage fractures or cooling fractures form regularly as the rock cools and contracts, giving the rocks separated by the fractures an elongated columnar shape with a cross-section of four to six angles.
They vary in diameter from a few centimeters to several meters, with some as long as 30 meters. The columns are typically parallel to each other and straight. Another world-famous columnar formation is the columnar formation at Devils Tower in Wyoming, USA, which formed about 40 million years ago and is about 382 meters long.
Fractures caused by contraction when magma or lava cools
The Giant’s Causeway, off the coast of Antrim in Northern Ireland, is another famous example, formed by volcanic activity about 60 million years ago and is home to more than 40,000 sculptures.
The word “columnar” means “joint,” and in geomorphic terms, a joint is a crack or grain in a rock. They are usually formed when hot magma or lava contracts during rapid cooling when it encounters cold air or water, and they usually form in clusters of columnar stones, hence the name. It is known for its characteristic long, vertically oriented columnar stones, which form unique landforms, but not necessarily vertically. They are most commonly found in the veins of volcanic rocks such as basalt and andesite, lava, and molten tuff, and are most common in basaltic lava, which has a high temperature and high fluidity.
Columnar fractures usually appear as regular rows of stones with hexagonal cross-sections, and many places have been developed as tourist destinations due to their unique shape. The hexagonal cross-section is believed to be due to the even distribution of cooling and contraction centers on the surface of the lava, with contraction occurring evenly around each contraction center. However, they do not necessarily appear as hexagons, but rather as squares, pentagons, and other shapes. They typically range in size from a few centimeters to several meters in cross-section, and the columns can be several meters long, with longer ones reaching tens to hundreds of meters.
They are usually found in volcanic rocks and are best seen in places where volcanic activity has been active, such as the Giant’s Causeway on the northern coast of Northern Ireland, the Garni Valley in Armenia, the Devil’s Tower in Wyoming, USA, and several coasts of Iceland.
Creation of Columnar Basalt
Columnar basalt are mainly formed as tensile fractures in igneous rocks that have been intruded shallow underground. They form when thermal stresses are exerted between the host and intrusive rocks, usually in eruptive igneous rocks or shallowly intruded rocks, by thermal contraction and cooling. When high-temperature lava or tuff solidifies, the temperature is much higher than the surrounding host rock, so the thermal contraction is quite large.
The molten contact between the two rocks does not cause any relative displacement, and as it cools, it creates significant thermal stresses between the two rocks. The vertical stress component parallel to the contact surface of the two rocks prevents the igneous rock from shrinking as much as thermal shrinkage would require, and this stress component is balanced by the compressive stress in the parent rock, the force of which is greater than that required by thermal shrinkage.
In general, tensile stresses in igneous rocks become parallel to the isothermal surface during cooling. Because rocks are more susceptible to tensile stresses than compressive stresses, tensile insulation readily forms at right angles to the isothermal surface.
The formation of scaphoidal fractures, the tortoiseshell-like cracks in clayey rice fields during summer drought, is still not fully understood. However, tensile stresses in two orthogonal directions require more than one insulation system. These insulation systems fill the spaces in the prismatic columns formed by the incisions if the columns have a triangular, quadrangular, or hexagonal cross-section.
A hexagonal column has the least amount of insulation per unit volume of each column. Therefore, it takes less energy to form a hexagonal cross-section than it does to form a prism with a different shape, so this shape is common.