Giant's Causeway in Northern Ireland

May 25, 2015


Photographer: John Adam
Summary Author: John Adam

BasaltThe Giant's Causeway is a huge deposit of columnar basalt found in County Antrim near the tip of Northern Ireland. The predominantly hexagonal-shaped columns were formed some 50 million years ago by cooling lava. In the Causeway rock, the overall jointing pattern (columnar joints), is primarily due to shrinkage of the semi-solid interior of the lava flow after cooling. The internal stresses induced by thermal contraction on horizontal surfaces lead to the formation of vertical and approximately parallel columnar joints. Three-pronged cracks are started at many points of the top surface with angles of approximately 120 degrees emanating from those points.
A crude and simplistic mathematical model (neglecting all the physics) would involve the regular tessellation of the Euclidean plane by hexagons, which (of the three regular polygons, equilateral triangles, squares and hexagons) have the minimum perimeter for a given area. This is superficially similar to the mud crack phenomenon, although that occurs on a much smaller scale and is limited to the surface layers of the mud. Physically, as cooling proceeds, the cracks (which started at the top and base of the flow) propagate inwards and solidify as three-dimensional polygonal columns. However, although hexagonal patterns are very common, there exist many irregular polygonal columns with 3-7 sides on the horizontal surface. The columns also shrink along their length and produce ball and socket convex/concave joints, dividing the columns vertically.