Evolving fracture patterns: columnar joints, mud cracks, and polygonal terrain

TL;DR

This paper explains how different crack patterns like rectilinear and hexagonal form and evolve in contracting layers, influenced by forces and repeated opening cycles, with implications for natural formations such as mud cracks and columnar joints.

Contribution

It demonstrates how both rectilinear and hexagonal crack patterns can originate from the same forces and describes the evolution process from rectilinear to hexagonal patterns in natural settings.

Findings

Hexagonal patterns often result from repeated crack opening and healing.

Rectilinear patterns evolve towards hexagonal with multiple cycles.

Crack patterns are guided by previous crack positions and slight variations.

Abstract

When cracks form in a thin contracting layer, they sequentially break the layer into smaller and smaller pieces. A rectilinear crack pattern encodes information about the order of crack formation, as later cracks tend to intersect with earlier cracks at right angles. In a hexagonal pattern, in contrast, the angles between all cracks at a vertex are near 120$^\circ$. However, hexagonal crack patterns are typically only seen when a crack network opens and heals repeatedly, in a thin layer, or advances by many intermittent steps into a thick layer. Here it is shown how both types of pattern can arise from identical forces, and how a rectilinear crack pattern evolves towards a hexagonal one. Such an evolution is expected when cracks undergo many opening cycles, where the cracks in any cycle are guided by the positions of cracks in the previous cycle, but when they can slightly vary their position, and order of opening. The general features of this evolution are outlined, and compared to a review of the specific patterns of contraction cracks in dried mud, polygonal terrain, columnar joints, and eroding gypsum-sand cements