Mudcrack

Mudcracks, also known as desiccation cracks, are sedimentary structures. They form when muddy sediment dries out and contracts. These patterns are important because they tell geologists about the history of an environment. They can show if a place was once a pond or a river channel. They can even show if an area was sunny or shady.

The formation of a mudcrack is a specific physical process. It begins when wet, muddy sediment is exposed to the air. As the water evaporates, the top layer of mud begins to shrink. This creates a condition called strain. The strain happens because the top layer wants to get smaller, but the material underneath stays the same size. When this strain becomes too great, the surface must break to relieve the pressure. This results in channel cracks that spread and join. These interconnected networks of shapes are called tessellations.

Geologists classify mudcracks based on several different features. One way to look at them is by their completeness. Complete mudcracks form a fully connected, tessellating network. Incomplete mudcracks are not connected to one another. They still form in the same area, but they do not join up. Another way to classify them is by their plan-view geometry. Orthogonal cracks meet at right angles. These can be oriented in rows or appear in random patterns. Non-orthogonal cracks form geometric patterns that do not meet at right angles. These often look like three-point star shapes or small polygonal tiles.

The shape of the crack changes depending on how the environment behaves. Most mudcracks are polygonal when viewed from above. In a cross section, they look like the letter V. The wide part of the V opens toward the top of the sediment. The narrow part tapers downward. If a mudfield is wetted and dried many times, the junctions change. Initial cracks often have T-shaped junctions. Repeated cycles can change these to Y-shaped junctions. This happens because Y-shaped junctions are thermodynamically favored. This is similar to how columnar jointing forms in nature.

Different environments produce different types of crack patterns. Mudcracks form in places that were once saturated with water. This includes abandoned river channels, floodplains, and dried ponds. The amount of sunlight also matters. Rapid drying in sunny environments creates widely spaced, irregular cracks. Shady environments produce closer, more regular crack patterns. Sometimes, the drying is so intense that mud curls form. This happens in very thin layers of mud. As the water evaporates, the layers separate and the top layer curls upward.

Scientists have studied these structures to understand the Earth. In 1982, J.R.L. Allen proposed a classification scheme. He focused on the completeness, orientation, and shape of the cracks. Other researchers, like R. Linholm, studied their geometric patterns. We even see these patterns in human-made materials. They can appear in ceramic glazes, paint films, and poorly made concrete. Modern scientists even study tiny crack patterns in technological thin films using micro and nanotechnologies.

Sometimes, mudcracks are preserved for millions of years in rock. They can be preserved on the top of a sediment bed. In this case, they look exactly as they did when they formed. They can also be preserved as casts on the bottom of an overlying bed. This happens when dried cracks are covered by fresh, wet sediment. Pressure pushes the new sediment into the cracks. The new material then dries and hardens. When erosion happens, the original cracks erode faster than the new filler. Geologists use these bottom-of-bed casts to find the original orientation of rocks. This helps them understand how rocks were folded or faulted by the Earth.