Large cracks in the ground, also known as fissures, are physical breaks in the Earth’s surface that vary dramatically in scale, origin, and permanence. These ruptures are reminders of the dynamic processes constantly reshaping the planet, from the superficial layer of soil to the deep, rigid crust. The formation of these features is governed by various forces, including climatic changes, hydrological activity that removes subsurface support, and immense geological stresses that fracture bedrock. Understanding why these cracks appear requires differentiating between the common, temporary breaks caused by weather and the larger, more significant fissures resulting from subterranean or tectonic movements.
Ground Cracks from Soil Desiccation and Moisture Loss
The most frequent cause of ground cracking is the desiccation, or drying out, of expansive soils, particularly those rich in clay minerals. Clay soil contains a high proportion of fine particles that absorb significant amounts of water, causing the soil volume to swell considerably when wet. When these soils are exposed to prolonged periods of drought or excessive evaporation, the moisture content plummets, and the soil mass begins to shrink.
This volumetric shrinkage generates internal tensile stress within the soil body, which eventually exceeds the soil’s tensile strength, causing it to fracture. The resulting cracks typically form a polygonal or hexagonal pattern on the surface, which is a characteristic response of a shrinking material. The depth and width of these cracks are directly proportional to the severity and duration of the moisture deficit.
The process of desiccation cracking is often exacerbated by repeated cycles of drying and wetting. As the soil rehydrates, the cracks close, but the previous fractures create planes of weakness that make the soil more susceptible to cracking during the next dry period. This cycle of volume change significantly affects the soil’s mechanical properties and can create pathways for increased water infiltration.
Subsidence Caused by Subsurface Extraction and Karst Activity
Larger ground cracks and fissures often arise from the loss of underlying support, a phenomenon known as subsidence. One major cause is human activity, specifically the excessive withdrawal of underground fluids like groundwater, oil, or natural gas. When large volumes of water are pumped from certain types of aquifers, the pressure that once supported the overlying soil and rock layers is reduced.
The removal of this support causes the unconsolidated sediments to compact and settle, leading to a permanent reduction in the total storage capacity of the aquifer system. This compaction manifests at the surface as a gradual sinking of the land over a large area. This process can also generate large, localized fissures at the edges of the subsiding zone as the land surface adjusts to the deeper-seated volume loss.
Natural processes also cause subsidence, particularly in regions underlain by soluble bedrock such as limestone, dolomite, or gypsum, a landscape known as karst topography. Slightly acidic rainwater and groundwater dissolve these rocks over thousands of years, creating underground cavities, caves, and voids. When the ceiling of a void can no longer support the weight of the overlying soil and rock, the ground collapses suddenly to form a sinkhole. This dissolution process, often accelerated by changes in the water table, can also lead to the formation of surface fissures that radiate outward from the area of collapse.
Deep Fissures Caused by Tectonic Stress
The largest, most permanent ground cracks and fault lines are the result of immense forces generated by the movement of the Earth’s rigid outer layer, the tectonic plates. These deep fissures represent structural breaks in the bedrock caused by the accumulation and sudden release of crustal stress. This stress builds up as tectonic plates slowly converge, diverge, or slide past one another.
When the stress exceeds the rock’s strength, the crust ruptures, creating a fault and releasing energy in the form of an earthquake. The resulting surface rupture can be a visible crack or a step in the ground, representing the physical displacement along the fault plane. Normal faults, where the crust is being pulled apart (extension), result in one block dropping relative to the other, while strike-slip faults involve blocks sliding horizontally past each other.
Tectonic fissures involve the entire lithosphere and are driven by deep-earth processes rather than surface conditions. While seismic activity can cause sudden, dramatic surface ruptures, slow crustal movement, known as aseismic creep, can also cause the ground to deform and fissure gradually over time. These features are the most enduring testament to the planet’s internal geological engine.