Large cracks in the ground form when soil or rock is pulled apart by forces stronger than the material can hold together. Those forces range from tectonic plates shifting deep below the surface to something as common as groundwater being pumped out for agriculture. The specific cause determines whether a crack is inches wide or several feet across, and whether it’s a one-time event or slowly growing.
Tectonic Forces and Earthquake Ruptures
The most dramatic ground cracks come from movements in Earth’s crust. When tectonic plates pull apart, enormous tensile strain builds up in the rock. Eventually that strain releases through faults and fractures that propagate upward to the surface. In rift zones like the Kenya Rift Valley, researchers have traced surface fissures in loose soil all the way down to deep rock fractures in volcanic bedrock, showing that what looks like a simple crack on the surface can be rooted in continental-scale geology.
Earthquakes create ground cracks more suddenly. When a fault ruptures, the energy radiates outward and can split the surface along the fault line and in surrounding areas. During the 2021 magnitude 7.4 Madoi earthquake in China, shear-tension fractures appeared across bands 50 to 100 meters wide, with individual cracks opening 20 to 30 centimeters. These cracks often appear in staggered, stair-step patterns called en echelon fractures, a telltale sign of seismic origin.
Groundwater Pumping and Land Subsidence
One of the most widespread causes of large ground cracks in the modern world is excessive groundwater withdrawal. When water is pumped from underground aquifers, the pressure that once held soil particles apart drops. Without that water pressure, the full weight of the overlying ground compresses the soil beneath it, and the surface sinks. This process, called land subsidence, doesn’t always happen evenly. Where the ground sinks unevenly, the surface stretches and eventually tears open.
The mechanics are particularly stubborn in clay-rich soils. Clay compresses under pressure but barely rebounds when water is added back. Studies show that after compression, the pore space in clay recovers only about one-fifth of the volume lost. The clay particles physically reorient and the pore shapes flatten, making the damage largely permanent. This is why refilling an aquifer doesn’t undo the subsidence or close the cracks.
Arizona is one of the best-documented examples. The Arizona Geological Survey classifies earth fissures as open ground fractures in loose sediments caused by tensional stress from land subsidence, which itself is powered by decades of heavy groundwater pumping. These fissures threaten roads, canals, gas lines, and buildings. In California’s San Joaquin Valley, overpumping has caused the land to sink at over one foot per year in many years since 2006, a record-breaking rate that continues to open new cracks and warp infrastructure.
Shrinking and Swelling Clay Soils
You don’t need tectonic forces or industrial-scale pumping to get significant ground cracks. Certain soils crack on their own during dry weather. All clay soils shrink as they lose moisture and swell when they get wet again, but soils rich in expansive clay minerals like smectite are especially reactive. The British Geological Survey notes that soils with high volume-change potential can cause foundation damage with even a small shift in moisture content.
During prolonged drought, these soils lose water from the surface downward. As they contract, the shrinkage creates deep polygonal cracks that can extend several feet into the ground. The Arizona Geological Survey distinguishes these “giant desiccation cracks” from subsidence fissures: they form in fine-grained sediments like clay, purely as a result of drought, and tend to appear in predictable patterns on flat terrain. Seasonal wet-dry cycles can reopen the same cracks year after year, gradually widening them.
Underground Mining
When material is removed from underground, the surface above eventually responds. Coal mining is the most common culprit. After a coal seam is extracted, the overlying rock and soil lose their support. The surface settles into a broad depression called a subsidence trough, and the edges of that trough experience strong horizontal tension. Research on longwall mining operations shows that surface cracks form primarily in these tensile zones, where the ground is being stretched rather than compressed.
These cracks start at the surface and propagate downward as mining advances. Engineers call them “downward fractures” to distinguish them from fractures that develop upward from the mined-out void. The surface crack keeps deepening until the horizontal stress in the soil drops to equal the soil’s natural cohesion, at which point the crack stops growing. In high-intensity coal mining, both types of fractures can develop simultaneously, sometimes connecting to create pathways that disrupt groundwater flow and surface drainage across a wide area.
How Ground Cracks Differ From Sinkholes
People often confuse large ground cracks with sinkholes, but they form through different processes. A sinkhole is a roughly circular depression that forms when underground rock dissolves (typically limestone, gypsum, or salt) and the surface collapses into the void. Sinkholes are most common in “karst terrain,” where soluble bedrock sits beneath the surface. They act as natural drains: rainwater flows in and disappears underground.
Earth fissures, by contrast, are linear. They form from horizontal stretching of the ground rather than from a cavity opening below. The U.S. Geological Survey groups sinkholes as one type of ground collapse within the broader category of land subsidence, which also includes aquifer compaction, underground mining effects, and permafrost thaw. Subsidence can affect areas thousands of square miles in size, while sinkholes tend to be localized.
What Ground Cracks Mean for Buildings
If you’re noticing cracks near your home, the width and pattern tell you a lot about whether the issue is cosmetic or structural. Hairline cracks less than 1/16 of an inch wide are common in concrete foundations and typically result from normal shrinkage as concrete cures. These are surface-level and don’t compromise structural strength.
Cracks that signal a real problem share certain characteristics:
- Width over 1/10 of an inch, especially if they’re expanding over time
- Horizontal cracks, with or without the wall bowing inward
- Stair-step cracks in brick or masonry, following the mortar joints
- Diagonal cracks or clusters of vertical cracks near one another
These patterns typically indicate that the soil beneath or around the foundation is shifting, whether from expansive clay, regional subsidence, or poor drainage. Large, spreading cracks that grow visibly over weeks or months point to active ground movement that will likely continue without intervention.