The Science of Sinkholes: Formations, Types and Global Hotspots
Suddenly, the ground under your feet starts to sink into a vast deep crater that can swallow up your vehicle, the highways, and buildings in mere seconds. It is not a scene from a science fiction film; it's the actual geological occurrence of a sinkhole.
Some sinkholes develop over several hundred years, but others can form suddenly and without warning. It is critical to modern urban planning and safety to understand the geographical zones at risk, the types of architecture likely to suffer sinkholes, and the conditions that can precipitate their appearance.
What is a Sinkhole?
A sinkhole, also known as a shake-hole, swallow-hole, or doline, is a hole or depression in the ground caused by the collapse of the upper layers of the earth. They are found mostly in landscapes with karst topography, where the underlying bedrock is composed of rocks that can dissolve in water, such as limestone, dolomite, gypsum, and salt.
Acidic rainwater percolates into the ground, over millennia, eating away the soluble bedrock to form vast underground caves. Once the roof in one of these concealed caves becomes too thin to withstand the weight of soil and buildings above, the ground collapses. Sinkholes are a common occurrence, but can form rapidly and catastrophically due to human activity such as heavy groundwater pumping, leaky underground water mains, and improper construction drainage.
The Three Basic Classes of Sinkholes
Sinkholes can be categorized according to the manner in which the mechanical failure takes place:
1. Cover-Collapse Sinkholes
They are the most serious and dramatic of sinkholes. They occur when the earth above, in the form of a cohesive soil (clay), is thick enough to cause a cavern to develop below the surface. As time passes, sediment comes in from below and forms a void in the cave in the shape of a hollow dome which progresses upward. The surface ground looks completely solid until the remaining roof is very thin and suddenly collapses without warning.
2. Cover-Subsidence Sinkholes
These voids are created over time in permeable, non-cohesive surface materials, such as sand. Groundwater slowly dissolves the underlying limestone, and the sand grains slowly fill the cracks in place of the lost limestone. This forms a surface depression that is noticeable over a few months or years, usually without any structural catastrophes.
3. Solution (Dissolution) Sinkholes
Solution sinkholes form when the water-soluble bedrock is exposed to the surface or is thinly covered with soil. The rainfall and surface runoff directly accumulate in the natural fractures and erode the rock from the top to the bottom. This creates a depression that spreads out and that will settle into a slow pond, sometimes forming a small, natural pond.
Where are Sinkholes Found?
While sinkholes can occur in any place where karst formations occur, there are locations on Earth that are particularly affected by these underground voids:
- China: Home to the world's largest and deepest natural sinkhole, the Xiaozhai Tiankeng (also referred to as the "Heavenly Pit"), located in Chongqing, plunging over 600 meters deep, housing its own isolated ecosystem at the bottom.
- United States: About 20% of the land mass is very prone to sinkholes. Florida, Texas, Alabama, Missouri, and Kentucky are among the states that are built on highly karstified platforms.
- Central America and Mexico: The Yucatán Peninsula is renowned for its cenotes, which are water-filled sinkholes. Furthermore, the Taam Ja' Blue Hole in Chetumal Bay has been charted by sea-going explorers to be a record-breaking underwater sinkhole that goes a thousand feet deep.
- Other Vulnerable Zones: Human-induced sinking of piping holes in unconsolidated volcanic ash is a common issue in Guatemala City, and in parts of Japan, Croatia, and the Khammouan mountains, there are massive karst structures.
Early Warning Signs of a Developing Sinkhole
Several surface indicators may be noted before a significant collapse occurs, and these can be observed by a homeowner and/or a municipal engineer, but are not visible from a distance:
- Structural Anomalies: New, deep cracks in interior walls, tiling, concrete foundation, or outdoor sidewalks.
- Surface Depressions: Subtle, circular tilting or slumping in lawns, fence posts, or backyard trees.
- Hydrological Changes: Abnormally large collections of water into small rainwater ponds in places where water never pooled before or abnormally dry soils in localized areas where water is draining into new underground crevices.
- Infrastructure Stress: Sudden changes in the water utility line pressure or uncovered underground pipes due to movement of the underlying earth.
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