What causes a sinkhole?
Sinkholes are all about water.
- Water dissolved minerals in the rock, leaving residue and open spaces within the rock. (This is called "weathering".)
- Water washes away the soil and residue from the voids in the rock.
- Lowering of groundwater levels can cause a loss of support for the soft material in the rock spaces that can lead to collapse.
- Changing groundwater gradients (due to removing or introducing water to the system) can cause loose material to flush out quicker from the voids and the surface to collapse in response.
- Any change to the hydrologic system (putting more water in or taking it out) causes the system to become at least temporarily unstable and can lead to sinkholes.
- Sinkholes can result from seasonal changes in the groundwater table, freeze and thaw of the ground, and extremes in precipitation (drought vs heavy rain).
Karst landscapes develop naturally through the weathering process so a sinkhole can be considered a natural occurence. But, human influence causes sinkholes to occur where they might not naturally have happened. Or, they may occur sooner or more abruptly than under natural conditions.
Typical activities that can lead to sinkholes are:
- Decline of water levels - drought, groundwater pumping (wells, quarries, mines)
- Disturbance of the soil - digging through soil layers, soil removal, drilling
- Point-source of water - leaking water/sewer pipes, injection of water
- Concentration of water flow - stormwater drains, swales, etc.
- Water impoundments - basins, ponds, dams
- Heavy loads on the surface - structures, equipment
Sometimes several factors combine to cause a sinkhole. A sinkhole left open to take more water may continue to grow and can contribute to the appearance of more sinkholes nearby.
A sinkhole is not a hole in the rock
A common misunderstanding is to think that a sinkhole is the hole in the rock. Actually, the sinkhole is what we see on the ground surface because of the hole in the rock below. The space in the rock (known as a void, solution cavity or cave) takes hundreds or thousands of years to form. Then, soil from above can move into the void in rock. If the soil is sticky, a void can form within the soil. As more soil washes down (over years or maybe just days), the void space moves toward the surface until it can't hold together anymore. When it collapses (or subsides), you see the sinkhole on the surface. Often, you can only see soil in the hole and not the actual hole in the rock itself because the rock is too far below.
This DEP sinkholes website is designed to provide very basic information on sinkholes in Pennsylvania. For a more advanced explanation about sinkhole formation and karst hydrology, consult the following sources:
Your local college or university library or bookstore may contain these and other technical books on geology, karst and sinkholes. They may also hold USGS publications and periodicals that aren't available in bookstores. There are almost no popular books on sinkholes sold at retail bookstores. Government documents can be obtained from the responsible agency at a low cost.