Sinkholes happen when rock at or near the surface of the ground dissolves over time and creates a depression. They tend to happen most often in areas where the underlying structure of the ground consists of especially soluble materials, such as limestone, carbonate rock or salt. Sinkholes may appear suddenly when the overlying soil collapses into cavities created by the dissolving rock, but more often, they form gradually and may go unnoticed for long periods of time.
Dissolution sinkholes form when rain and surface run-off water infiltrate soluble rock near the surface of the ground. As the surface water works its way downward through existing cracks in the rock, small amounts of the rock dissolve in the water and are carried away from the site through fissures and fractures in the bedrock. Over time, the dissolving rock wears away and creates a depression at the surface. The resulting depression often collects surface water and forms a pond, and the concentration of water at the surface makes the rock dissolve faster and makes the sinkhole bigger.
A subsidence sinkhole forms when the soluble rock is covered by a layer of soil or sediment. As in a dissolution sinkhole, water infiltrating from the surface dissolves the rock, but in this case, the dissolution creates a cavity in the rock. The soil or sediment above the rock slowly, usually over a long period of time, settles into the cavity in the rock, creating a depression at the surface.
Subsidence sinkholes are most common in areas where the soil above the soluble rock is sandy. Water moves downward through sand more quickly than it does through heavy soils, and the loose sand readily falls into the cavities in the rock.
The formation of a cover-collapse sinkhole begins in much the same way as a subsidence sinkhole, but in the case of a cover-collapse sinkhole, the layer of soil above the dissolved rock remains in place until it collapses suddenly into the underlying cavity. In some cases, the sinkhole collapse occurs when the cavity in the rock is covered by a gradually thinning layer of rock. When this layer grows to thin to support the weight of the soil above it, it collapses, and the soil falls suddenly into the cavity.
In other cases, the collapse occurs in the soil layer, which has remained intact as the rock cavity forms below it. Water infiltration gradually enlarges a void in the soil above the rock cavity, and when the void expands enough to reach the surface of the ground, the sinkhole opens suddenly. This type of formation is most common where the overlying soil contains clay, which is able to hold together as the void forms beneath the surface.
Human activity can cause sinkholes to form or accelerate their development. Any kind of construction activity that alters the natural pattern of water drainage has the potential to direct water toward areas conducive to sinkhole formation, and moving groundwater away from a developing sinkhole through pumping or drainage may also accelerate a hole's formation or collapse. New construction of structures, water features or ponds may also hasten a sinkhole collapse by placing extra weight on top of existing rock cavities.