Sinkholes are common where the rock below the land surface is limestone, carbonate rock, salt beds, or rocks that can naturally be dissolved by groundwater circulating through them. As the rock dissolves, spaces and caverns develop underground. Sinkholes are dramatic because the land usually stays intact for a while until the underground spaces just get too big. If there is not enough support for the land above the spaces then a sudden collapse of the land surface can occur. These collapses can be small, or, as this picture shows, or they can be huge and can occur where a house or road is on top.
Types of Sinkholes
Occur when rainfall and surface water percolate through joints in limestone. The dissolved material is carried away from the surface and a depression gradually forms. Cavities may be formed beneath the surface. These are the most common type of sinkhole in Florida.
Develop gradually when the covering sediment is permeable and contains sand. The small sediment and sand will drain into the underlying bedrock creating a vacated space allowing overlying sediment to settle. As the dissolution and infilling continue a noticeable depression will form. The slow downward erosion eventually forms a small surface depression.
These may develop abruptly (over a period of hours) and can cause catastrophic damages. These types of sinkholes occur where covering sediments contain a significant amount of clay which drains into a cavity in the underlying bedrock. New cavity will form in the overlying sediment and as erosion and dissolution continue the overlying cavity will migrate toward the surface. The cavity will eventually breach the ground surface creating a sudden and dramatic collapse.
Sinkholes can also be created by poor land use practices resulting from ground water pumping and construction and development practices. Development increases water usage, alters drainage pathways, overloads the ground surface and redistributes soil. According to FEMA, the number of human-induced sinkholes has doubled since 1930 and insurance claims for damages as a result of sinkholes has increased 1,200 percent from 1987 to 1991, costing nearly $100 million.
Common causes of other types of sinkholes includes collapsed drain pipes, washouts from poor drainage, and decaying organic matter such as covered tree stumps and roots. These types of sinkholes tend to be noticed while still small but their size will be determined by what is causing them.
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