Sinkhole Activity

Types of Sinkholes

• There are three main types of sinkholes, but they can all be dangerous because they can range from tens to hundreds of meters deep, according to "Geomorphological Hazards and Disaster Prevention" by Irasema Alcàntara-Ayala. Locations prone to sinkholes include places with limestone bedrock such as Florida and Pennsylvania. Collapse, suffosion and gradual sagging can all form sinkholes, but the most feared is sudden collapses. These occur when the overl ying soil falls into the weakened structure below it. Suffosion sinkholes result from the settling of minerals farther down into the ground over time, slowly sinking the land on top. Gradual sagging happens when different layers of rock under the ground sag, showing up on the surface as a depression in the ground.

Natural Activities Promoting Development

• Sinkholes are naturally occurring, but the root cause is the water in the rock. Groundwater levels that fall quickly draw water out of the rock above. As the water level underground falls, the water pulls minerals out of the limestone rock it passes through, hastening weathering and weakening of the rock. Not all types of rock will create sinkholes. They are more prevalent in karst (limestone) bedrock, soft enough to be quickly weathered by water seeping underground. Periods of drought, heavy rain or freezing and thawing cycles all can change the level of groundwater, which can accelerate the formation of sinkholes.

Human Activities Accelerating Sinkhole Development

• Human activity plays a significant role in the development of many of the most dangerous type of sinkholes – collapse sinkholes, according to Irasema Alcàntara-Ayala in "Geomorphological Hazards and Disaster Prevention." Water main breaks, construction of large buildings on weakened ground and the draining of natural underground aquifers for human consumption all can change the pressure above the ground or the support system below the ground, resulting in collapse.

Indications of Sinkhole Activity

• Sinkholes leave clues during their development -- even the collapse type. Watching for these signs on your property before building can be a signal to choose another construction site. Look for signs of lowering in the soil, such as newly exposed portions near the bottoms of fence posts or trees, new locations of ponding in a yard or foundation problems on an existing building. Should any of these signs be present, contact your state's environmental department for information on controlling the collapse or getting a geologic review of the property.