MEMPHIS, Tenn. (WMC) - In order to understand sinkholes we need to first start with erosion because it plays a major role in how sinkholes develop. Erosion is the process by which flowing water or wind removes soil, rock, or dissolved material from the Earth’s crust and then transports it to another location.
This is not the same as weathering because that involves no movement.
Water is the key because it has the ability to erode many things. The ability of water to move soil and rocks will greatly depend on the flow of the water.
The faster and more turbulent the flow the more erosive. Some particles like gravel or cobblestone are the most erosive resistant meanwhile silt or clay is more easily dissolved.
Erosion can occur on the surface or below the surface (subsurface). Sinkholes are rare but happen in areas where the most common rock below the land surface is limestone, carbonate rock, salt beds, or rocks. These are rocks that can naturally be dissolved by groundwater circulating through them. These rock types are known as evaporites. Evaporite rocks underlie about 35 to 40 percent of the United States, according to experts, in many areas they are buried at great depths.
Over time groundwater that flows below the surface can dissolve the rocks which can create a space, a void or tunnel. These spaces are how many caves were formed. These spaces and tunnels can change how the water flows by creating more space for the water to flow and pile up. As the water flows more freely and faster this can cause more erosion and more holes. As the holes or spaces get larger you can get sinkholes. Sinkholes occur when lots of weight sits on soft ground and causes the ground to cave in.
This process described is a natural process but not all sinkholes are formed naturally. Some are man-made due to underground piping like drainage pipes, water, sewer or stormwater drainage pipes. Sometimes sinkholes form due to bad construction.
Settling or shifting of these underground pipes can allow internal erosion to start. An abundance of rain or a water main break can cause erosion, which over time can lead to the land surface becoming so thin that it caves in. The water from the pipes can take away the sediment in and around it or from below. As the sediment is washed away from the inside, this can allow the pavement on top to cave in. This process can take weeks or years.
Sinkholes are pretty rare but can happen suddenly without warning. As these evaporite rocks dissolve, gaps develop underground. Sinkholes can happen so suddenly because the land typically stays intact for a while until the underground spaces get too big. If there is not enough foundation for the land above the spaces, then the land surface can cave in suddenly.
Unlike potholes, which are usually much smaller and are caused by failure of paving materials with roads, parking lots, and airports.
When it comes to size, sinkholes can be a few feet wide and deep, to 2,000 feet wide and deep!
There are some very large and old sinkholes that are thousands of years old in some areas of the U.S. Alabama claims to have the largest recent collapse sinkhole. It is known as the “Golly Hole” and is located in Shelby County in the central part of the state. It collapsed suddenly in 1972. The sinkhole is about 325 feet long, 300 feet wide.
The most damage from sinkholes tends to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania.