Subsidence/Sinkholes – Types and Classifications

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Sinkholes can happen in areas which rock dissolution forms underground cavities. These rock types are evaporites (salt, gypsum, and anhydrite) and carbonates (limestone and dolomite). Evaporite rocks underlie about 35 to 40 percent of the United States, though in many areas they are buried at great depths. The following are some of the more important types of sinkholes:[1]

Dissolution sinkholes are the most common and they also are the slowest to occur. Dissolution of the limestone or dolomite is most intensive where the water first contacts the rock surface. Aggressive dissolution also occurs where flow is focussed in preexisting openings in the rock, such as along joints, fractures, and bedding planes, and in the zone of water-table fluctuation where groundwater is in contact with the atmosphere.[2]

Below are graphics and descriptions of the process forming these types of sinkholes:

Rainfall and surface water percolate through joints in the limestone. Dissolved carbonate rock is carried away from the surface and a small depression gradually forms.

On exposed carbonate surfaces, a depression may focus surface drainage, accelerating the dissolution process. Debris carried into the developing sinkhole may plug the outflow, ponding water and creating wetlands.

Gently rolling hills and shallow depressions caused by solution sinkholes are common topographic features throughout much of karst areas.

Process of Forming Dissolution Sinkholes[3]

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Cover-subsidence sinkholes tend to develop gradually where the covering sediments are permeable and contain sand. In areas where cover material is thicker or sediments contain more clay, cover-subsidence sinkholes are relatively uncommon, are smaller, and may go undetected for long periods.

In areas where cover material is thicker or sediments contain more clay, cover-subsidence sinkholes are relatively uncommon, are smaller, and may go undetected for long periods.[4]

Below are graphics and descriptions of the process forming these types of sinkholes:

Granular sediments spall into secondary openings in the underlying carbonate rocks. A column of overlying sediments settles into the vacated spaces (a process termed "piping"). The slow downward erosion eventually forms small surface depressions 1 inch to several feet in depth and diameter. Dissolution and infilling continue, forming a noticeable depression in the land surface.
Process of Forming Cover-Subsidence Sinkholes[5]

 

 

 

 

 

 

 

 

 

 

 

 

Cover-collapse sinkholes may develop abruptly (over a period of hours) and cause catastrophic damages. They occur where the covering sediments contain a significant amount of clay. Over time, surface drainage, erosion, and deposition of sinkhole into a shallower bowl-shaped depression.

Over time, surface drainage, erosion, and deposition of sediment transform the steep-walled sinkhole into a shallower bowl-shaped depression.[6]

Below are graphics and descriptions of the process forming these types of sinkholes:

Sediments spall into a cavity. As spalling continues, the cohesive covering sediments form a structural arch. The cavity migrates upward by progressive roof collapse. The cavity eventually breaches the ground surface, creating sudden and dramatic sinkholes.
Process of Forming Cover-Collapse Sinkholes[7]

 

 

 

 

 

 

 

 

 

 

 

 

 


References:

  1. USGS – The USGS Water Science School – Sinkholes: http://water.usgs.gov/edu/sinkholes.html
  2. USGS – The USGS Water Science School – Sinkholes: http://water.usgs.gov/edu/sinkholes.html
  3. Image Source: http://water.usgs.gov/edu/sinkholes.html [Accessed: January 14, 2014]
  4. USGS – The USGS Water Science School – Sinkholes: http://water.usgs.gov/edu/sinkholes.html
  5. Image Source: http://water.usgs.gov/edu/sinkholes.html [Accessed: January 14, 2014]
  6. USGS – The USGS Water Science School – Sinkholes: http://water.usgs.gov/edu/sinkholes.html
  7. Image Source: http://water.usgs.gov/edu/sinkholes.html [Accessed: January 14, 2014]

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