Geology Home Page physical geology historical geology planetary gems
Roger Weller, geology instructor
What is a sinkhole?
A sinkhole is a funnel shaped hole or natural compression in the land surface in which water drips downwards toward the bottom of the sinkhole. Sinkholes vary in size from less than a meter to several hundred meters in diameter and depth. Sinkholes are also known as sink, shake hole, swallow hole, swallet, doline, or cenote.
Photos courtesy of Wikipedia
How is a sinkhole caused and /or formed?
Sinkhole’s are caused when rocks underground (ex. limestone,
carbonate rock, salt beds) are naturally dissolved by water circulating through
Water washes away the soil and residue from the voids in the rock.
As the rock dissolves, caverns and spaces begin to develop underground, as the
space underground enlarges the earth above can crack and sink into the vacant
space underground. Any changes in the water system can cause the ground to
become unstable and could lead to sinkholes. Some examples of this are the
ground freezing and thawing, change in water tables, extremes in precipitation,
Diagram courtesy of watermatters.org
diagram courtesy of www.depweb.state.pa.us
Processes carried out by humans also have an effect on the formation of sinkholes. The presence of sinkholes has been correlated to land-use practices; such as:
· Ground-water pumping
· New home development practices
· Changes to natural water draining patterns
· Mining (abandoned mines)
· Drilling water wells
What types of sinkholes are there?
· Collapse sinkholes form when surface materials suddenly sink into an underground cave or cavity. The cave/cavity forms slowly over time as groundwater moves along cracks in the water soluble bedrock and enlarges them. This type of sinkhole forms with little warning and leaves behind a deep, steeply sided hole. The actual collapse can happen in two different ways:
1. When a cavity gets very large, the roof becomes too thin to support the weight of any overlying rock or sediment, so it collapses into the cavity.
2. Sometimes caves are able to support the weight of the sediments above due to being filled with groundwater. But when groundwater levels are lowered, then the overlying sediment will first erode then collapse into the cavity.
Caves and/or cavities begin to form underground; there is no evidence of ground subsidence
Erosion process begins
Cavities continue to grow larger; the roof of cavern becomes thinner and weaker, allowing more water to flow down into the rocks
Diagrams courtesy of www.earthtech.net
During the dry season groundwater levels drop, the weight of the surface becomes too heavy for the roof of the cavern to support and collapses into the cavern.
· Solution sinkholes form where water soluble bedrock is exposed as the surface and is subjected to weathering. Water at the surface is collected in natural depressions and slowly dissolves a sinkhole
Drawing courtesy of www.sinkholes.com
· Cover subsidence sinkholes are similar to solution sinkholes except for the water soluble rock is covered by a thin layer of soil and/or sediment. Surface water flows in dissolving cavities where the bedrock has the most cracks, and the overlying sediment gradually moves downward into the expanding cave. This type of sinkhole tends to form naturally and is not greatly influenced by human activities.
1. The limestone contains fractures, but no subsidence has occurred
2. Over time cracks and cavities grow larger and water flowing through the rock causes erosion. Sediments carried in the water fill the voids in the rock
Drawing courtesy of www.earthtech.net
3. Upper layer sediments continue to fill in the openings, causing a depression at the land surface
What are some warning
signs of sinkholes?
Well drilling or sudden alterations to the land can cause a rapid sinkhole which may not give any warning signs prior to collapse. Some possible warning signs of naturally occurring sinkholes are:
• Fresh exposures on fence posts, foundations, and trees that result when the ground sinks;
• Slumping, sagging or slanting fence posts, trees or other objects; doors and windows that fail to close properly;
• Ponding small ponds of rainfall forming where water has not collected before;
• Wilting vegetation wilting of small, circular areas of vegetation. This happens because the moisture that normally supports vegetation in the area is draining into the sinkhole that is developing below the surface;
• Turbidity in well water muddy water in nearby wells during early stages of sinkhole development;
• Structural failure cracks in walls, floors, and pavement; cracks in ground surface
Where do sinkholes occur?
Sinkholes are common where the rock below the land surface is Limestone,
carbonate rock, salt beds, or any rocks that can be naturally dissolved by
groundwater. The map below shows areas of the United States where certain rock
types are present. In these areas the formation of underground cavities are
possible causing sinkholes to occur.
The most damage from sinkholes tends
to occur in Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and
Map courtesy of www.sinkholeinfo.net
The State of Florida is known for
having frequent sinkholes, especially the central part of the state. Many
sinkholes are also found in Northern Michigan, in some parts of the state you
can find up to five deep sinkholes within two miles of each other and also found
underwater. Sinkholes are found throughout the world and not limited to the
Picture courtesy of
Picture courtesy of
GUATEMALA CITY, Guatemala, February 2007 - A 330-foot-deep sinkhole killed three people, swallowed about a dozen homes, and forced the evacuation of nearly 1,000 people in a crowded Guatemala City neighborhood. Officials blamed the sinkhole on recent rains and an underground sewage flow from a ruptured main. Photo courtesy of www.cbsnews.com/images