Types of Sinkholes


Source: USGS

SINKHOLES and the AQUIFER
Sinkholes originate beneath the surface when groundwater moves through the limestone and erodes large voids, or cavities, in the bedrock. When water fills a cavity, it supports the walls and ceiling, but if the water-table drops, the limestone cavity is exposed to  further erosional processes that
eventually result in the collapse of the cavity, causing a surface indenture, or sinkhole. The sinkhole becomes a primary site of recharge, where surface water can enter the aquifer and replenish the groundwater supply.
 
SURFACE CHARACTERISTICS
On the surface, sinkholes may develop progressively as subtle, bowl-shaped depressions, or they may collapse suddenly into steeply sided, water-filled craters. The shape of the sinkhole, and the speed that it forms, depend on the size of the subsurface cavity and the thickness of the overburden (sediments or organic matter that rest on the limestone bedrock).
 
Three general types of sinkholes occur in Tennessee and Florida: collapse, solution, and subsidence.
 
COLLAPSE SINKHOLES
Collapse sinkholes are the most common type in Florida. They happen suddenly where the overburden is thick with soils and heavy clay. Collapse sinkholes are deep, steeply-sided holes in the ground. They are frequently triggered by fluctuations in the water-
table. As water levels fluctuate, the


Source: USGS

roof  of the cavity is stressed and weakened. When the water-table drops too far, the cavity walls are unsupported and the ceiling becomes too weak to hold the heavy overburden. Eventually, the ceiling collapses and a sinkhole is formed. A conical debris mound left on the sinkhole floor is all that remains of the cavity ceiling.

If the water-table rises, the collapse sinkhole can fill with water, and overflow like a spring. An off-set sinkhole will have an upstream and downstream conduit as water flows into the sink and siphons underground. If the water-table drops below the sinkhole, it will remain dry and accumulate sediments and vegetation.


SOLUTION SINKHOLES
If the overburden is thin or absent, the surface of the limestone bedrock is broken down by erosion from wind and surface water. A bowl-shaped depression, or solution sinkhole, naturally forms slowly and continuously as chemical and physical processes erode the rock.


Source: USGS

SUBSIDENCE SINKHOLES
Subsidence sinkholes form gradually where the overburden is thin. The dissolving limestone is replaced by sand granules that fall into the depression and fill the holes. They appear as a concave
depression in the ground. Subsidence sinkholes are usually only a few feet in diameter and depth because the development of the cavities in the limestone is retarded since they are filled with clay and sand. As the sediments fill the depression, they restrict the flow of water through the bottom and the hole begins to retain water. As water accumulates, a lake is formed.

SINKHOLES and LAKE FORMATION
Many ponds and lakes in Florida are a result of sinkhole formation. The characteristics of a sinkhole lake can give clues as to how it was formed. A circular lake indicates that the lake evolved


Source: USGS

from a collapse sinkhole. A shallow circular lake results from impermeable sediments washing into a subsidence sinkhole. If a lake rests above groundwater level, it is above a confining bed.
 
 

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