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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