, also known as a sink
, swallow hole
, is a natural
depression or hole in the surface topography caused by the removal
or bedrock, often both, by water.
Sinkholes may vary in size from less than a meter to several
hundred meters both in diameter and depth, and vary in form from
soil-lined bowls to bedrock-edged chasms. They may be formed
gradually or suddenly, and are found worldwide. These terms are
often used interchangeably though many will distinguish between
those features into which a surface stream flows and those which
have no such input. Only the former would be described as sinks,
swallow holes or swallets.
A special type of sinkhole - formed by
rainwater leaking through the pavement and carrying dirt into a
ruptured sewer pipe.
Mechanisms of formation may include the gradual removal of slightly
soluble bedrock (such as limestone
percolating water, the collapse
roof, or a lowering of the water table
. Occasionally a sinkhole may exhibit
a visible opening into a cave below. In the case of
exceptionally large sinkholes, such as Cedar Sink at Mammoth Cave
National Park, USA, a stream or river may be
visible across its bottom flowing from one side to the
Sinkholes are common where the rock below the land surface is
, carbonate rock
, or rocks that can naturally be dissolved by circulating
ground water. As the rock dissolves, spaces and caverns develop
underground. These sinkholes can be dramatic because the surface
land usually stays intact until there is not enough support. Then a
sudden collapse of the land surface can occur.
Sinkholes can be human-induced. New sinkholes have been correlated
to land-use practices, especially from ground-water pumping,
construction, and development practices. Sinkholes can also form
when natural water-drainage patterns are changed and new
water-diversion systems are developed. Some sinkholes form when the
land surface is changed, such as when industrial and runoff-storage
ponds are created. The substantial weight of the new material can
trigger an underground collapse of supporting material, thus
causing a sinkhole.
Sinkholes may capture surface drainage
running or standing water, but may also form in currently high and
dry locations. The state of Florida in the USA
is known for having frequent sinkholes, especially in the central
part of the state.
southern Italy also has numerous sinkholes. Sinkholes can be formed
in retention ponds from large amounts of rain.
Sinkholes are usually but not always linked with karst
landscapes. In such regions, there may be
hundreds or even thousands of sinkholes in a small area so that the
surface as seen from the air looks pock-marked, and there are no
surface streams because all drainage occurs sub-surface.
Sinkholes have been used for centuries as disposal sites for
various forms of waste
. A consequence of this
is the pollution
resources, with serious health
implications in such areas. In contrast, the
Maya civilization sometimes used
sinkholes in the Yucatán Peninsula (known as cenotes) as places
to deposit precious items and sacrifices.
also form from human activity, such as the rare but still
occasional collapse of abandoned mine in
places like West
More commonly, sinkholes occur in urban
due to water main
collapses when old pipe
give way. They can also occur from
the overpumping and extraction of groundwater and subsurface
Many sinkholes are also found in Northern Michigan. These are
prominent in Alpena County
Northeast Michigan. In Lachine, Michigan there are five sinkholes
that are found to be very deep , and within two miles of each
other. Alpena's visitor information cites their sinkholes as an
attraction for visitors to the area. In August 1998 a 16 year old
Alpena boy survived a 200+ ft fall in an open sinkhole 3/4 a mile
off of Leer road in Lachine Michigan (The Alpena News
8-21-1998). A majority of sinkholes
in Alpena are also found underwater. Many divers explore these on a
When sinkholes are very deep or connected to caves, they may offer
challenges for experienced cavers
. Some of the most
spectacular are the Zacatón cenote in
Mexico (the world's deepest water-filled sinkhole), the Boesmansgat sinkhole in South Africa, Sarisariñama tepuy in Venezuela, and in the
town of Mount Gambier, South Australia.
Sinkholes that form in coral reefs and
islands that collapse to enormous depths are known as Blue Holes
, and often become popular diving
States, the most damage from sinkholes tends to occur in
Florida, Texas, Alabama, Missouri, Kentucky, Tennessee, and Pennsylvania. Palmyra, Pennsylvania is the sinkhole capitol of America.
sediments that cover
buried cavities in the aquifer systems are delicately balanced by
fluid pressure. The water
below ground is actually helping to keep the surface soil in place.
Groundwater pumping for urban water supply and for irrigation can
produce new sinkholes in sinkhole-prone areas. If pumping results
in a lowering of groundwater levels, then underground structural
failure, and thus, sinkholes, can occur.