Geoscience Reference
In-Depth Information
During stage 1, the process of carbonation is concentrated
in the deposits of limestone that occur just below the water
table. The resulting dissolved sediments are then carried by
underground streams to the surface channel. These kinds of
streams evolve because water concentrates along joints in the
rocks where it can move freely. These cracks slowly enlarge
through dissolution, and the water can thus flow more quickly
as an underground stream. Over time, the process of carbon-
ation and associated groundwater flow creates myriad passage-
ways and caverns at the water table boundary. In stage 2, the
stream has cut a valley into the rock, lowering the water table.
Groundwater cuts large channels and caverns into the rock as
it flows through to the stream. At this time, these features are
submerged in groundwater. Finally, in stage 3, as the stream
continues to downcut and deepens its valley, the water table
also lowers still farther. As a result of this lower water table,
the formerly submerged passageways of stage 1 are now in the
unsaturated zone and thus become open caves and caverns.
Once stage 3 is complete, the cave or cavern has formed
and the dominant processes shift to those that occur within the
chamber. In order for the cavern to be preserved, some kind of
more resistant cap rock must be present to minimize the amount
of water percolating into it and keep the roof from collapsing. If
such preservation occurs, features such as stalactites (hanging
rods), stalagmites (upward-pointing rods), columns, and drip
curtains can begin to form within the cave or cavern (Figure
15.12). These unusual formations evolve in caves where large
quantities of surface water percolate downward into the roof
of the open cavern. As this water moves through the overly-
ing rock, the solution becomes enriched in dissolved carbon-
ates before it slowly drips into the cave. Stalactites in the roof
of a cave form in much the same way that an icicle lengthens
downward during a winter thaw. Stalagmites, in contrast, de-
velop when water drips to the cave floor. This water evaporates,
leaving the calcium carbonate behind in a deposit that slowly
grows upward over time. An easy way to remember the dif-
ference between the two forms is that the “c” in stalactites can
mean “ceiling,” whereas the “g” in stalagmites means “ground.”
Karst Topography
Karst landforms can also evolve at the surface. The most com-
mon surface landform is a
sinkhole
, which is a depression that
occurs in regions of cavernous limestone (Figure 15.11). Sink-
holes form when caves enlarge so much that their ceilings col-
lapse due to the force of gravity, causing the surface rocks and
sediment to sink, often suddenly. The rapidity of sinkhole for-
mation was tragically exemplified in March 2013 when a sink-
hole developed at night beneath the bedroom of a sleeping man
in Florida. The hole, which measured 6 m (20 ft) across and
about 9 m (30 ft) deep, opened instantaneously and swallowed
the victim. His body was never found. Sinkholes are particu-
larly common in parts of Florida because much of the peninsula
is underlain by limestone that easily dissolves.
Sinkholes can also form due to excessive groundwater ex-
traction because caves that were previously filled with water
become empty. As a result, nothing but air is present below the
ceiling of the structure and it collapses under its own weight.
Sinkholes frequently enlarge for a period of time after their
initial formation because surface water tends to funnel into
them. Sometimes this surface flow becomes a
disappearing
stream
, which flows down into a sinkhole from the surface
and then enters underground channels. Often, this downward-
flowing stream causes a pipe to evolve that connects the sink-
hole with the caverns and passageways beneath.
At other times sinkholes remain detached from the under-
lying cavern and simply fill with water, appearing as ponds on
the landscape and in topographic maps (Figure 15.13). In re-
gions where karst activity occurs in densely populated regions,
sinkholes are a significant geological hazard because they can
unexpectedly swallow homes or portions of highways. A spec-
tacular form of karst topography occurs in humid subtropical
areas where thick beds of limestone and high water tables exist.
This kind of karst is called
tower
(or
haystack
)
karst
and re-
sults in dramatic landscapes. In order for tower karst to evolve,
karst processes must have operated for a long period of time,
during which a variety of subterranean caves and passageways
developed. Over time, these features gradually enlarge so that
Figure 15.12 Drip formations at Carlsbad Caverns in New
Mexico.
Here, dripping water enriched in calcium carbonate
has formed numerous stalactites, stalagmites, and columns in the
Papoose Room. This form of deposition occurs when percolat-
ing groundwater dissolves limestone and drips, leaving calcium
carbonate behind.
Sinkhole
A topographic depression that forms when underly-
ing rock dissolves, causing the surface to collapse.
Disappearing stream
A surface river or stream that flows
into a sinkhole and subsequently moves into an underground
river system.
