Essay Outline 2
Question: Describe 2 different limestone formations in detail.
Naturally formed caves evolve in various ways, mainly as a result of the solvent action of water and compounds in it. Known technically as caves of solution, such chambers are most common in limestone formations, particularly in regions that have ample rainfall.
- The surface water in such regions contains carbon dioxide and humid acids derived from the organic constituents of the soil. Attacking the soluble limestone, this acidic water dissolves and carries the limestone away in solution.
- Over long periods of time, such action results in the formation of subterranean chambers. The depth of such chambers depends on the depth of the water table.
- If after several unusually wet years the water table is rising, old cave chambers become flooded and new ones begin forming at higher levels. Likewise, during a long dry spell, chambers will begin forming at lower levels, closer to the declining water table.
- Over thousands of years, fluctuations such as these produce multi-level cave systems, as in Mammoth Cave National Park in Kentucky, where a subterranean stream flows through the lowest level. Underground rivers erode and transport sediments and rock fragments in a manner analogous to that of surface streams. If such action has been predominant, the cave is said to have been formed by mechanical abrasion.
The presence of caves in limestone regions may be detected by means of clues provided by the topography of the land.
-In such a region the roofs of large caverns may collapse and leave depressions and troughs at the surface of the ground.
-Natural bridges, another phenomenon of cave regions, may remain after the collapse of a tunnel bearing an underground stream. The Natural Bridge in Virginia is a classic example of this type of formation.
-In the phenomenon known as disappearing streams, which is a common feature in areas underlain by caves, whole watercourses may vanish down sinks, or sinkholes, leading to the underground caverns. The sinks are indicative of caves below.
-Because of the capture of the surface waters by the subterranean drainage system, some cave regions have a rather dry, dusty, poorly vegetated appearance. Such regions are said to have a karst topography, a name derived from a famous cave region along the Adriatic Sea in Italy and Slovenia.
In caves of solution, the dissolved lime carbonate is often precipitated in such a fashion as to form grotesque deposits.
-The best-known structures are the stalactites, which hang like icicles from the roofs of caves, and the stalagmites, which extend upward from the cavern floors. If the two growths meet and join, a pillar forms, helping to support the roof.
-Less well-known forms of carbonate deposition include flowstone and dripstone. Depending on dissolved mineral impurities brought into the cave by the groundwaters, the formations vary in color from alabaster white to hues of dusky red and brown. The dripstone formations may be exceedingly thin and translucent. Among rare formations is the helictite, a twisted, flowerlike variety of stalactite.
Sinkholes, sometimes referred to as dolines, are the most common surficial landforms found in karst landscapes. The three main morphological types of sinkholes are bowl-shaped, funnel-shaped, or well-shaped. The size of individual sinkholes may range from a few meters to in their dimension, several hundred meters in horizontal dimension, and over 100 meters in depth. Sinkholes can occur in close proximity to each other or completely isolated, it all depends on the porosity and permeability of the underlying karst bedrock. The five major types of sinkholes are solution, subsidence, collapse, alluvial stream sink, and subjacent karst collapse sinkholes. They are all are named for the processes leading to their formation.
a) Solution sinkholes (Fig 1) form in places such as joint intersections. Within the intersections, surface runoff accumulates, leading to the solution of the soluble bedrock. The solutes then move downward through the bedrock joints and bedding planes. The movement of the solute through the bedrock inhibits the solution process to reach deeper into the underlying strata. The result is a funnel-shaped hole on the surface. Water often fills the sinkholes, creating marshes and ponds.
figure 1
b) Subsidence sinkholes (Fig 2) occur in karst areas covered by soils or other unconsolidated materials. They appear to occur very quickly , due to the fast collapse of the overlying materials. The reasoning behind this is that most of the solution process takes place under the surface. Upon formation, a cylindrical hole is created. The hole is then eroded over a period of time into more of a bowl shape. These sinkholes are eroded and widened very easily sue to the presence of soils and unconsolidated materials located on the rims of the hole. Subsidence sinkholes are also prone to become ponds of varied sizes.
figure 2
c) Collapse sinkholes (Fig 3) form either from sudden mass movements of karst bedrock due to sudden drops in the water table or when the roof of an underground cavern or cave collapses, or both. Unlike solution sinkholes, collapse sinkholes can be angular in shape with vertical walls. Over time, abrasion will erode the walls and create a round or oval-shaped sinkhole. One common reason for a sudden drop in the water table is excessive groundwater pumping. Over time, collapse sinkholes often fill with water .
figure 3
d) Alluvial stream sinkholes (Fig 4) are created by streams sinking through deposits of alluvium on the surface of the landscape into the underlying soluble karst bedrock. The same natural processes that lead to the formation of subsidence sinkholes can also lead to alluvial stream sinkhole formation. These sinkholes are often prone to filling of water.
figure 4
e) The formation of subjacent karst collapse sinkholes (Fig 5) can be due to cave or cavern roof collapse as well as the continual removal of overlying rocks near widened joints in the karst bedrock. They are formed by processes very similar to those responsible for subsidence sinkholes.
figure 5
