LANDFORMS MADE BY UNDERGROUND WATER

INTRODUCTION

The whole process of circulation of water between the land, sea and atmosphere is known as the Hydrological Cycle.

The rainwater is distributed on the earth in various ways:
1. Some is evaporated immediately.
2. Some is absorbed by the plants and later transpired.
3. Some flows into streams and rivers, eventually reaching the seas and oceans. This is known as runoff.
4. A considerable portion of water from precipitation percolates downwards into the soil and rocks forming what is known as Ground Water.
The ground water is present in the joints and pore spaces and plays an important part in weathering and mass movement besides being an important natural storage.
Of all the world’s water, only six-tenths of 1% is found underground. Nevertheless, the amount of water stored in the rocks and sediments beneath the earth’s surface is a vast and significant natural resource. The US geological survey estimates that the quantity of water in the upper 800 meters of the continental crust is about 3000 times greater than the volume of water in all rivers at any one time, and nearly 20 times greater than the combined volume in all lakes and rivers. In many parts of the world, wells and springs provide the water needs not only for great number of people, but for crops, livestock and industry as well. In addition, subsurface water is important as an equalizer of stream flow and as an agent of erosion. It is the work of subsurface water that creates Caverns (subterranean passage) and Sink Holes. (Strahler 214, GOH 42)

GEOLOGICAL WORK OF GROUNDWATER

The groundwater does flow or travel but its flow is so slow that it has no energy to erode the surface over which it flows. Therefore, the work of groundwater is of three types: (Gupta 313)

a. Solution (Dissolution of matter)
The rainwater, as it moves into the earth gets mixed up with such chemicals as CO2, Nitrogen and Sulphur. This water becomes powerful enough to act chemically on rocks through which it travels and in the course of time dissolves a large part of them (weathering) so that they become hollow and ultimately sink downwards.
This underground water is most active over chalk and limestone and therefore its effects are more marked in limestone regions.

b. Deposition
When a large proportion of minerals has been dissolved by the water, the capacity of water to dissolve any further comes to an end and it begins to aggrade the minerals that it has dissolved. The fact is that as the temperature becomes higher, the water is transformed into vapors leaving behind its deposits of dissolved minerals. Besides, if the temperature becomes low, solution power of water declines and aggradation begins. Sometimes the oxides – specially the CO2 – which make the water powerful enough to do the solution work begin to escape and the groundwater, rendered powerless in that way, has to deposit its load held in solution.

c. Petrification
The groundwater often replaces the original mineral substances by another. This process is known as Petrification and most commonly it is seen that carbonate of lime is replaced by silica or iron sulphide. In this way, the form, shape and content of any mineral is entirely changed. The petrification done by underground water is a very useful activity. It helps in gathering useful minerals scattered at various places. It dissolves those minerals and holds them in solution. Then it deposits them at one place and thus facilitates their exploitation later on.

EROSIONAL LANDFORMS ASSOCIATED WITH GROUNDWATER

The primary erosional work carried out by groundwater is that of dissolving rock. Since soluble rocks, specially LIMESTONE, underlies millions of square kilometers of the earth’s surface. It is here that groundwater carries on its rather unique and important role as an erosional agent. Although nearly insoluble in pure water, limestone is quite easily dissolved by water containing small quantities of Carbonic Acid. Most natural water contains this weak acid because rainwater readily dissolves CO2 from the air and from decaying plants. Therefore, when groundwater comes in contact with limestone, the carbonic acid reacts with calcite in the rocks to form calcium bicarbonate, a soluble material that is then carried away in solution.

Karst Topography

Many areas of the world have landscapes that to a large extent have been shaped by the dissolving power of groundwater. Such areas are said to exhibit Karst Topography. The term is derived from a plateau region located along the northeastern shore of the Adriatic Sea in the border areas between Yugoslavia and Italy where such topography is strikingly developed. In the United States, Karst landscapes occur in many areas, including portions of Kentucky, Tennessee, Alabama, Southern Indiana, and Northern Florida. Generally, arid and semi-arid areas do not develop Karst topography. When solution features exist in such regions, they are likely to be remnants of a time when more humid conditions prevailed.
The Karst Topography shows a series of stages in its development. (Strahler 482, GOH 76)
In the first stage of the Karst Topography only fretting and fluting of the surface takes place so that lapies, swallow/sink holes and dolines are formed. The surface drainage disappears through these to a place beneath the surface and a sort of subterranean reservoir is formed.
In the second stage, underground chambers, ponors and caverns are formed and enlarge swallow holes, dolines and sink holes collapse or are so breached that more and more water is added to the underground reservoir. The water table is reduced in height. Intermittent drainage of this stage makes the surface topography dry and barren. In late maturity, dolines and swallow holes coalesce to form uvalas, whose roofs are very thin. These become 50 to 60 miles wide.
The third and the final stage is marked by the collapse of the roofs of these subterranean caverns, formation of Poljes with residual hums standing here and there and revival of surface drainage. The water has done its job and it can percolate no further. With the collapse of the roofs of the caverns, it is ultimately exposed to view once more and along with it is exposed the underlying impervious strata.

Characteristic Features of Karst Topography

Lapies

As rain water mixed with CO2 proceeds over a region of limestone rock, it begins to dissolve certain portions of the rock. The system of joints is widened by solution and the surface is fretted (grooved) and fluted (hollowed). This is known as Lapies in French, Karrens in German and Bogaz in the Serbian language. (Gupta 313)

Sink Holes

Gradually, lapies are further widened to form sink holes and swallow holes, through which surface streams disappear, and begin to flow beneath the ground.
Sink holes commonly form in one of two ways. Some develop gradually over many years without any physical disturbances to the rock. In these situations, the limestone immediately below the soil is dissolved by downward-seeping rainwater, that is freshly charged with CO2. These depressions are usually gentle slopes. By contrast, sink holes can also form suddenly and without warning when the roof of a cavern collapses under its own weight. Typically the depressions created in this manner are steep-sided and deep. When they form in populous areas they may represent a serous geologic hazard.
In the limestone areas of Florida, Kentucky, and Southern Indiana, there are literally tens of thousands of these depressions varying in depth from just a meter or two to a maximum of more than 50 meters.

Dolines

If the dissolution is very intense, the sink holes may become very wide and such wide and deep funnel shaped sink holes are called Dolines. The typical doline is found in the Karst Region along the Adriatic Sea and its characteristic feature is a funnel-shaped top, the diameter being 30 to hundreds of feet. Sometimes it is deeper than broader. In barren regions their sides are steep and bottoms deep. On the other hand in regions with a vegetation cover, dolines are shallow and have rounded outlines.
Limestone Caverns
Among the most spectacular results of groundwater’s erosional handiwork is the creation of limestone caverns. Most are relatively small, yet some have spectacular dimensions. In the United States, Carlsbad Caverns in southeastern New Mexico and Mammoth Cave in Kentucky are famous examples.
Although caverns may develop in folded, faulted, and steeply dipping limestone layers, most caverns occur in areas of flat-lying strata.
In the first stage, the action of calcium carbonate is concentrated just below the water table. Products of solution are carried along in the ground-water flow paths to emerge in streams and leave the region in the stream flow. In a later stage, the stream has deepened its valley and the water table has been correspondingly lowered to a new position. The caverns system previously excavated is now in the zone of aeration. Evaporation of percolating water on exposed rock surfaces in the caverns now begins the deposition of carbonate matter, known as travertine. Encrustation of travertine take many beautiful forms – stalactites, stalagmites, columns, drip curtains, and terraces. (Strahler 481)

Natural Bridge

Beneath the surface of the earth the underground drainage is able to dissolve out very long and wide caverns. Sometimes roofs of such caverns are incaved, and rock bridges may be formed. These are known as natural bridges and are circular in outlines.

Uvalas

Due to the collapse of cavern roofs and coalescence (coming together) of dolines and swallow holes, often very vast depressions are formed which are known as Uvalas or Poljes. Uvalas follow the system of fissures in their outline. On their floors develop Blind Valleys with steep sides and terminating in a steep wall where the surface stream disappears.

Poljes

Poljes are basins closed from all sides by steep walls. In outline they resemble an ellipse but their floors are almost flat with independent drainage. These depressions result when a Karst land suffers from Block Faulting movement at a sufficiently late stage in the youth of its cycle. They may be called Grabens of Rift Valleys of the Karst land.

Natural Tunnels

When a considerable portion of the roof of the tunnel is retained over the valleys (originally cavern), it is called a Natural Tunnel.

Hums

In the midst of Uvalas and Poljes, residual masses of limestone rise here and there. These are known as Hums.

DEPOSITIONAL LANDFORMS ASSOCIATED WITH GROUNDWATER

Depositional work by the groundwater goes on in the caverns.

1. Drip Stones
The features that arouse the greatest curiosity for most cavern visitors are the stone formations that often exhibit quite bizarre patterns and give some caverns a wonderland appearances. These features are created by seemingly endless dripping of water over great spans of time. The calcite that is left behind produces the lime stone we call Travertine. These cave deposits, however, are also commonly called drip stones, and obvious reference to their mode of origin.
Although the formation of caverns takes place in the zone of saturation, the deposition of drip stone is not possible until the caverns are above the water table in the zone of aeration. This commonly occurs as nearby streams cut their valleys deep, lowering the water table as the elevation of the river drops. As soon as the chamber is filled with air, the conditions are right, the decoration phase of the cavern building to begin.

2. Stalactite
Of the various drip stone features found in caverns, perhaps the more familiar are stalactites. These icicle-like (ice cube like) pendants (hanging ornaments) hang from the ceiling of the cavern and from where water seeps through crater (valley) above. The water containing limestone drops from the roof of the cavern. As the water evaporates it leaves behind solidified calcium carbonate (CaCO3). The stalactites are sharp, slender, downward growing pinnacles (peaks) that hang from the cave roof. They look like a set of concentric rings of CaCO3 placed over one another with the broadest one at the roof and the narrowest being at the downward extremity. The stalactite is appropriately called a Soda Straw. (GOH 76)

3. Stalagmites
The drops of water that fall on the floor of the cavern after trickling down the roof still contain some lime which is left behind on the floor after the water has evaporated. Thus on the same lines as a stalactite, a pillar begins to rise upwards from the floor. This is known as Stalagmite. It is conical in shape and comparatively thicker. Because the drops, as they fall down the roof, are scattered away, the stalagmite is blunt and irregular in form but at its top it has generally a crater like depression.

Environmental Problems Associated with Groundwater
As with many of our valuable natural resources, ground water is being exploited at an ever increasing rate. Overuse threatens the ground water supply in some areas. In other places, ground water withdrawal has caused the ground and everything resting upon it to sink. Still other localities are concerned with the possible contamination of their ground water supply.

1. Sinking of Grounds

The ground may sink when water is pumped from wells faster than the natural recharge processes can replace it. This effect is particularly pronounced in areas underlain by thick layers of unconsolidated sediment. As water is withdrawn, water pressure dorps and the weight of the overburden is transferred to the sediment. The greater pressure packs the sediment grains tightly together and the ground subsidies.
Many areas may be used to illustrate land subsidence resulting from the excessive pumping of ground water from relatively loose sediment. A classic example in the United States occurred in the San Joaquin Valley of California [Atlas 14-B3]. Here, in a region of extensive irrigation, the water table beneath the valley has gradually been drawn down by as much as 30 meters. As a consequence, the land has subsided by 3 m in some places.

2. Pollution of Groundwater

The pollution of ground water is a serious matter, particularly in areas where aquifers supply a large part of the water supply.

a. Sewage

A very common source of ground water pollution is sewage, which results from an ever-increasing number of septic tanks, as well as inadequate or broken sewer systems, and barnyard wastes.
If water contaminated from bacteria enters the ground water system, it may become purified through natural processes. The harmful bacteria may be mechanically filtered out by the sediment through which the water percolates, destroyed by chemical oxidation and / or assimilated (absorbed) by other organisms. In order for purification to occur, however, the aquifer must be of the correct composition. For example, extremely permeable aquifers such as highly fractured crystalline rock, coarse gravel, or cavernous limestone have such large openings that contaminated ground water may travel long distance without being cleansed. In this case, the water flows too rapidly and is not in contact with the surrounding material long enough for purification to occur. On the other hand, when the aquifer is composed of sand or permeable sand stone, the water can sometimes be purified within distances as short as a few tens of meters. The openings between sand grains are large enough to permit water movement, yet the movement of the water is slow enough to allow ample time for purification.

b. Sanitary landfills and garbage dumps

Sanitary landfills and garbage dumps are another source of pollutants that may endanger the ground water supply of an area. As rain water oozes through the refuse, it may dissolve a variety of organic and inorganic materials, some of which may be harmful. If water containing material is leached from the landfill reaches the water table, it will mix with the ground water and contaminate the supply. Since ground water movement is slow, the polluted water may go undetected for a considerable time. When the problem is finally discovered, the volume of contaminated water may already be very large. Thus, even if the source of pollution is eliminated immediately (which is most unlikely), the problem could linger for many years until the contaminated water has migrated from the area of use.