Essay Outline 3

Question: Explain the conditions and processes responsible for the formation of underground caves.

Conditions required:
- Soluble limestone
- Thick enough to sustain the roof of the cave, as a thin limestone may not provide enough mechanical strength
- Jointing which is not too close not too near to have mechanical strength and also to enable circulation of water.
- Balance between permeability and mechanical strength.
- High rainfall or presence of underground river

Main factors in cave formation:
- Solution
- Mechanical erosion by underground streams
- Occasional roof collapse

Through the tectonic widening of existing joints
- Groundwater flow initiated by tectonic stresses on the rock which opened fractures and created the driving force for fluid flow.
- Piping: removal of fine particles by water driven by pressure through poorly consolidated material, important in producing fracture porosity

Through corrosion by water and hydraulic action along joint planes (underground river action)
- Erosion and abrasion in limestone caves
- Many caves contain large amounts of clay, gravel, cobles and boulders which could not have been dissolved from the limestone
- Cave filling material appears to have been transported by moving water from a sediment source outside the cave.

Through solution in the phreatic zone below the water table by slowly moving ground water
- Caverns form by dissolution of limestone at water level
- When water table drops, old caverns are abandoned while new caverns form at new water level
- Ground water circulating in zone below water table dissolves the limestone and forms cavities by attacking joints and bedding planes.
- When water table falls, vadose water pecolates through rock, enlarging cavities.

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

For more information, please visit
http://www.uwec.edu/jolhm/EH/Below/Matt%20Below%20-%20GEOG%20361-sinkhole.htm

LIMESTONE CAVE FORMATIONS

General Description

Cave formations are created when acid reacts with limestone or a rock containing 80% or more calcium carbonate. These formations are found on the walls, ceilings and floors of caves. Cave formations are called speleothems, from the Greek word "spelaion",cave and "thema" meaning deposit.

Conditions Required

A number of conditions need to be present for speleothems to form within a karst environment:

1) The types of rocks within and surrounding the cave need to have an 80% content of calcium carbonate which is usually limestone, dolomite or a similar type of rock.

2) The bedrock also needs to be highly fractured or jointed so the water can flow through or follow these joints or fractures.

3) The bedrock also needs to be relatively close or at the surface.

4) A relatively moderate annual rainfall (>500 mm).

5) To form the speleothems,vegetation cover is needed. Vegetation enhances the Karst process by producing more available acids. A few variable factors including humidity, temperature and air flow through the cave also play an important role in speleothem formation.

Caves and their features form when rainwater follows the cracks or joints in the rock material, usually limestone or dolomite. The rainwater combined with carbon dioxide forms a weak acid called carbonic acid. This weak acid once in contact with the limestone begins to dissolve the limestone. This process slowly creates larger and larger crack and joints. The acid can remain at a consistent level, but often is strengthened due to increased amounts of carbon dioxide absorption from vegetation and soil surrounding the area. As more and more limestone is dissolved large tunnels, networks of tunnels and joints, and actual caves become established. Once the caves and network of tunnels have formed different types of cave formations begin to evolve. Many different speleothems are common in caves including stalactites, stalagmites, columns, rimstone pools, cave pearls, and baconstrips.

Types of Limestone Cave Features (Speleothems)

- Stalactites

- Stalagmites

- Columns

- Rimstone Pools

- Cave Pearls
- Baconstrips

Formation of Stalactites

Just one drop of water on the ceiling of a cave is all that is needed for a stalactite to start forming. Each drop contains a small amount of dissolved limestone that has been acquired from flowing through the cracks and joints of the bedrock. Once this drop is hanging suspended from the ceiling some of the contained carbon dioxide escapes the droplet. Due to this carbon dioxide escaping, the droplet can’t hold as much limestone so a thin external ring is formed. After the drop falls, a small layer is left as a residual. After multiple drops have fallen the dripstone forms a hollow stalactite. These stalactites are called soda straws. As growth continues the soda straw regularly becomes plugged by the deposition. The limestone rich droplets are now forced outside of the soda straw creating the droplets to leave a small “paper trail” of limestone. This results in the cone shaped dripstone. Average growth rates for dripstones (stalactites) are about ½ inch for every 100 years. When multiple soda straws or dripstones join together a baconstrip is formed. These features usually form along a joint where multiple dripstones and stalactites can form.

Formation of Stalamites

Stalagmites are formations that are created from the ground up. These formations form from the drops that have fallen from stalactites or dripstones. Even though the drops left some dissolved limestone with the stalactite (dripstone) some still remains in the droplet. As this droplet falls and hits the bottom of the cave or tunnel the droplet scatters. This process allows more carbon dioxide to be lost hence another dripstone formation on the cave floor has begun to form. After many drops have landed on the same spot a stalagmite has formed (Figure 3).

Formation of Columns

Columns form after thousands if not millions of years of stalactite and stalagmite formation (Figure 4). When both of these two formations finally grow into one another a column is formed. This as stated earlier can take a very long time and in some cases never happens due to cave or environmental changes occurring.

> Click here to view an animation of the formation of limestone cave :DDD

http://www.classzone.com/books/earth_science/terc/content/visualizations/es1405/es1405page01.cfm?chapter_no=visualization

Dolines

• General Description

A doline, sink or sinkhole is a closed depression draining underground in karst areas. It can be cylindrical, conical, bowl- or dish-shaped. The diameter ranges from a few to many hundreds of metres.

The name doline comes from dolina, the Slovenian word meaning valley. So this was originally a colloquial Slovenian word which was used by the geologists to describe a geologic feature.

• Characteristics

- Size: It may vary in size from less than a meter to several hundred meters in diameter and depth.

- Shape: It can be cylindrical, conical, bowl- or dish-shaped. The diameter ranges from a few to many hundreds of metres.

• Contributing Factors

1. Soluble rock at or near the surface (Ca Carbonate, rock salt, gypsum, etc.)
2. Dense rock (little interstitial porosity) which is highly jointed. Chalk develops poor karst due to high porosity.
3. High hydraulic gradient produced by steep topography or entrenched rivers. Moving water corrodes much faster than standing water.
4. High rainfall
5. High biological activity
6. Temperature
- High temperature: Increases biochemical activity so that more CO2 and organic acids are formed
- Low temperature: Cold water has a higher potential to become more acid
- However, in cold regions there is much less CO2 available to dissolve in water because of the low biochemical activity. In permafrost regions the water is frozen. Acid water is concentrated in the active layer and is only.

• Climate and karst

Polar Regions: Karst is poorly developed

Reasons:
1. Low rainfall and short runoff season
2. Limited infiltration in permafrost regions
3. Cold temperatures result in low biochemical activity.

Cold Humid Mid Latitudes: Well developed karst characterized by limestone sinks and closed depressions

Reasons:
1. Greater precipitation
2. Infiltration is uninhibited
3. Greater biochemical activity
a. More CO2
b. more organic acids



Subhumid and semiarid steppe and savanna grasslands: Little to no karst development

Reasons:
1. Very low precipitation
2. During hot dry seasons groundwater tends to move upward and deposit carbonate rather than dissolve it. (Caliche/duricrust: hard crust on arid soils formed by the precipitation of CaCO3 )

**Limestone in arid regions forms ridges and cliffs rivaling sandstone in its stability.



Tropical Rainforests: Well developed karst characterized by residual hills. Region where karst is best developed.

Reasons:
1. Very high rainfall
2. Warm temperatures and thick vegetation results in high concentration (partial pressure) of organic acids and CO2
3. Groundwater flows through the ground in large quantities and is very aggressive



Other controlling factors:
Besides climate and lithology, other factors which strongly influence the nature of karst landscapes are:

1. Base-level fluctuations: caused by tectonic activity or changes in sea level
(i.e. eustatic lowering during glaciation)
2. Structure: solution exploits fracture systems and other planes of weakness that are structurally controlled
3. Stratigraphy: thickness of limestone and permeability of adjacent units
4. Geologic history (is the landscape active or exhumed?)
5. Time

• Types of Dolines

1. Solution doline: Funnel-shaped doline formed by solution along a joint or along the intersection of several joints. Regolith drapes the floor of the doline.

2. Collapse doline: Steep-sided sink formed by collapse into a subterranean cavity. An underground cavern forms. Eventually the overlying rock is longer collapses.






3. Subjacent karst: occurs where the soluble rock is locally breached by erosion over a minor part of its thickness and karst features may already be expressed at the surface at springs and/or collapse features






4. Subsidence doline: Similar to solution doline but overlying soil has washed into a subsuface cave system.

ESSAY OUTLINE 1

Question: State the geological conditions that limestone has which allows for karst landscape formation.

- Chemical Composition: composed largely of the mineral calcite (calcium carbonate: CaCO3). Limestone often contains variable amounts of silica in the form of chert or flint, as well as varying amounts of clay, silt and sand as disseminations, nodules, or layers within the rock. The primary source of the calcite in limestone is most commonly marine organisms. These organisms secrete shells that settle out of the water column and are deposited on ocean floors as pelagic ooze or alternatively is conglomerated in a coral reef. It is alkaline in nature.

*Hence soluble by surface and groundwater.
Though it must be present near or at the surface and less than 50 feet of loose soil covering this soluble rock.

- Porosity: Low porosity due to the compaction of the sediments under high pressure during its formation.

*provides mechanical strength to support karst features such as caves. Consolidation also affects the surface slope development where weak limestone, due to lack of mechanical strength resulting from high porosity.

- Rock Jointing:
-Types of Rock Jointing: Presence of styolites, vertical joints and extensive bedding planes due to the processes of lithification.

- Permeability: High permeability due to the bedding planes, styolites (due to iron oxide content) and vertical joints which act as avenues of weakness.

*Jointing MOST IMPORTANT FACTOR because they permit rocks to holds more water and facilitate groundwater circulation through the system by increasing permeability, which is more important than porosity in the formation of karst.

- Specific Location: Travertine is a banded, compact variety of limestone formed along streams, particularly where there are waterfalls and around hot or cold springs. Calcium carbonate is deposited where evaporation of the water leaves a solution that is supersaturated with chemical constituents of calcite. Tufa, a porous or cellular variety of travertine, is found near waterfalls. Coquina is a poorly consolidated limestone composed of pieces of coral or shells.

*High relief is necesary in order that there can be development of verticaland underground circulation of drainage. Space must be available for these movements of water hence the height of the karst area above the sea level or above the level of through-flowing rivers must be great enough for a full circulation of underground water to develop. SPECIFIC location is then important.

Other factors that are undermined by the more important factors stated above in relation to karst formation:

- Rock Hardness: Relative hardness rating a H:3 on the Moh’s scale (comparable to that of a penny)

- Rock Texture: The texture varies from coarse to fine. Most limestones have a granular texture. Their constituent grains range in size due the variation in chemical composition where Limestone being a sedimentary rock has a mixture of rock components in them.

- Rock Colour: Pure limestone is white or almost white. Because of impurities, such as clay, sand, organic remains, iron oxide and other materials, many limestones exhibit different colors, especially on weathered surfaces.

• Formation of Limestone
  

Limestone is the most abundant of the non-clastic sedimentary rocks. Limestone is produced from the mineral calcite (calcium carbonate) and sediment. The main source of limestone is the limy ooze formed in the ocean. The calcium carbonate can be precipitated from ocean water or it can be formed from sea creatures that secrete lime such as algae and coral.

Most sedimentary rocks are formed when weathering crumbles the parent rock to such a small size that they can be carried by wind or water. Those particles suspended in water collide with one another countless times gradually becoming smaller and more rounded. When water is moving quickly due to flooding or due to a rapid change in elevation, larger particles can be carried by the streams and rivers but when the water slows down the particles begin to settle out. The particles dropped from water and wind are called sediment. As layers become thicker, the bottom particles get squeezed closer and closer together under the crushing weight.

Many of the organisms that live in the oceans have shells and skeletons that are made of calcium carbonate. Their decay releases a natural cement into the water. The cement glues the particles of rock back together forming sedimentary rock. People who live in homes with hard water see the amazing affect of this calcium carbonate glue in their showerheads as the holes gradually become closed until the spray is reduced to a trickle.

Limestone Landscapes

We will be delving into the, geologic characteristics of Limestones, its affects on the formation of subsurface landscapes, Tower and Cone Karsts and finally the formation of caves.

• Limestone Characteristics

Limestone rocks are sedimentary rocks that are made from the mineral calcite which came from the beds of evaporated seas and lakes and from sea animal shells (as can be seen from the picture above). This rock is used in concrete and is an excellent building stone for humid regions. [http://www.fi.edu/fellows/fellow1/oct98/expert/limestone.htm]

Hardness:

- Relative hardness rating a H:3 on the Moh’s scale (comparable to that of a penny) http://www.rocksandminerals.com/hardness/mohs.htm

<- Sawn Limestone Block Texture

<- Weathered Limestone Texture

2. Rock Texture:
-The texture varies from coarse to fine.

-Most limestones have a granular texture.

-Their constituent grains range in size due the variation in chemical composition where limestone being a sedimentary rock has a mixture of rock components in them.

3. Rock Colour:
-Pure limestone is white or almost white.

- Because of impurities, such as clay, sand, organic remains, iron oxide and other materials, many limestones exhibit different colors, especially on weathered surfaces.

4. Chemical Composition:

- Composed largely of the mineral calcite (calcium carbonate: CaCO3).

- Limestone often contains variable amounts of silica in the form of chert or flint, as well as varying amounts of clay, silt and sand as disseminations, nodules, or layers within the rock.

- The primary source of the calcite in limestone is most commonly marine organisms. These organisms secrete shells that settle out of the water column and are deposited on ocean floors as pelagic ooze or alternatively is conglomerated in a coral reef.

- It is alkaline in nature.

5. Rock Jointing:
-Types of Rock Jointing: Presence of styolites, vertical joints and extensive bedding planes due to the processes of lithification.

6. Porosity:

- Low porosity due to the compaction of the sediments under high pressure during its formation.

7. Permeability:

- High permeability due to the bedding planes, styolites (due to iron oxide content) and vertical joints which act as avenues of weakness.

8. Specific Location:

-Travertine is a banded, compact variety of limestone formed along streams, particularly where there are waterfalls and around hot or cold springs.

- Calcium carbonate is deposited where evaporation of the water leaves a solution that is supersaturated with chemical constituents of calcite.

- Tufa, a porous or cellular variety of travertine, is found near waterfalls.

- Coquina is a poorly consolidated limestone composed of pieces of coral or shells.


Formation of Limestones

In Layman’s Terms: For thousands, even millions of years, little pieces of our earth have been eroded--broken down and worn awayby wind and water. These little bits of our earth are washed downstream where they settle to the bottom of the rivers, lakes, and oceans. Layer after layer of eroded earth is deposited on top of each. These layers are pressed down more and more through time, until the bottom layers slowly turn into rock.

Click http://www.fi.edu/fellows/fellow1/oct98/create/sediment.htm for a simple animation.