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.