Litho: Limestone Flashcards
Describe a limestone and its formation
- sedimentary rock
- formed by diagenetic processes. Calcite deposition occurs in deep sea conditions and limestone forms from the accumulation of the calcareous remains of animal and plants plankton where the voids are filled in by mud or chemical precipitates.
What are the 3 characteristics of limestone
Chemical composition
Structural control - low primary high secondary permeability
Little weathered end product
What are the chemical composition of limestone as a characteristic
(4 points)
- dominated by calcite/ calcium carbonate
- Carboniferous limestone is physically hard (due to compaction and diagenesis), hence resistant to physical weathering
- chemically unstable - prone to carbonation
Calcium carbonate + very weak carbonic acid (co2 + rainfall) -> calcium bicarbonate (removed in solution) + calcium - results in solutional landforms which are peculiar to the humid tropics and temperate regions
What is the structural control of limestone
Hint: talk bout age of rock too
Low primary high secondary permeability. Rock is impenetrable by pore spaces, water only enters via fissures such as joints and beddings. Selective weathering occur along these lines of weaknesses, leading to solutional landforms like dolines and caves. The older the rock, the lower the primary and higher the secondary permeability due to lithification (compression and cementation) of the rock with increasing age and subject to tectonic forces causing fissure.
Explain the little weathered end product as a characteristic of limestone
It leaves behind little impurities since most of the end product of carbonation is carried away in solution. This means soil mantles are usually thin or non existent and hollows are allowed to further enlarge to become closed depressions, caves etc. (Contrasts granite which leaves behind much regolith)
Name the 3 types of enclosed depressions
Doline, uvala, cenote
Describe dolines and how they are formed.
- medium sized closed depressions in temperate areas.
- percolating rainwater will solutionally enlarge the upper most fissures of the highly fractured rock
- rainfall will be drained there which enhances selective weathering at that location
- ground will subside, producing a depression
- throughflow flow towards base of hollow which enlarges the depression
Describe uvalas
A ‘depression within a depression’ which is a bigger doline with uneven pitted floors and several sub basins
What are cenotes and how are they formed
Cenotes are depressions with circular, smooth-walled vertical shafts floored by water or boulders. They develop where a water filled cave lies just below the surface. If the cave interjects a zone of highly fractured rock, these fractures become enlarged and rock weakened by both percolating rainwater and upward solution of cave water. Collapse occurs when the overlying rock is weakened sufficiently. (Eg. Yucatan, Mexico)
Which climate are cockpit and tower karsts found? Give one example of cockpit karsts.
Humid tropics
Chocolate hills in the Philippines
How are cockpit karsts formed?
Depressions develop where joints are concentrated. (Same mechanism as dolines) However, in the tropics, the torrential nature of rain fall allows for surface gully if where water stays on the surface of the ground for brief periods during storms. As more gullying occurs, elongated depressions extend headward along the joints so they eventually interconnect, forming cones separated by cockpits.
How are tower karsts formed and what are their characteristics?
Tower karsts are taller cones. Further erosion of cockpits will lower the floor vertically until the base level for erosion is reached. This may lead to the clogging of the flood with impermeable residue from the impurities of limestone.
What limestone landforms are found in glaciated areas?
Karen features - limestone pavements are formed when clints (blocks) and grykes (grooves) created by the joint structure are accentuated by the chemical action of water at grykes. They develop to become spitzkarren over time as grykes are deepened by runnelling.
Name the 2 types of drainage features found in the karst landscape
Karst gorges
swallow holes, dry valleys and blind valleys
What are karst gorges? How are they formed?
Karst gorges are major rivers which commonly originate in non carbonate rocks and flow across limestone terrains without sinking underground as fissures have not been sufficiently enlarged to engulf rivers. The valleys are characterised by vertical cliffs and scree slopes and a narrow valley floor. The lack of mass movement in thin soiled limestone maintains verticality of the slopes.
