What is limestone and how is it formed? How is Limestone formed?



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What is limestone and how is it formed?

How is Limestone formed?

Task:

  • Using the information on the next slide you should come up with no more than 3 sentences that explain how limestone is formed and exposed to the elements.
    • It doesn’t have to be as detailed but needs to show an understanding of the origins of limestone.
  • Limestone is formed from the fossilised remains of countless marine plants and animals, such as corals, which lived during the Carboniferous period, 345 - 280 million years ago.
  • These former sea beds were moved from the Equator due to continental drift and forced above sea level 280 - 230 million years ago.
  • During Glaciation the topsoil is scoured from the landscape exposing the limestone underneath to weathering.
  • Formation
  • Carboniferous limestone landscapes are the result of:-
  • The rock is divided into blocks as a result of breaks between
  • the rock beds (bedding planes)
  • Vertical cracks (joints) are created as the limestone dries out and
  • pressure is released.
  • bedding plane
  • joint
  • permeability - water passes easily through the rock by following the bedding planes and joints.

Limestone Features

Limestone features

  • Pot holes, swallow holes and shake holes
  • Carboniferous
  • limestone
  • features
  • Limestone
  • pavements
  • Limestone
  • gorges
  • Caves and
  • caverns
  • Stalactites and
  • stalagmites
  • Intermittent
  • drainage

Now to make a cheat booklet:

Limestone landscapes

  • Malham, in the Yorkshire Dales, is famous for its limestone scenery. One feature that is particularly prominent is the limestone pavement.
  • .
  • Limestone Pavement - scraped clear of soil by repeated glaciation.

Limestone landscapes

  • We can think of the limestone pavement as a chocolate bar:
  • Limestone is an alkali rock and is weathered by rain which is acidic.
  • Any water that lies in cracks within the rocks, dissolves the limestone over a long period of time, causing the cracks to widen.
  • Gryke
  • Clint

Limestone pavements

  • Limestone pavement, showing its characteristic clints (blocks) and grikes (gaps).
  • CLINT
  • CLINT
  • GRIKE
  • GRIKE

Limestone Pavement

  • Before weathering
  • After weathering
  • Joints
  • Bedding planes
  • Clints
  • Grikes
  • Gaping Gill
  • Clapham
  • Beck
  • Head
  • limestone
  • pavement
  • 2009 Higher Paper 1, Question 6 b
  • Choose any one Carboniferous Limestone feature described in your answer to part (a) and, with the aid of annotated diagrams, explain how it was formed.
  • Marks 6
  • Probably the most obvious (sensible!) Carboniferous Limestone feature to choose
  • would be a limestone pavement although some candidates may focus on limestone
  • caves and their associated underground landforms such as stalactites, stalagmites
  • and rock pillars.
  • Answers which fail to make use of diagrams should score a maximum of 4.
  • Sufficiently well annotated diagrams ought to be able to earn full marks.
  • In explaining the formation of a limestone pavement, for example, candidates
  • could refer to such points as:
  • • the part played by glacial erosion (abrasion) in scraping away any overlying
  • soil cover and thus exposing the horizontally-bedded, rectangular blocks of
  • limestone.
  • • joints formed in the limestone as it dried out and pressure was released.
  • • these joints/lines of weakness are more prone to chemical weathering than the
  • surrounding limestone. The limestone is dissolved over time by rainwater
  • (weak carbonic acid) leaving deep gaps (grykes) and intervening blocks
  • (clints).
  • • continued weathering (both physical and chemical) will further deepen and
  • widen the grykes.
  • Assess out of 6.
  • Max 2 marks for describing features if no explanation given. 6 marks

Swallow Holes and Pot Holes

  • Because limestone is a permeable rock, there are few surface streams.
  • Streams that flow onto limestone quickly fall into one of the many enlarged joints on the surface and disappear underground.
  • Where a river goes underground it is called a swallow hole or a pot hole.

Swallow holes and pot holes

  • The stream disappears into the ground

Limestone features

Swallow Hole

Limestone features

Caverns

  • Caverns form where some of the underground limestone is dissolved more quickly than the rock around it. This happens when the rock has many joints and bedding planes close together.
  • These cracks allow through lots of water, which dissolves away the rock completely and a cavern forms.

Limestone features

Stalactites

  • Form in the following way:
  • The water that drips into the cavern is laced with calcium carbonate that has dissolved on its passage through the rock.
  • The water drips from the cavern roof very slowly and some of it evaporates leaving behind calcite deposited on the cavern roof.
  • The water continues to drip, evaporating as it does so, and the deposits build up to form fingers of calcite that grow downwards into the cavern.

Stalactites

  • They grow by only a few millimetres a year.
  • They grow slowly, partly because the water cannot hold much dissolved limestone and partly because the caverns are cool and not much evaporation takes place.

Stalagmites

  • Some of the water drips onto the cavern floor where it may also evaporate.
  • It leaves behind calcite here as well, which is deposited on the cavern floor.
  • As more water drips down, more is deposited forming fingers of dripstone that grow upwards from the cavern floor.
  • These are called Stalagmites.

Stalagmites

  • Sometimes stalagmites and stalactites join together to form a limestone pillar.

Caves of Drach

Limestone gorge

Stalactite revision

Limestone landscapes

Limestone landscapes

  • Carboniferous limestone features

Key ideas

  • Limestone solution is the key to understanding all types of limestone features. It is a form of chemical weathering.
  • Limestone features include caves, scars, gorges, limestone pavements, stalactites, stalagmites, clints, grykes, swallow holes and intermittent drainage.
  • Land uses in limestone areas include tourism, quarrying, farming, forestry and military use. Many of the groups that use the land are in conflict with each other.
  • Carboniferous limestone is a particularly tough form of limestone which formed about 350 million years ago and is found in the UK mostly in the north of England and Ireland.

Glossary

  • Alkali – Any substance which, when dissolved in water, has a pH greater than 7.
  • Acid – Any substance which, when dissolved in water, has a pH less than 7. The opposite of an alkali.
  • Calcium carbonate – The chemical composition of limestone.
  • Carboniferous limestone – A type of limestone normally occurring in thick beds, formed in the carboniferous period.
  • Chemical solution – The process of weathering in which limestone rock is dissolved by acidic rain.
  • Chemical weathering – The process by which rocks are decomposed, dissolved or loosened by chemical means.

Glossary

  • Clints – Solid blocks of rock that make up limestone pavements.
  • Grykes – The cracks between the clints in limestone pavement, formed by chemical solution.
  • Limestone – A sedimentary rock, formed by deposits of marine creatures.
  • Limestone pavement – A surface area of limestone weathered in such a way as to form clints and grykes.
  • Weathering – The natural decomposition of rocks into smaller particles over time.
  • Yorkshire Dales – An area in North West Yorkshire, where the underlying rock is principally carboniferous limestone.


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