Explain the formation of limestone features
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Over 70% of Jamaica's land is limestone, which is composed mainly of calcium carbonate. Limestone weathers through carbonation as rainwater mixes with carbon dioxide to form a weak acid that dissolves calcium carbonate. This process forms several distinctive karst landforms, including clints and grikes (flat limestone surfaces separated by narrow grooves), sinkholes, swallow holes, poljes (flat-floored limestone basins), and resurgent streams. Cockpit country is known for its tower karst and cockpit landscapes formed by the intersection and erosion of limestone joints. Underground streams and caves form as carbonic acid dissolves limestone along joints and bedding planes. St
Explain the formation of limestone features
- 1. EXPLAIN THE FORMATION OF LIMESTONE FEATURES IN THE CARIBBEAN
- 2. Over 70% of Jamaica’s total land surface is made up of limestone rock. Limestone is mainly composed of the mineral calcium carbonate. The island experiences a tropical marine climate with rain falling most of the year, and high temperatures for most of the year. This abundance of water, heat and calcium carbonate are the exact requirements for the chemical weathering process of carbonation. In carbonation, rain mixes with carbon dioxide gas in the atmosphere to produce a weak acid called carbonic acid. Limestone is a porous rock, this means it allows water to pass through it. This acid enters the structure of limestone through its cracks, pores, joints and bedding planes where it chemically changes the insoluble calcium carbonate minerals in limestone to soluble calcium bicarbonate.
- 3. Surface features are those visible above ground, they include: Clints and Grikes Sink holes Swallow holes Poljes Resurgent streams Cockpits Tower Karst Underground features are those found under the Earth’s surface. They include: Underground stream Caves Drip Features The Cockpit country which stretches through the parishes of St. Ann and Trelawny, and Lluidas Vale in St. Catherine, Jamaica are known for their karst landscape. Lets look at how these landforms are created and observe some examples from the sites mentioned.
- 4. SURFACE FEATURES CLINTS AND GRIKES Carbonic acid enters areas of weakness in limestone (joints, cracks, pores, bedding planes) wearing these areas down to form narrow grooves in between the limestone pavement. These narrow grooves are called grikes. The flat limestone surfaces are called clints. Can you identify the clints and grikes from the picture?
- 5. SINK HOLE Watch the video to see how sinkholes are formed. Question: How is water able to form a hole or void in limestone?
- 6. SURFACE FEATURES SWALLOW HOLE A swallow hole like the one seen in the photograph (at right) of the One Eye River, Wallingford,Cockpit country, Jamaica, marks the area where a surface stream disappears in the limestone to flow underground. Swallow hole
- 7. POLJE The polje such as the one seen in the photograph at Lluidas Vale, St. Catherine, Jamaica. The picture shows that a polje is a long basin with a flat floor and steep walls. Rivers brought down alluvial soil and deposited it inside the basin to form the floor of the polje. Farming is therefpre practiced on this polje since the soil is fertile. Polje Floor
- 8. RESURGENT STREAM Rivers which flow underground through limestone will be forced to come back up on the surface when they meet upon impermeable rock such as granite as shown in the picture of the Dornock river head in Stewart Town, Trelawny, (Cockpit country) This is the largest river rising in Jamaica and is the resurgence for two rivers which flow underground – Quashie’s river and the Cane River. It forms the source of the Rio Bueno river. Resurgence Point
- 9. COCKPITS AND TOWER KARST Cockpits are small rounded hills interspersed with depression (Ottley, Gentles, 2006). They are formed wherever limestone joints from a criss cross or star shaped pattern Carbonic acid then enters the limestone where the joints intersect. These areas are eroded to form solution basins. The unweathered portion of the limestone will stand out as a cockpit hill. Go to www.cockpitcountry.com/landscapeimgs/film.cc. swf to watch a movie on cockpit formation. Overtime, continuous chemical weathering on limestone will wear away the sides so that they look almost vertical. At this stage, the cockpit is called a tower karst. Example, in Warsop, Trelawny (Cockpit country) as shown in photograph. Tower Karst
- 10. UNDERGROUND FEATURES UNDERGROUND STREAM Observe the map of the One Eye River on the next page. Observe where it breaks at Wallingford, indicated by the brown arrow. That area shows where it sinks underground in the limestone rock (also shown in the picture) to become an underground stream. It also shows the extent of its underground journey (Source of map: UK D0OS map sheet 19959).
- 12. CAVES AND DRIP FEATURES According to Rahil (1994) calcium carbonate in underground water flowing along the bedding planes and joints of limestone rock will dissolve the rock to form large underground passages called caves. When the roof of a cave collapses, a gorge is formed. Carbonated water passing through cracks and pores in the roof of a cave will form drip features as shown in the photograph taken in the Swansea Caves, Lluidas Vale, St. Catherine, Jamaica. Stalactite Stalagmite
- 13. Carbonated water passing through cracks in the roof of a cave will dissolve the calcium carbonate in the roof. Water droplets mixed with calcite will settle on the roof . Each droplet which hangs from the ceiling will lose water and carbon dioxide by evaporation causing a thin layer of calcite to be left behind. “Successive drops add a layer after layer building up the calcite (Rahil, 1999). These deposits on the roof will build up into a long column of calcium bicarbonate which hangs downwards called stalactites (see picture in Swansea cave). Some of the water mixed with calcite will drip slowly from the roof to the floor. As the water and carbon dioxide exaporates, these will build up and grow upwards forming stalagmites as shown in the photograph in Swansea caves. Overtime, a stalactite and stalagmite will join together since they are directly aligned from top to bottom. This will form a pillar. The picture of the drip features show that the stalactite and stalagmite may become a pillar in the near future.
- 14. REFERENCES Ottley, Gentles “Longman Geography for CSEC, Hodder Education, 2015 Rahil V. “New Caribbean Geography with Map Reading and CXC Questions, Eniath’s Printing Company Ltd, 1994 The One Eye River, UK D0OS map, sheet 1959