Limestone

Limestone is a sedimentary rock made up mainly of calcium carbonate (CaCO₃) in the form of calcite. Its calcium carbonate content generally falls between 50–90%. Most limestones form in warm, shallow, and calm marine settings, where marine organisms can easily build shells and skeletons from dissolved calcium carbonate.

The purity of limestone depends on the proportion of calcium carbonate it contains. High-purity limestone is usually light in colour, such as the white limestones found in Jamaica. If the rock contains impurities like clay, sand, or volcanic debris, it may appear yellow, brown, or even black.

Limestone is typically softer than igneous or metamorphic rocks, though its hardness is not uniform. The strength of any given limestone depends on its origin, composition, and age.

Varieties of Limestone

1. Biological Limestones

• Originates from the build-up of shells, skeletons, and organic debris in shallow seas.
• Over time, these deposits become compacted and cemented into solid rock.
• A common form is chalk, a fine-textured, soft, white limestone.
• Caribbean example: Chalk deposits in Jamaica and Barbados.
• Other example: Chalk cliffs of Dover, UK.

2. Chemical Limestones

• Produced by the precipitation of calcium carbonate from water.
• Often acts as a natural cement that binds biological grains together.
• Includes travertine, typically found in caves and hot springs.
• Caribbean example: Travertine formations in Harrison’s Cave, Barbados.
• Other example: Travertine terraces at Pamukkale, Turkey.

3. Dolomitic Limestones

• Formed when some of the calcium carbonate is altered into dolomite (CaMg(CO₃)₂) by magnesium-rich groundwater.
• This process, known as dolomitization, makes the rock harder and more resistant.
• Caribbean example: Dolomitic deposits in Cuba.
• Other example: Kaibab Limestone in Arizona, USA.

4. Evaporative or Cave Limestones

• Created when water carrying dissolved calcium carbonate evaporates, leaving behind deposits.
• Leads to the formation of stalactites, stalagmites, and flowstone in caves.
• Caribbean example: Limestone features in Harrison’s Cave, Barbados.
• Other example: Carlsbad Caverns in New Mexico, USA.

5. Crystalline Limestone (Marble)

• When limestone is subjected to intense heat and pressure, it undergoes metamorphism.
• This process causes calcite to recrystallize into larger crystals, transforming the rock into marble, which is harder and less porous.
• Caribbean example: Marble outcrops in Puerto Rico.
• Other example: Carrara Marble of Italy.

Physical Properties

• Texture: can range from fine-grained (chalk) to coarse (coquina).
• Porosity: contains pores that allow water to be absorbed.
• Permeability: water can pass through both pores and interconnected cracks/joints, making limestone a pervious rock.
• Strength: softer when young or pure; harder when dolomitized or mixed with impurities.

Weathering Processes

1. Chemical Weathering (Carbonation)

Limestone is vulnerable to carbonation. Rainwater combines with carbon dioxide to form weak carbonic acid, which reacts with calcium carbonate to produce soluble calcium bicarbonate:

CO₂ + H₂O → H₂CO₃
CaCO₃ + H₂CO₃ → Ca(HCO₃)₂

• Human activity (burning fossil fuels, deforestation, agriculture) increases CO₂ in the atmosphere, speeding up this process.
• Acid rain has damaged many limestone buildings and monuments.
• Caribbean example: Karst weathering in Puerto Rico and Jamaica.
• Other example: Erosion of limestone structures in Europe.

2. Physical Weathering

In colder regions, freeze-thaw cycles expand cracks and break limestone into fragments.

3. Biological Weathering

Roots penetrate cracks, animals disturb the rock, and human quarrying contributes to breakdown. Pollution also increases chemical decay.

Key Caribbean Locations

• Jamaica: Cockpit Country (karst limestone).
• Puerto Rico: Karst regions and marble deposits.
• Barbados: Harrison’s Cave and chalk deposits.
• Cuba: Dolomitic limestone formations.