Karst Landscapes and Limestone Pavements

2023 Marking Scheme: Physical Geography Landforms

• Question Structure and Grading Requirements:     - Total Structure: 2 marks for a labelled diagram and 14 Significant Relevant Points (SRPs) for the written explanation.     - Diagram Rules:         - A valid diagram is worth $2$ marks.         - A diagram without labeling receives $0$ marks.         - Up to $2 \times \text{SRP's}$ can be awarded for additional relevant information provided on the labelled diagram, provided this information is not already included in the written account.     - Identification Credits:         - Identify the process of chemical weathering: Credit $1 \times \text{SRP}$.         - Identification of a named landform: Credit $1 \times \text{SRP}$.         - Identification of a specific named example: Credit $1 \times \text{SRP}$.     - Content Constraints:         - If the answer merely describes chemical weathering without referencing the shaping of the surface or underground karst landscape, the maximum marks awarded are $2 \times \text{SRP's}$.         - If both surface and underground features are explained, they are marked separately and the highest mark is awarded.         - The question is not geographically restricted to Ireland.

• Marking Annotations and Meaning:     - Dg: Valid diagram ($2$ marks).     - Process: Process of chemical weathering identified ($2$ marks).     - N: Landform named ($2$ marks).     - Ex: Example provided ($2$ marks).     - \checkmark (Tick): Valid information worth $2$ marks ($1 \times \text{SRP}$).     - Surplus SRP: Additional valid information points beyond the required total.

Characteristics of Karst Landscapes

• Lithology and Rock Type:     - The primary rock type associated with karst landscapes is Carboniferous limestone.     - Limestone is a sedimentary rock that is hard, well-jointed, and typically grey in colour.     - Chemical Composition: It consists of at least $50\,\%$ calcium carbonate, also known as calcite.
• Vulnerability:     - Due to its high calcium carbonate content, limestone is particularly vulnerable to the chemical weathering process known as carbonation.
• Landscape Features:     - This weathering creates unique surface features, such as limestone pavements, and underground features, such as dripstone features.
• Primary Example: The Burren, Co. Clare, Ireland.

The Chemical Weathering Process: Carbonation

• Stage 1: Acid Formation:     - Carbon dioxide ($CO_2$) from the atmosphere or the soil mixes with rainwater ($H_2O$) as it falls.     - This reaction forms a weak carbonic acid ($H_2CO_3$).     - Chemical Formula:$CO_2 + H_2O \rightarrow H_2CO_3$

• Stage 2: Chemical Reaction with Limestone:     - When the carbonic acid falls onto or percolates through the limestone, it reacts with the insoluble calcium carbonate ($CaCO_3$).     - The acid converts the insoluble rock into soluble calcium bicarbonate ($Ca(HCO_3)_2$).     - Chemical Formula:$H_2CO_3 + CaCO_3 \rightarrow Ca(HCO_3)_2$

• Stage 3: Removal (Solution):     - Solution is the process where these dissolved minerals (calcium bicarbonate) are carried away by water.     - Regular rainfall ensures that this weathering process remains ongoing.

Surface Feature: Limestone Pavement

• Definition: A large, flat area of exposed limestone rock on the Earth's surface.
• Formation Process:     - Soil Removal: First, the protective soil cover is removed by erosion or human activity, exposing the limestone bedrock.     - Weathering: The exposed surface is then attacked by carbonation.     - Permeability: Limestone is permeable, meaning it allows water to pass through it.     - Structural Weakness: Carbonic acid focuses its weathering on the rock's vertical joints and horizontal bedding planes.

Detailed Anatomy of a Limestone Pavement

• Grikes: These are the vertical joints that have been widened and deepened by carbonation and solution. They can reach dimensions of up to $30\,cm$ wide and $2\,m$ deep.
• Clints: These are the rectangular blocks of limestone that remain between the grikes.
• Karren: Small hollows that form on the surface of the clints where carbonic acid settles and dissolves the rock.
• Fluting: When karren (hollows) overflow, the carbonic acid runs down the sides of the clint, dissolving the vertical edges further and forming furrows. This specific type of erosion is called fluting.
• Landscape Characteristics: The combination of grikes, clints, and karren creates the distinct appearance of the limestone pavement.

Associated Karst Features and Ecology

• Other Surface Karst Features:     - Swallow Hole: A depression in the ground where a surface stream disappears into an underground passage.     - Disappearing Stream: A stream that vanishes from the surface into an underground system.     - Doline: A depression formed by the collapse of an underlying cave system.     - Turlough: A seasonal lake that forms on a bed of limestone depending on the water table level.

• Ecology of the Karst Landscape:     - A thin layer of soil often develops within the grikes of a limestone pavement.     - These grikes provide a microhabitat for unusual flora and fauna.     - Named Flora Example: Spring Gentian.     - Named Fauna Example: Burren Blue Butterfly.

Academic Activity: Regional Study

• Task: Compile a list of information regarding The Burren, Co. Clare, specifically for use in Geomorphological regions essays (relevant to examination questions 4, 5, and 6).