Karst (Limestone) Topography – SS3 Geography Lesson Note (First Term)

Karst topography is a landscape formed when soluble rock, mainly limestone and dolomite, is dissolved by rainwater. The rainwater contains carbon dioxide, which forms a weak carbonic acid. Water enlarges underground cavities, and the collapse of the rock above forms sinkholes and surface depressions. Karst regions also have caves, disappearing streams and underground drainage.

Behavioural objectives

By the end of the lesson, students should be able to:

1. Explain the meaning of karst (limestone) topography and identify karst regions in Nigeria and the world.
2. State the characteristics of karst topography.
3. Describe the surface and underground features of karst regions.
4. Describe the importance of karst regions to man.

What is karst (limestone) topography?

Karst topography, often associated with limestone bedrock, is characterised by unique landforms created by the dissolution of soluble rocks, particularly limestone and dolomite. It is shaped by chemical weathering. Rainwater containing carbon dioxide forms a weak carbonic acid that gradually dissolves the soluble rock.

How does karst topography form?

Over time, as water percolates through the limestone, it enlarges underground cavities and creates a network of interconnected voids. This subsurface drainage often results in the collapse of the overlying rock, forming sinkholes and surface depressions.

Where are karst regions found?

Karst regions in Nigeria

In Nigeria, limestone deposits at Ewekoro (Ogun State), Sagamu (Ogun State), Nkalagu (Ebonyi State), Gboko (Benue State), Calabar and Sokoto are used for the cement industries.

Karst regions worldwide

Karsts are found in widely scattered parts of the world. They include the Causses of France, the Kwangsi area of China, the Yucatán Peninsula, and the Middle West, Kentucky and Florida in the United States.

What are the characteristics of karst topography?

Karst topography is characterised by distinctive landforms resulting from the dissolution of soluble rocks, typically limestone or dolomite. Key features include sinkholes, caves, disappearing streams and underground drainage systems. The terrain often has an irregular surface with unique geological formations shaped by the dissolution of rock over time.

What are the surface features of karst regions?

1. Sinkholes: Depressions in the ground formed when underground caverns collapse, creating visible holes on the surface.
2. Poljes: Large, flat-floored depressions surrounded by hills or mountains, often serving as catchment areas for water.
3. Dolines: Smaller depressions, similar to sinkholes, but usually with vegetation or water at the bottom.
4. Uvalas: Expanded depressions formed by the merging of dolines.

What are the underground features of karst regions?

1. Caves: Extensive underground systems created by the dissolution of soluble rocks, offering unique habitats.
2. Subterranean rivers: Underground watercourses flowing through caves or passages, contributing to the shaping of the landscape.
3. Speleothems: Formations such as stalactites and stalagmites, resulting from the deposition of minerals in caves.

Why are karst regions important to man?

1. Karst regions often act as important water reservoirs, supplying freshwater through underground aquifers.
2. Unique ecosystems in caves and surface features support diverse flora and fauna, including rare and specialised species.
3. Karst landscapes attract tourists because of their geological formations, caves and scenic beauty, which contributes to local economies.
4. Many caves in karst regions have historical and cultural significance, and contain artefacts and ancient rock art.
5. The fertile soils in sinkholes and dolines are suitable for agriculture, so farming is possible in these regions.

Evaluation

1. What is karst topography?
2. Identify three surface features and three underground features of karst regions.
3. Describe the formation of karst topography.
4. Mention five ways in which karst regions are important to man.

Assignment

Classwork: Answer the evaluation questions above.

Take-home: Name four places in Nigeria where limestone deposits are found and state what the deposits are used for.

Instructional techniques

• Identification
• Explanation
• Questions and answers
• Demonstration
• Videos from the source

Instructional materials

• Videos
• Loudspeaker
• Textbook
• Pictures

Closing the lesson: The teacher commends the students positively.