Karst Landscapes - Stages of Karst (Leaving Cert Geography): Revision Notes
Karst Landscapes - Stages of Karst
Karst landscapes develop through a process of chemical weathering, primarily carbonation, which gradually dissolves limestone bedrock. This development occurs in three distinct stages, each creating unique landforms and drainage patterns.
Karst landscape development is an extremely slow process that occurs over thousands of years, with some landscapes taking millions of years to reach their final stage.
The three stages of karst development
Karst landscapes evolve over thousands of years, progressing through three main stages as weathering and erosion gradually transform the limestone terrain.
Stage 1: Youthful karst
During the youthful stage, limestone bedrock remains largely protected beneath layers of soil or impermeable rock. Surface water from rivers and glaciers slowly erodes these protective layers, exposing the limestone underneath.
Key characteristics:
- Limestone is gradually exposed at the surface
- Rivers and streams flow normally above ground
- Carbonation begins to act on exposed limestone
- Limestone pavements start to form as joints are widened
- Shallow surface features like swallow holes begin to appear
Limestone pavement - flat areas of exposed limestone where chemical weathering has widened the natural joints, creating a distinctive pattern of blocks (clints) separated by deep cracks (grikes).
Stage 2: Mature karst
The mature stage represents the most active period of underground development. Rivers disappear below ground through swallow holes, creating an extensive network of caves and underground passages.
Key characteristics:
- Rivers disappear underground through swallow holes
- Extensive cave and cavern systems develop
- Dripstone features form inside caves (stalactites, stalagmites, pillars)
- Dry valleys are carved out by rivers that later disappeared underground
- Surface drainage becomes increasingly rare
Swallow hole - a depression or hole in the ground where surface water disappears underground into limestone bedrock. This is the key feature that marks the transition from youthful to mature karst.
The Burren in County Clare, Ireland, is an excellent example of mature karst landscape, covering approximately 250 km².
Stage 3: Old karst
In the final stage, the limestone has been so extensively weathered that cave roofs begin to collapse. Only the most resistant limestone blocks remain standing above the surrounding lowlands.
Key characteristics:
- Cave and cavern roofs collapse due to continued weathering
- Most limestone has been dissolved and removed
- Isolated blocks of resistant limestone remain as tower karst or hums
- Rivers flow on the surface again over impermeable rock
- Dramatic vertical limestone towers dominate the landscape
Tower karst (hums) - isolated pillars or towers of limestone that remain after the surrounding rock has been weathered away, creating spectacular vertical landscapes.
The most famous example of tower karst can be found in Guilin, southern China, where dramatic limestone towers rise from flat plains.
Case study: The Burren, County Clare
Case Study Example: The Burren - A Mature Karst Landscape
The Burren provides an excellent example of a mature karst landscape, demonstrating many characteristic features of this stage of development.
Location and formation
The Burren is located in north-west County Clare, Ireland, covering approximately 250 km² of distinctive limestone terrain. Carbonation has shaped the surface over millions of years, creating an extensive limestone pavement system.
The area was formed roughly 350 million years ago when Ireland was submerged beneath a shallow tropical sea. Layers of marine limestone were deposited and later exposed when sea levels fell during ice ages.
Key karst features in the Burren
Turloughs: These seasonal lakes appear during wet winter months when the water table rises above ground level, then disappear during drier summer periods. Many place names beginning with 'Poll' or 'Poul' (from the Irish word for 'hole') indicate the locations of these features.
Cave systems: The most famous is Aillwee Cave, which has become a popular tourist attraction. These caves formed as underground streams carved passages through the limestone bedrock.
Swallow holes: Rivers such as the Casletown River disappear underground at specific points, then re-emerge elsewhere as springs at points of resurgence.
Dry valleys: These valleys were carved by surface rivers that later disappeared underground, leaving behind steep-sided valleys with no permanent surface water.
Evidence of karst processes
Small streams flowing from Slieve Elva disappear into swallow holes after flowing across shale onto limestone bedrock. During winter months, many dry valleys temporarily fill with water before the flow disappears underground again during summer.
The limestone pavement shows clear evidence of chemical weathering, with distinctive patterns of clints (limestone blocks) separated by deep grikes (solution-widened joints).
Key Points to Remember:
- Karst development occurs in three stages: youthful (protected limestone), mature (underground drainage), and old (tower karst formation)
- Chemical weathering through carbonation is the primary process dissolving limestone bedrock
- The Burren represents mature karst with features like turloughs, caves, and limestone pavement
- Surface drainage disappears during the mature stage as rivers flow underground through swallow holes
- Tower karst landscapes like those in Guilin, China, represent the final stage where only resistant limestone blocks remain standing