External Landform Processes and Related Features
Senior 6 · Geography
External Landform Processes and Related Features
Lesson 2 of 5
25 min
In some limestone country a river flows across the fields and then simply vanishes into a hole in the ground. Kilometres away it reappears from the foot of a cliff. Between those two points it has been flowing through caves that it dissolved itself. Landscapes like this, shaped mainly by solution rather than by surface rivers, are called karst, after the Kras plateau of Slovenia where they were first studied. They show more clearly than anywhere else what chemical weathering can do.
Definition
Karst: a landscape formed mainly by the solution of soluble rock, usually limestone, with few surface streams, many closed hollows and underground drainage through caves.
Limestone: a sedimentary rock made mostly of calcium carbonate, . It is permeable along its joints and bedding planes, though the rock between them lets little water through.
Figure 4: a karst landscape in cross-section.
| Landform | Description | Formation |
|---|---|---|
| Limestone pavement: clints and grikes | Bare, flat limestone surface cut into blocks (clints) by deep, narrow cracks (grikes) | Solution widens the joints, often under a soil cover that is later removed |
| Swallow hole (sink) | A shaft where a surface stream disappears underground | Water from impermeable rock flows onto limestone and dissolves a widened joint |
| Doline (sinkhole) | A closed, bowl-shaped hollow, metres to hundreds of metres across | Solution at the surface, or collapse of a cave roof |
| Uvala and polje | Larger hollows: an uvala forms where dolines merge; a polje is a large flat-floored basin, kilometres long | Merging of dolines; solution guided by faults |
| Dry valley | A valley with no permanent stream | Cut when the water table was higher or the ground frozen; the river now flows underground |
| Gorge | A deep, steep-sided valley | River erosion, or collapse of a long cave roof |
| Karst towers and pinnacles | Steep isolated hills or sharp spires of limestone | Long-continued solution in hot, wet climates leaves the most resistant blocks standing |
| Caves and passages | Underground openings along joints and bedding planes | Solution and erosion by underground water |
| Stalactites, stalagmites and pillars | Icicle-like growths from the roof, mounds rising from the floor, and columns where they join | Deposition of calcium carbonate from dripping water |
| Resurgence | Where an underground river emerges | The water reaches impermeable rock and flows out at the surface |
Definition
Stalactite: a deposit of calcium carbonate hanging from a cave roof. Stalagmite: a deposit rising from the cave floor beneath a drip. Pillar (column): the two joined. Memory hook: stalactites hold tight to the ceiling; stalagmites might reach them one day.
Cave deposits form by the reverse of carbonation. Water seeping through the soil carries dissolved calcium hydrogencarbonate and extra carbon dioxide. When it drips into the cave air, which contains less carbon dioxide, some of the gas escapes, and calcium carbonate is deposited:
Each drop leaves a thin ring of calcite on the roof; the drop then falls and leaves more on the floor directly below.
Worked example 1: how long does a grike take?
Problem. Suppose solution lowers a bare limestone surface by 40 mm per 1,000 years but deepens the joints twice as fast (illustrative figures). How long would a grike take to reach 2 m deep?
Solution. The joints deepen by mm per 1,000 years.
Karst landforms take tens of thousands of years or more, which is why the deepest are found where limestone has been exposed for a long time.
Worked example 2: a stalactite's age
Problem. A stalactite is 1.5 m long. If it grew at an average of 0.1 mm per year (an illustrative rate; real rates vary widely), how old is it?
Solution.
Growth depends on the drip rate, the carbon dioxide content of the water and of the cave air, and temperature, so an age worked out this way is only an estimate.
| Opportunities | Problems |
|---|---|
| Limestone is quarried for cement, lime (used to treat acid soils and to make mortar) and building stone | Quarries scar the landscape and produce dust and noise |
| Caves, pavements and pinnacles attract tourists | Surface water is scarce, because rain sinks underground |
| Underground water can be tapped from springs and wells | Pollution enters the groundwater quickly through open joints, without being filtered by soil |
| Short grass on thin soils supports grazing | Thin soils limit crops; collapsing dolines can damage roads and buildings |
Warning
Not every cave is a limestone cave. Caves also form in lava flows, where the surface of a flow cools and hardens while molten lava drains out from beneath it, leaving a tube. Such lava tubes have no stalactites of calcium carbonate and are not karst. Check the rock before you call a cave "karst".
Tip
Exam technique. For each karst landform, link it to its process: solution (pavements, dolines, caves) or deposition (stalactites, stalagmites). A labelled cross-section like Figure 4, showing where water enters, flows and emerges, earns marks quickly in a "describe and explain" question.
Illinois State Geological Survey: "Ask a Geologist: What is Karst?". A geologist explains karst, sinkholes and caves, using examples from Illinois, USA. As you watch, trace the path of the water on Figure 4.
Practice
Summary
Sources and credits
Figure 4, the worked examples and the exercises were drawn and written by Ganzaa.
Video: "Ask a Geologist: What is Karst?", Illinois State Geological Survey, embedded from YouTube.
This lesson adapts CC BY-SA material and is shared under CC BY-SA 4.0.
