Karst topography is a distinctive landscape formed mainly by the chemical action of water on soluble rocks. Which type of rock usually underlies and gives rise to a classic karst landscape?

Introduction / Context:
Karst landscapes are characterised by features such as sinkholes, caves, underground streams and disappearing rivers. These features result from the dissolution of certain types of rocks by slightly acidic water over long periods. This question asks which rock type usually underlies a classic karst landscape, and therefore is most closely associated with karst landforms.

Given Data / Assumptions:
• The term karst refers to a specific type of solutional landscape. • Options include limestone, quartz, granite and all of the above. • We focus on the most typical and widely cited rock associated with karst. • Groundwater is assumed to be slightly acidic due to dissolved carbon dioxide.

Concept / Approach:
Limestone is a sedimentary rock composed largely of calcium carbonate, which dissolves readily in weak carbonic acid present in rainwater and groundwater. Over time, this dissolution creates underground caverns, enlarged joints and characteristic surface features. This process is the essence of karst topography. Quartz and granite are much more resistant to chemical weathering and do not normally form classic karst features. Therefore, limestone is the rock that usually underlies karst landscapes.

Step-by-Step Solution:
Step 1: Recall that karst topography forms where soluble rocks such as limestone are dominant and groundwater circulation is active. Step 2: Recognise that limestone is made mainly of calcium carbonate, which reacts with carbonic acid in water to form soluble bicarbonate ions. Step 3: Understand that this dissolution process enlarges fractures and bedding planes, leading to caves, sinkholes and subsurface drainage. Step 4: Note that quartz is very resistant to chemical weathering and usually forms hard, erosion resistant rocks such as sandstone and quartzite, not deeply dissolved karst formations. Step 5: Granite is an igneous rock made mostly of quartz, feldspar and mica; it tends to weather into grus and core stones but does not dissolve to create classic karst features. Step 6: Therefore, limestone is the typical rock underlying karst landscapes and must be chosen as the correct option.

Verification / Alternative check:
Physical geography texts almost always define karst topography by referring to soluble rocks, particularly limestone and sometimes dolomite. Examples of famous karst regions around the world, such as in Slovenia, China and parts of India, are all largely underlain by limestone. They rarely cite quartz or granite as the controlling rock for such landscapes. Multiple choice questions in exam books consistently pair the word karst with limestone, which confirms this answer.

Why Other Options Are Wrong:
Quartz is chemically stable and does not readily dissolve in weak acids; therefore, it is not suitable for forming classic karst landforms. Granite undergoes physical and chemical weathering but tends to form tors and exfoliation domes rather than sinkholes and cave systems, so it is not a typical karst rock. All of the above is incorrect because karst landscapes are not equally produced on quartz and granite; they are especially associated with limestone and related carbonate rocks.

Common Pitfalls:
Some students may be tempted by the all of the above option, assuming that any rock could produce karst with enough time. This is not true in the classical geomorphological sense. Others confuse general chemical weathering with the specific solutional processes that form karst, leading them to consider granite. To avoid this, remember that in basic geography, karst equals limestone terrain with sinkholes and caves.

Final Answer:
Classic karst landscapes are usually underlain by limestone.