Listening Section 4: Note & Table Completion: Question 3

Transcript-based practice (no audio).

Lecturer: Today's lecture looks at karst landscapes, terrain shaped almost entirely by the slow dissolution of soluble rock, most commonly limestone. Limestone is mainly calcium carbonate, and although it looks solid and permanent, it's actually vulnerable to a very ordinary chemical reaction. Rainwater absorbs carbon dioxide from the atmosphere as it falls, forming a weak carbonic acid, and when that slightly acidic water seeps into cracks in limestone bedrock, it gradually dissolves the rock along those fractures, widening them over thousands of years.

The most visible surface feature of a karst landscape is the sinkhole, a bowl-shaped depression that forms when dissolved rock beneath the surface collapses, sometimes gradually and sometimes, alarmingly, all at once. Karst regions are also known for their unusual drainage patterns: instead of flowing across the surface the way rivers normally do, a stream in karst terrain will often vanish abruptly into an opening called a swallow hole, continuing its journey entirely underground, sometimes for many kilometres, before re-emerging somewhere else as a spring.

It's this underground drainage that carves out cave systems, and inside those caves, the same dissolving process runs in reverse. As mineral-rich water drips from a cave ceiling, a tiny amount of calcite is left behind each time a drop evaporates, and over centuries this builds a downward-growing icicle-like formation called a stalactite. Where the drips land on the cave floor, the same mineral accumulates upward instead, forming a stalagmite. Given enough time, a stalactite and a stalagmite can even meet in the middle and fuse into a single column. Collectively, geologists refer to all of these mineral deposits (stalactites, stalagmites, and related shapes) as speleothems.

On the surface, dissolution can also produce a striking feature called a limestone pavement: a flat expanse of bedrock that chemical weathering has carved into a grid of rectangular blocks. The blocks themselves are called clints, and the narrow, often deep fissures separating them are called grikes. These grikes frequently shelter plant species that would otherwise struggle to survive fully exposed on open pavement.

Finally, I want to flag a serious practical concern. Karst aquifers supply drinking water to a substantial share of the world's population, but they are unusually vulnerable to contamination. In most rock types, water percolating downward is naturally filtered as it passes slowly through layers of soil and sediment. In karst terrain, however, water often travels rapidly through open fractures and cave conduits with little or no filtration, meaning a pollutant introduced at the surface can reach, and contaminate, a drinking water supply within hours rather than years.

Complete the notes below. Write NO MORE THAN TWO WORDS AND/OR A NUMBER for each answer.

Karst Landscapes. Lecture Notes

Formation

• Main rock type shaped by karst processes: (1) ______________
• Weak acid formed when rainwater absorbs carbon dioxide: (2) ______________

Surface features

• Bowl-shaped surface depression: (3) ______________
• Opening where a stream disappears underground: (4) ______________

Cave formations

• Downward-growing cave formation: (5) ______________
• Upward-growing cave formation: (6) ______________
• Collective term for cave mineral formations: (7) ______________

Limestone pavement

• Fissures separating pavement blocks: (8) ______________

Water supply

• Main risk to karst aquifers: (9) ______________