What this lesson teaches
I can identify cockpit karst, tower karst and caves in the humid tropics.
Syllabus 9173, Topic 2.2, 2. Karst Landforms in the Humid Tropics. What karst landforms can be found in the humid tropics?
I can explain the processes and factors that form tropical karst landforms.
Syllabus 9173, Topic 2.2, 2. Karst Landforms in the Humid Tropics. What factors influence the formation of karst landforms in the humid tropics?
Make a guess
Water drips from a cave roof and a mineral spike slowly grows down from the ceiling. Where does the mineral come from?
- The roof was eroded into a spike by drips.
- Sand blown in from outside the cave.
- Calcite that comes out of the drips.
- Ice freezing in the cave.
Show the answer
Calcite that comes out of the drips.
The water dissolved limestone above. In the cave it gives off carbon dioxide, so calcite comes out of the water and builds a stalactite.
Water dissolves limestone to form caves, and changes in the water's chemistry then deposit calcite as speleothems inside.
Caves are connected underground openings that carry water at different depths. Rainwater picks up carbon dioxide from air and soil, forming weak carbonic acid. As it moves through cracks and layers in limestone, it dissolves the rock. Over time, openings widen into tunnels and cave passages.
Many caves form at or below an old water table. If rivers cut down, the land rises or groundwater falls, passages can be left dry above the water table. Underground streams also erode, and roof collapses enlarge chambers.
Water dripping into an air-filled cave often holds more carbon dioxide than the cave air. It gives off carbon dioxide, so calcite comes out of the water and builds up. These mineral deposits are speleothems.
Stalactites hang down from the ceiling. Stalagmites grow up from the floor where drops land. When they meet, they form a column. Thin films of flowing water create flowstone. Not every cave has many speleothems.
- Cave
- Connected underground openings carrying water.
- Speleothem
- Calcite deposit inside a cave.
- Stalactite vs stalagmite
- Hangs from the ceiling vs grows from the floor.
Step by step
Dissolve limestone
Carbonic acid widens cracks.
Create an air-filled space
The passage drains as water levels fall.
Deposit calcite
Losing carbon dioxide makes speleothems grow.
Real examples to use
Gunung Mulu National Park, Sarawak, Malaysia
A UNESCO World Heritage Site since 2000, Mulu has some of the largest known limestone caves, including the Sarawak Chamber, one of the biggest cave chambers in the world. Heavy tropical rain and acidic water dissolving the limestone formed them.
Worked example: Same calcium, two places
Calcium dissolved from a widening limestone crack can later become part of a stalactite. Removal and deposition are two parts of the same journey. A stalactite is a deposit, not an erosion feature.
Watch out for this
Stalactites form when cave roofs are eroded into spikes.
They are deposits. Calcite builds up from mineral-rich drips.
Check your understanding
A mineral mound grows upward beneath a drip. What is it, and how does it form?
- A stalactite, formed by erosion of the cave floor.
- A stalagmite, formed by calcite from the water.
- A cockpit, formed by river abrasion of the floor.
Show the answer
A stalagmite, formed by calcite from the water.
Yes. It is a deposit growing from the floor.