Dissolution enlarges cave openings. Changed water chemistry can then precipitate calcite as speleothems inside air-filled passages.
Rainwater picks up carbon dioxide from air and soil, forming carbonic acid. As it follows joints and bedding planes through limestone, it dissolves carbonate. Continued removal of dissolved products enlarges connected openings into conduits and cave passages.
Many passages develop where groundwater circulates at or below a former water table. Changes in river level, uplift or groundwater level can leave passages above the current water table. Underground streams can also erode and transport sediment, while roof collapse may enlarge chambers. A cave's shape records drainage history as well as its present water supply.
Water entering an air-filled cave may contain more dissolved carbon dioxide than is in equilibrium with the cave air. It loses carbon dioxide, allowing calcium carbonate to precipitate as calcite. These mineral deposits are speleothems. Evaporation can contribute, but calcite deposition need not wait for the entire droplet to evaporate.
Stalactites grow down from ceilings. Stalagmites grow up where drops reach the floor. If they meet, a column forms. Flowing films of mineral-rich water can create flowstone. Growth depends on water chemistry, drip supply and cave conditions; not every cave has abundant speleothems.
Step by step
Dissolve limestone
Carbonic-acid water widens connected fractures.
Create an air-filled space
A passage may drain as local water levels change.
Deposit calcite
Loss of dissolved carbon dioxide allows speleothems to grow.
Worked example: The same calcium, two settings
Calcium carried out of an enlarging limestone joint can later become part of a cave stalactite. Removal and deposition are different parts of the pathway. Calling the stalactite an erosional feature reverses the final process.
Watch out for this
Stalactites form because cave roofs are eroded into spikes.
They are deposits. Calcite accumulates from mineral-rich dripping water.
Check your understanding
A mineral mound grows upwards beneath a drip point. What is it, and what forms it?
- A stalactite formed by roof erosion.
- A cockpit formed by river abrasion.
- A stalagmite formed by mineral precipitation.