Grades 6–8 · Earth & Space
How can water slowly open a cavern inside limestone?
A limestone cavern can be opened by water that moves through tiny spaces in rock. The water does not have to smash the rock like a hammer. It can dissolve part of the mineral and carry the material away in solution. Repeated over a long time, this can enlarge fractures into passages.
In the diagramDissolving mineral can open a limestone passageWater acquires carbon dioxide in air and soilEnlarged passageDissolved ions leaveEarlycrackIdeal reaction count: 1 calcite unit dissolved1 calcium ion + 2 bicarbonate ionsPassage sizes and reaction counts are symbolic.
Limestone commonly contains calcite, a mineral made of calcium carbonate. Water entering the ground acquires carbon dioxide from the air and especially from biological activity in soil. Some of the dissolved carbon dioxide reacts with water, making the water weakly acidic.
This water can react with calcite. A useful overall account is: calcium carbonate plus carbon dioxide and water gives dissolved calcium ions and bicarbonate ions. One calcite formula unit supplies one calcium ion and two bicarbonate ions in this account. The rock's material has not disappeared; it is now present as dissolved substances.
Water moving onward can transport those substances away and bring in water able to dissolve more rock. Chemistry, water pathways and changing groundwater levels influence where a cave grows. Water already saturated with calcite may not dissolve more under unchanged conditions. Other caves form through different processes, so this carbonic-acid pathway explains an important family of caves, not every underground chamber.
About this explanation
Starting knowledge: Understand dissolved substances, fractures and a chemical reaction.
Sources
- National Park Service — Cave Geology in Depth
- National Park Service — Solution Caves