Essay Outline 3

Question: Explain the conditions and processes responsible for the formation of underground caves.

Conditions required:
- Soluble limestone
- Thick enough to sustain the roof of the cave, as a thin limestone may not provide enough mechanical strength
- Jointing which is not too close not too near to have mechanical strength and also to enable circulation of water.
- Balance between permeability and mechanical strength.
- High rainfall or presence of underground river

Main factors in cave formation:
- Solution
- Mechanical erosion by underground streams
- Occasional roof collapse

Through the tectonic widening of existing joints
- Groundwater flow initiated by tectonic stresses on the rock which opened fractures and created the driving force for fluid flow.
- Piping: removal of fine particles by water driven by pressure through poorly consolidated material, important in producing fracture porosity

Through corrosion by water and hydraulic action along joint planes (underground river action)
- Erosion and abrasion in limestone caves
- Many caves contain large amounts of clay, gravel, cobles and boulders which could not have been dissolved from the limestone
- Cave filling material appears to have been transported by moving water from a sediment source outside the cave.

Through solution in the phreatic zone below the water table by slowly moving ground water
- Caverns form by dissolution of limestone at water level
- When water table drops, old caverns are abandoned while new caverns form at new water level
- Ground water circulating in zone below water table dissolves the limestone and forms cavities by attacking joints and bedding planes.
- When water table falls, vadose water pecolates through rock, enlarging cavities.