How do limestone caves form?

Dissolving limestone

Caves, and other karst features, are produced because limestone is soluble in water which contains dissolved carbon dioxide and organic acids. As it falls, rain picks up atmospheric carbon dioxide. When the water passes through the soil, more carbon dioxide, from plant roots and decaying vegetable matter, becomes dissolved in the water, along with complex organic acids, called humic acids. This ground water is easily able to dissolve limestone.

Limestones which contain large cave systems have few pore spaces.  Therefore, ground water moves along planes of weakness in the rock (joints, bedding planes and faults). This concentrates solution to structurally controlled zones, rather than allowing solution to be spread evenly throughout the rock. So, the formation of large, complex cave systems depends on:

1. the solubility of the rock,
2. mechanical strength of the rock, and
3. the restriction of the solution to structurally controlled zones.

Phreatic zones

Solution cavities in limestone may be completely water filled.  This is called the phreatic zone. Water in the phreatic zone may be relatively still (nothephreatic), or it may be moving quickly under pressure (dynamic phreatic).

Still phreatic conditions result in three-dimensional solution, forming either:

• large dome-shaped chambers or
• complex three dimensional mazes.

The rock may become like a Swiss cheese, producing what is called spongework.

In dynamic phreatic conditions, water in caves behaves like water in a system of pipes, and can flow up hill under pressure, forming loops. Moving phreatic water produces pressure tubes of circular or elliptical cross-section.

The boundary in the rock, between water-filled cavities and air-filled cavities is called the water table.

Vadose zones

The part of the rock mass where water does not fill the pore spaces, is called the vadose zone. In the vadose zone, water behaves much as it does in surface streams. It cuts down into the rock and may meander.

Complex cave systems usually start by phreatic solution. When downcutting outside the caves allows the water to drain, vadose stream action can take place. This results in cave passages with rounded phreatic ceilings and vadose canyons cut in their floors. There are many examples of this type of cave at Jenolan. The main passages of Imperial Cave, Chifley Cave and River Cave have this form.

Epiphreatic solution

Much cave development occurs just below the water table. This type of development is called epiphreatic. Water just below the water table often moves rapidly. It produces wide, flat, solution ceilings such as those in the western part of the Grand Arch. Arch caves like the Grand Arch, Devils Coach House cave and Carlotta Arch owe their wide flat ceilings to epiphreatic solution.

Breakdown

Collapse of rock from cave roofs, known as breakdown, is an important process in the late stage of cave development. Some breakdown occurs when the cave is drained of water, and the rock mass adjusts to contain a large cavity. Other breakdown is produced where solution undermines rock with horizontal joints. The large zone of breakdown which formed the Exhibition Chamber in the Lucas Cave was caused when solution undermined a block of limestone which was bounded on three sides by cave passages and contained horizontal joints.

Ongoing cave development

At Jenolan Caves, cave development continues today. In the Jenolan Underground River and the River Styx, new caves are being excavated by phreatic processes below the water level.  At the same time, old caves are being filled with sediment or, in a few cases, far from the showcave system, are undergoing breakdown.