Caves showing two different chemistries: some exchange air with the surface, others are sealed. The CO2 differences create different mineral formations in adjacent chambers.
No machine-readable author provided. The bellman assumed (based on copyright claims)., Public domain, via Wikimedia Commons
A tale of two atmospheres
The Jenolan Caves are is not uniform. They are a network of more than 300 distinct underground environments, some of which are geochemically isolated from others. This separation creates a site for observing how different atmospheric conditions affect the growth of crystalline cave decorations, or speleothems. The primary difference between these cave environments is ventilation.
"Open system" caves, such as the Imperial and Chifley caves, have a steady exchange of air with the surface. This happens through a "chimney effect," where temperature differences between the cave and the outside drive air circulation. As a result, the carbon dioxide (CO2) levels in these caves are close to that of the outside atmosphere. When rainwater, made slightly acidic by atmospheric and soil CO2, seeps through the limestone bedrock, it dissolves calcium carbonate. Upon entering the low-CO2 cave chamber, this dissolved CO2 rapidly degasses from the water. This chemical change causes the mineral calcite to precipitate, forming the familiar stalactites, stalagmites, and flowstones seen in most tourist caves.
The sealed chambers
"Closed system" caves are poorly ventilated or completely sealed. Here, CO2 from microbial activity and decaying organic matter in the soil above accumulates to high concentrations. With no fresh air to flush it out, CO2 levels can become many times higher than normal. The water seeping into these chambers is saturated with dissolved calcium carbonate, but the high ambient CO2 in the cave atmosphere prevents it from degassing.
This stable, high-CO2 environment changes the rules of crystal formation. Instead of rapid precipitation driven by CO2 loss, slow evaporation becomes the dominant process. This allows for the formation of aragonite, a different crystal form (polymorph) of calcium carbonate. Aragonite often grows into delicate, needle-like forms called anthodites or frostwork. In some cases, where magnesium is also present in the water, soft, paste-like deposits of hydromagnesite known as "moonmilk" can form. These distinct mineralogies mean that two adjacent caves, separated by only a thin wall of rock, can display entirely different crystalline structures, one shaped by degassing and the other by evaporation.
The limestone that hosts the caves dates back to the Silurian Period, around 430 million years ago, when the area was a shallow sea. Scientific dating of clay deposits within the caves shows they began forming at least 340 million years ago, making Jenolan the world's oldest known open cave system.
💡Fun Facts
The limestone bedrock of the caves contains fossils of Silurian-era marine life, including corals, brachiopods, and nautiloids.
Aragonite and calcite have the same chemical formula (CaCO3) but different crystal structures; aragonite is less stable and tends to convert to calcite over geological time.
Some caves, like the Chifley Cave, are actively monitored for real-time CO2 levels to study the fine-scale patterns of speleothem growth.
The cave system is extensive, with over 40 kilometers of mapped passages and more than 300 entrances.
Jenolan Caves is currently closed due to major road damage from flooding events. The reopening date is yet to be confirmed. Check the official website for the latest updates.
Admission
When open, admission is by paid guided tour for specific show caves. Prices vary depending on the cave. Self-guided options may also be available.
Accessibility
Most cave tours involve many stairs and uneven surfaces. Some caves, like the Imperial Cave, are considered more accessible with fewer stairs. Limited wheelchair access may be possible with prior arrangement.
