Jenolan Caves

World’s Oldest Caves Never Stop Changing

October 14, 2020

Australia in the Carboniferous Era

October 11 to 18 is Earth Science Week. This annual international celebration highlights how important and relevant it is to learn about our planet. Every day, we are faced with environmental and economic concerns, such as acid rain, water supply, the greenhouse effect, and waste disposal. A knowledge of Earth Science enables us to make better informed decisions about our planet’s energy and minerals. The Earth constantly changes, and Earth Science helps us understand why.

Australia now.People come to Jenolan for many reasons, including adventure, to experience the natural world, the magic of the caves, the wildlife, the quirky history, relaxing overnight stays, great food and wine. But when they leave, they also take away some fundamental knowledge of how these limestone caves formed. Exploring Jenolan caves is not only fun, but a learning experience, so we can understand the land on which we live, and know a bit more about how and why it constantly changes. Change is the key word.

We were amazed in 2006, when scientists announced that Jenolan Caves is the world’s oldest cave system yet discovered. In a study published in the June issue of the Australian Journal of Earth Sciences (Vol. 53, 377-405), scientists from CSIRO, the University of Sydney and the Australian Museum showed that Jenolan Caves has been existing and changing for at least 340 million years.

In 1999 geologists estimated that the caves might be between 90 and100 million years old. Dr Armstrong Osborne, a senior lecturer at Sydney University, had long suspected that the caves are older, but was surprised to find they date back to the Carboniferous (290 to 354 million years ago). To put it into context, the Blue Mountains began to form 100 million years ago. Dinosaurs became extinct 65 million years ago, and Tasmania was joined to the mainland 10,000 years ago.

Slowly, relentlessly changing over millions of years, a vast coral reef compressed into limestone, the limestone rose out of the ocean and became part of a high mountain range.  The action of water carved caves out of the limestone. Clouds of volcanic ash, containing potassium, settled in the caves and crystalised into clay minerals.  Some of the potassium was radioactive, and began changing to Argon gas. The gas remained trapped in the crystalised clay minerals.  All these changes took many millions of years.

Fast forward to 2005. Using X-ray diffraction and scanning electron microscope imaging, the Australian Museum identified eight clay areas in the Jenolan Caves that were suitable for dating – areas where the Argon gas atoms could still be trapped.  Knowing that it takes 1,250 million years for all the potassium isotopes to change into Argon, the CSIRO scientists used a variation of potassium-argon dating to work out the age of the clay minerals. They could tell that the clay was 340 million years old, and had to have been deposited in the caves after the caves were formed. This work was done by the CSIRO’s Petroleum Resources division, using a technique they developed previously to help oil exploration companies find oil deposits. It is a good example of how industry-focused techniques can be used to solve geological mysteries. And see how it’s all about constant change?

School students can delve a bit deeper into Earth Science at Jenolan - more concepts that involve change. They might learn about fossils and how they change people’s understanding of Earth’s history or about how plate tectonics slowly created the Great Dividing Range. They might learn about the various gases that accumulate underground or how we balance conservation and sustainability with changing human activity and needs.  They may learn about the communities of interdependent organisms that live in and around the caves, the rare animals and the effect of changes to the ecosystem. They might learn about the formation of limestone and caves, how they grow and slowly change through weathering, erosion, deposition, and water.  They might learn about chemical reactions that involve energy transfer, like how acidic water changes limestone and the how calcite crystal forms and grows in caves.

So, although we may feel that there is nothing as solid and unchanging as the ground beneath our feet, Earth Science shows that the opposite is true. Our planet is dynamic. Everything about it moves and changes, even the world’s oldest caves - Jenolan.


Sources:

2006 Media Release

Carboniferous clay deposits from Jenolan Caves, New South Wales: implications for timing of speleogenesis and regional geology

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