Todays Earth Science lesson brings you to The Yarrangobilly Caves in Kosciuszko National Park, it is one of the most beautiful limestone Karst systems in Australia and can be explored by the whole family. Six of the caves have been developed and are open for public viewing, including South Glory, North Glory, Jersey, Jillabenan, Harrie Wood and Castle. We have left it up to you which one(s) you visit, it is not necessary to go to the published coords. WARNING –You may want to spend all day here, it is REALLY cool!
For further information: Click here
Some background information:
What is Limestone?
Limestone is a sedimentary rock composed primarily of calcium carbonate (CaCO3) in the form of the mineral calcite. It most commonly forms in clear, warm, shallow marine waters. It is usually an organic sedimentary rock that forms from the accumulation of shell, coral, algal and fecal debris. It can also be a chemical sedimentary rock formed by the precipitation of calcium carbonate from lake or ocean water.
The limestone at Yarrangobilly is of Late Silurian age and formed approximately 440 million years ago. The limestone occurs in a belt that is approximately 14 kilometres long and 1.5km wide and is overlain by slates, shales, sandstone and conglomerates of the Ravine Beds. The Yarrangobilly karst contains an outstanding collection of features, including gorges, arches, blind valleys, springs, pinnacle fields and over 250 caves.
What is Karst?
Karst is a distinct landform shaped largely by the dissolving action of water on carbonate rock such as limestone, dolomite and marble. This process typically occurs over thousands or millions of years, resulting in a variety of surface and below-ground features, including gorges, sinkholes, underground streams and caves.
Karst features interact with the environment to produce complex ecosystems supporting highly specialised plants, animals and micro-organisms.
For further reading about Karst caves Click here
What is a Speleothem?
A Speleothem is commonly known as a cave formation, is a secondary mineral deposit formed in a cave. Speleothems typically form in limestone or dolostone solutional caves.
How do Caves become so ornate?
Many of Yarrangobilly's caves are well-decorated. Several caves contain sections of rockfall and rock fills thought to be related to ice-wedging associated with previous ice ages. In Jersey Cave, thick flowstone sequences span half a million years and provide the longest continuous fire history record from a single site in Australia.
In limestone caves, (just like the ones you are visiting today) after the natural process of erosion and excavation, a simple but slow natural process is responsible for the decoration of the bare, dull walls. Falling rain picks up atmospheric carbon dioxide. On passing 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 easily dissolves limestone. The solution slowly descends into the cave and deposits the dissolved calcium carbonate, sometimes including impurities of iron and other minerals. On reaching the cave, the water droplets fall to the floor, leaving behind a tiny deposit of calcite crystal. This is known as precipitation.
The simplified chemistry involved is:
The Acidic Solution
Water + Carbon Dioxide = Weak Carbonic Acid Solution
H²0 + CO² = H²CO³
Dissolution of Limestone
CaCO³ + H²CO³ = Ca² + 2(HCO³)
Calcite Deposition
Ca² + 2(HCO³) = CO²CaCO³ + H²O
Major types of limestone formations (speleothems):
Dripstones:
Straws are thin walled hollow formations that resemble drinking straws. As water drips slowly from the roof of the cave, it deposits a microscopic ring of calcite crystal. These rings continue to build and can form straws many centimetres long.
Stalactites are also formations that grow downwards from the cave roof. Nearly all stalactites start their life as a straw. When the straw becomes blocked with calcite or impurities, a stalactite starts to develop and thicken over the years, from the solution which runs down its outer surface.
Stalagmites are solid dripstones that grow upwards from the cave floor, from each drop of water from the roof or from stalactites overhead.
Columns or Pillars are formations that develop from stalactites or stalagmites that extend from floor to roof. Straws are thin, long, hollow and glasslike. Straws eventually become stalactites and hang thickly from the cave roof.
Flowstone:
Shawls - Water reaching the roof of a cave does not always form drops. Sometimes it trickles down a rockface, depositing a narrow strip of calcite, that eventually results in a thin sheet, growing at an angle from the wall. Shawls often contain interesting folds, which occur because the initial trickle turned from side to side in its downward path along the rockface. The rich coloured banding that is often seen, is caused by other minerals in the solution, such as iron oxide.
Flowstones: These attractive formations occur when flowing water leaves a film of calcite. They cover the original rock or mud floor, often to a considerable thickness. Sometimes the lower portions hang free, making a fringe or shawl of stalactites.
Pore Deposits:
Helictites are formed by water slowly entering the caves through pores and cracks in the limestone. Helictites are named from the Greek word 'helix', meaning 'a twist'. They are small irregular growths which proceed in any direction contrary to gravity. The most accepted theory for their growth and development is a combination of capillary action and hydrostatic pressure. The saturated solution eminates from a pore so slowly that it doesn't form a drop. Evaporation occurs and a minute layer of crystal is deposited. A capillary tube slowly develops, through which the solution is drawn, extending the helictite. Because no drop forms, gravity has no effect and the helictite can develop in any direction, defying gravity.
Subaerialcave corals are another form of pore deposit that develops as the solution seeps from cave walls, depositing layers of calcite crystal. Capillary tubes do not form, and the growth can develop over large areas. They have a rough texture and may resemble cauliflower.
Pool Deposits:
Rimstone dams or gours are fragile vertical walls that build up as cave pools overflow, depositing calcite at the edges. At the point of overflow, calcite is precipitated as CO² loss occurs.
Dogtooth Spar is a pool deposit that forms under extremely still conditions, where there is little or no water movement. This allows the supersaturated solution to form large crystal faces and perfect form. Dogtooth Spar also forms in vugs (mineral lined rock cavities) which are later exposed as the cave develops.
Cave Pearls are spherical and develop in saturated pools. The calcite forms around a nucleus, such as a tiny pebble or grain of sand. Calcite crystal builds up as the pearls are agitated, usually through drops of water from the cave surroundings.
Lily Pads or Shelfstone form around the edges of cave pools or existing dripstone formations, such as stalactites, stalagmites and columns. They usually develop under still water conditions and a constant pool level.
Other noteworthy Yarrangobilly features:
Fossils: Significant sub-fossil deposits have been found in caves at Yarrangobilly. These include the remains of the vulnerable broad-toothed rat (Mastacomys fuscus), endangered smokey mouse (Pseudomys fumeus) and long footed potoroo (Potorous longipes), and the extinct thylacine (Thylacinus cynocephalus).
Thermal Pool: A well loved feature of Yarrangobilly Caves is the thermal pool. Water seeps through porous rock to a depth of approximately 760 metres where it is heated and then forced up through cracks to emerge as a warm spring. The spring discharges at approximately 100,000 litres per hour and at a constant temperature of 27 degrees centigrade. A bathing pool was first constructed at the thermal spring in the late 1890s and the present pool is 20m long and up to 2.5m deep.
To log this Earth Cache we require you to consider the information above then wander around and discover at least one of the caves, then message us with the following answers to the best of your ability;
1. In which direction does a stalactite form and which direction does a stalagmite form?
2. Flowstones and Dripstones are very common here in the caves, consider the cave(s) you explored, which was more prevalent , state the cave you visited and give a reason why this would be the case?
3. On your journey through the cave(s) identify a pore deposit and describe what you see, what gives it the characteristics of a pore deposit?
4.Can you find a cave pearl? Describe its size and colour?
5. A photo of your team, GPS near/in one of the caves with your log and answers. This is such a beautiful place feel free to upload as many photos as you like. (optional)
You are welcome to log your answers straight away to keep your TB's and Stats in order but please message us with your answers within 48 hours. Cachers who do not fulfil the Earth Cache requirement will have their logs deleted.
Sources: Wikipedia, www.jenolancaves.org, www.environment.nsw.gov.au, geology.com.
*This cache although virtual was placed with permission of Simon Allender - Ranger - Riverina Highlands Area.
