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How Caves Form

Introduction

The caves at Jenolan are what are known as limestone caves. Quite simply that means that they are formed within a rock known as limestone. Of the caves that are known throughout the world, limestone caves tend to be amongst the longest and most complex in nature. They are also the type of cave that most people think of when they think of caves. There are other types of caves though. Lava tubes are common in areas where there once lava flows. The lava tubes at Undara in Queensland are particularly well known. Another common type of cave is the overhang and there are many examples of these throughout the Blue Mountains, formed in sandstone. In planning a trip to Jenolan, some sidetrips may be made as the Blue Mountains are crossed to see a number of overhangs. Some of these may be quite large such as Kings Cave at Linden. Nevertheless, overhangs are nowhere near as large and involved as the limestone caves of Jenolan.

What is Limestone

Limestone is a sedimentary rock which simply means that the rock formed as a result of material being built up by its constituents being laid down over a period of time and gradually becoming cemented together. Many people will state that limestone is the ancient remains of coral reef and whilst this may be true in some instances it need not always be the case. Generally limestone is formed in a warm shallow sea, rich in life. As organisms grow, those that have skeletons or live in shells, will exude calcium carbonate - a prime constituent in their structures. The calcium carbonate settles to the seabed where it forms a calcareous ooze. Mixed in with this are skeletal remains of sealife and corals. As the rockmass is lifted up out of the ocean water is able to start seeping through the sediment, dissolving the calcareous material and then reprecipitating it, forming a cement which binds the rock mass together. The action of the water flowing through the rock mass not only cements it together but also can cause concentrations of various minerals. This concentration of minerals can lead to differing types of cave formation later on. Through the process of uplift fractures are introduced into the rock which allow water to penetrate more easily through parts of the rock. For more information on minerals check out www.minerals.net

The Process of Cave Formation

As rainwater falls through the sky, carbon dioxide dissolves into the water causing it to become slightly acidic. The formula for this is shown in the figure below.

Chemical Equations

As the water seeps through the soil it becomes more acidic in nature. One of the important qualities of limestone is that it is easily dissolved in a weak acidic solution when compared to other rock types. The water enters fractures in the rock, dissolving the rock as it progresses. Fractures are widened over time and as underground spaces grow there is further cave development as surrounding rock collapses. As the water table drops dry spaces remain and these are what are inspected on cave tours. Cave development continues both through the action of surface water entering the cave environment as well as the main body of water flowing through the rock mass.

 

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