Limestone caves are a natural cavity formed through various external and internal processess. Caves can range from few metres to kilometres in depth.
In the Waitomo region, the Waitomo Caves are formed from the sedimentary rock limestone dissolving over millions of years. The Waitomo Caves started forming over 23 million years ago, with the formation of limestone. Limestone is what causes New Zealand’s karst landscape, and places such as the Waitomo Caves. Limestone is generated by marine organisms, as these organisms contain calcium carbonate in their shells and bones, which is what limestone is made out of. For this to occur, the waters in which the limestone is built up need to be shallow, warm and not fast-moving to attract marine life. 23-34 million years ago, New Zealand had perfect conditions for limestone to develop. Most of New Zealand was covered by a shallow sea, and the climate was warmer.
This provided an ideal place for marine life to thrive, particularly invertebrate organisms with calcareous shells (shells that contained the calcium carbonate needed for limestone.). After an organism had died, it’s decaying matter would accumulate on the seabed. Their skeletons and shells contain calcium carbonate.
This is then mixed with other sea life such as algae and sponge to form layers of limestone. Limestone consists of over 50% calcium carbonate, and can also contain silica, clay, silt and sand. Over time, thick sedimentary layers of these shell fragments develop. As more layers of limestone deposit, the rock becomes heavier, and water on top of the limestone also adds pressure. This results in compaction and cementation, as minerals in the water cement the CaCO3 particles together. This rock is then uplifted and transported to form our karst landscape.
Tectonic movements cause this movement. New Zealand lies on the edge of the Pacific and Australian plates. The Pacific plate moves west and southwest by 30-60 mm a year, as it goes under the Australian plate. In the compression zone, lots of pressure builds up between the two plates as convection currents below cause friction on the plates.
Convection currents occur because of the heating and cooling of particles in the Earth’s mantle. As these particles are heated by the core, the particles are lighter and less dense. As these lighter particles start to rise, they are cooled down as they move away from the heat source. This causes the particles to become more dense, as they have cooled down, and in turn become heavier. Therefore, these particles start to fall down, where they arrive at the heat source and the cycle starts over again.
These currents cause the tectonic movement that in turn cause the limestone to be uplifted. The currents below the Australian and the Pacific plates move in different directions. This is what causes the compression zone, which is the area where the two plates meet. In this area, the Australian plate pushes upwards whilst the Pacific plate subducts underneath.
This builds pressure, which causes earthquakes and faults, as well as joints and fractures. These joints and fractures are crucial to the formation of limestone caves. As New Zealand was moved above the sea level by these plates moving, the limestone within its surface is now exposed to carbonic acid.
Carbonic acid is a weak acid that forms limestone caves, as well as speleothems. As an acid is dropped onto a sample of limestone, the limestone fizzes and dissolves. This happens over a long period of time to caves and speleothems. Carbonic acid is formed when rainwater absorbs the carbon dioxide in the atmosphere. This carbon dioxide gets into the atmosphere through the respiration of organisms. As the carbon dioxide is absorbed into the water, the water turns slightly acidic and becomes a weak carbonic acid. This acid then passes through the cracks, joints and fractures caused by the tectonic movements of the plates. These cracks in the earth allow the acid to reach the limestone.
Once it has reached the limestone, it slowly creates cavities that grow to form caves. It creates these cavities as it slowly dissolves the limestone. These small cavities will grow as more carbonic acid falls down the joints and fractures, and can link up with other cavities to form interconnected cave systems. Because the acid is very weak, it can take thousands of years to carve out the cave systems we see today. Therefore, as limestone forms over millions of years through the deposit of marine organism’s bones and shells contains calcium carbonate, it is then moved above the Earth by tectonic movements between the two plates, Australian and Pacific, which is caused by the convection currents that occur underneath the Earth’s crust. These movements also create joints and fractures, which allow slightly acidic water to travel down to the limestone and dissolve it, forming the Waitomo Caves.