Speleothems formed in limestone caves are crystals that grow through multiple external and internal processes. Speleothems are found in limestone caves such as Waitomo Caves, and can be formed on the roof of the cave, or on the cave floor. Speleothems that are formed on the ceiling of the cave are called stalactites whereas when formed on the cave floor they are called stalagmites. Stalactites can be seen on the cave roof in figure 1. Speleothems need limestone to be able to form. As mentioned before, limestone takes millions of years to layer, compact and cement to eventually form the rocks we see today. Limestone is formed through the deposit of calcareous shells and bones onto the seafloor. These shells contain calcium carbonate which limestone is made up of. For this to occur, the water needs to be shallow, warm and slow moving to attract this type of marine life. Millions of years ago, New Zealand was a perfect place for limestone to start layering as New Zealand was underwater. As thick layers of the shells and bones developed, this added pressure and resulted in compaction and cementation into limestone. Eventually the rock is uplifted by tectonic movements and is exposed to new external processes that affect the rock. Tectonic movements are caused by the convection currents beneath them. As mentioned before, convection currents are a cycle in which particles are heated, and become less dense and lighter. This causes the particles to rise, where they cool down as they are further away from the heat source and become denser and heavier. This causes the particles to fall, and are now closest to the heat source where they are heated again and the cycle repeats. This causes friction to the Australian and Pacific plates that New Zealand is on the edge of. The currents below these plates move in different directions to each other, and cause the Pacific plate to subduct underneath the Australian plate which moves upwards. In the compression zone, where the two plates meet, lots of pressure builds up and can be released through an earthquake. This causes joints and fractures within the Earth which is crucial to the formation of speleothems. After New Zealand has been uplifted due to these movements, carbonic acid is formed and falls down these joints and fractures within the Earth to reach the limestone. As humans, animals and other organisms respire, they breathe out carbon dioxide. This carbon dioxide becomes part of Earth’s atmosphere. Water in the atmosphere can react with the carbon dioxide to form carbonic acid, a weak acid that can slowly dissolve limestone. This water travels down the joints and fractures to reach limestone, and start dissolving it. This forms cavities, which grow as more carbonic acid dissolves the limestone over long periods of time to form cave systems, such as the Waitomo Caves. As well as creating caves, carbonic acid also aids in creating speleothems. Speleothems are formed when the carbonic acid on the cave ceiling drips down. When the carbonic acid reaches the cave, it loses the carbon dioxide to the air of the cave. The water is now a lot less acidic, and starts depositing the limestone it was carrying (from dissolving the limestone earlier) into crystals. At first a long, thin and hollow straw grows. (fig. 2) The internal channel of the limestone becomes blocked, therefore the carbonic acid flows on its outer surface. This forms successive layers of the limestone, with the innermost layer being the oldest and the outermost layer being the layer most recently formed. These stalactites are typically pointed, and producing a carrot like shape. When water drips down from these stalactites, it lands on the cave floor. This starts the process of forming stalagmites. Stalagmites grow from the drips of the stalactites. These crystals typically produce a more round, stumpy shape. (fig. 3) Limestone columns result when stalagmites and stalactites meet. The rate of growth in which speleothems grow depends on the amount of carbon dioxide is in the air. It can take 1500 years for 1 centimetre of crystals to grow, and is a very slow process that can also be affected by the amount of carbonic acid that runs down the cracks into the cave. To conclude, limestone is formed over millions of years underwater through marine organisms shells and bones containing calcium carbonate accumulating at the seafloor, which is cemented and compacted into limestone. This limestone is then uplifted above the sea through tectonic plate movements, which are caused by convection currents. Then, as humans and animals respire, they release carbon dioxide into the air which reacts with water to form carbonic acid, which seeps through joints and fractures caused by tectonic movements and convection currents. Then the carbonic acid reaches the caves, and forms speleothems, which decorate the walls of the Waitomo Caves.
September 13, 2019 0 Comments