Chinoyi Caves EarthCache

Chinoyi Caves - a scenic and geological wonder.

To qualify to log this cache the following questions need to be answered:
1. In your own words - why do you think the colour of the water is the most incredible blue?
2. From your observations is the predomonant rock inside these caves very hard or soft and porus, and which minerals is it likely to composed of ?
3. One aspect of this water remains constant in all weather and at all times - what is this?
4. While standing at "the sleeping pool" What would you estimate the circumference of the open area immediately above from which the sky is visible?
Purely optional and not nescessary to log this cache would be the posting of any photos of your visit.

Email your answers to zambesiboy@gmail.com - logs without email answers will be deleted as will any logs deemed to be "armchair caching".

The Great Dyke in the southern Zimbabwe Craton is one of the most remarkable rock formations in the world. A “craton” is a term geologists use to describe Earth’s oldest continental regions. These thick and very hard layers of rock are more than 2 billion years old, and form the cores of most of Earth’s continents. Over the millennia, the cratons comprising Earth’s landscapes were transported, twisted, folded, eroded, and split by the tremendous geological forces continually reshaping our world’s land surfaces.
Zimbabwe’s Great Dyke, running vertically through the center of this scene, probably formed slowly, over centuries, as molten rock forced its way up from the Earth’s interior through the Zimbabwe Craton. The dyke stretches about 530 km across the craton and varies from between 3 and 12 km wide long its length. The dyke is extremely rich in platinum and chromium deposits that support the Zimbabwe economy.

This earth cache is not directly about the Great dyke but the world famous caves that are situated as you approach the Great Dyke when travelling North West from Harare. A future Earth cache will focus on the Dyke itself.

The caves are situated on the edge of this "Great Dyke" near the town of Chinhoyi in Zimbabwe. Chinhoyi Cave is the only show cave in Zimbabwe. The cave system is composed of limestone and dolomite, ( The makeup of the limestone is largely from the minerals Calcite and Argonite). The descent to the main cave with its pool of cobalt blue water is very impressive. They are situated about 8 kilometres (5 miles) north of Chinhoyi town itself, and 128 kilometres (80 miles) north-west of the capital of Zimbabwe, Harare. The caves are named after a local chief who used them as a refuge from Ndebele raiders. These caves are the most extensive cave system in Zimbabwe that the public can access and were designated a National Park in 1955.

usually tens of thousands of years Chinhoyi Cave is the second longest cave and the fourth deepest. The Chinhoyi dolomite karst is the largest karst area in Zimbabwe.  Karst topography is a geological formation shaped by the dissolution of a layer or layers of soluble bedrock, usually carbonate rock such as limestone or dolomite. This takes place over a very long time to even much more than that!
The cave has several fossil passages with noticeable speleothems (secondary mineral deposits). Several larger daylight windows are unique for this cave. But the extraordinary sight is the underground lake  popularly called Sleeping Pool or Chirorodzira (Pool of the Fallen)., which is sounded to be 172m deep. US Navy divers reached a depth of 135m in the lake.
The daylight from above and the high amount of soluted limestone gives the crystal-clear water a translucent aquamarine colour. The fish in the lake are a result of human intervention, they were freed here in order to keep mosquitoes at bay.

Divers have discovered a submarine passage leading from the Bat Cave, a subchamber of the Dark Cave to another room known as the Blind Cave. Diving is possible in the caves all year round, with temperatures that never vary beyond the 22 to 24 °C (72 to 75 °F) range with zero thermocline. Visibility is high, and 50 metres (160 ft) and above is not unusual. This site is often visited by diving expedition teams of technical divers that perform ultra deep diving. It is not uncommon for dives in excess of 100 metres (330 ft) to be made here by experienced technical divers.

The forming of karst caves is very complicated and it is still a topic of geological research. But some basic aspects are very simple: water containing carbon dioxide CO2 is able to dissolve limestone.
Unlike with salt and gypsum, water is not able to dissolve limestone without a little help. And this helper is carbon dioxide CO2. Carbon dioxide is a very common gas, you know it from sparkling water, softdrinks, and beer. They all contain it, it makes them bubbling. Natural water also contains CO2, but much much less, as it is normally not bubbling. Just fill some water in a bottle and after some hours you will see small gas bubbles at the glas of the bottle.
Carbon dioxide is in the the air, about 0.03% of our atmosphere is this gas. Animals and humans breathe air, consume (burn) the oxygen and exhale carbon dioxide. Many carbon dioxide is produced by biologic processes in the soil. When the rain water falls, it first absorbs some carbon dioxide from the atmosphere, but then, oozing through the soil, it absorbs much more biogene CO2.

In the water, the CO2 breaks up in two pieces, thus producing a (very weak) acid:
CO2 + H20=H2CO3=H+ + HCO3-
When this water gets in contact with limestone, the acid solutes the limestone.
H+ + HCO3- + CaCO3=Ca++ + 2 HCO3
This happens on the rock surface, just at the border between rock and soil. But when the limestone, which is originally water proof, contains cracks produced by tectonic forces - which is very common - the water oozes into the rock and starts to widen the cracks and solute caves inside the layers.
With the small amount of CO2, the amount of soluted limestone is very small and it takes several ten thousand years or more to form a cave.
The limestone in the spring water in karst areas is very common and well known to the people living in those areas. If you heat this water (e.g. for making coffee, in the washing machine,...) the CO2 escapes and the limestone gets solid again. This process is similar to the forming of many speleothems in caves, like drip stones.

In geological time the area is quite dynamic because of the processes described above.