What’s a Cave?
A cave is a natural cavity in the ground that is large enough that some portion of it will not receive direct sunlight and large enough for a human to enter.
Like everything, there’re several types and categories of caves based on their origin. Primary caves are developed as the host rock is solidifying like lava tubes or coral caves; secondary caves are carved out of the host rock after it has been deposited or consolidated in with there is the most part of them.
We’ll not focus in an extensive description of all the types of caves, instead we’ll focus in only some of them, the most commons ones.
Eolian Caves or Wind Caves.
When the caves formation is cause by the erosion of the wind in sandstone cliffs, these caves are known as Eolian Caves or Wind Caves.
If the cave is created with erosion away weak areas along sea cliffs, in which can be any size from crevices to large chambers, are called as Sea Caves
When colonies of coral in shallow water expand and unite, they form lacy or bulbous walls around an open area. When the shoreline is pushed up or the sea level falls, the cave is exposed. Waves and wind erode the coral, enlarging the cave, sometimes even destroying it.
Long tunnels form near the snouts of glaciers between glacial ice and the underlying bedrock. Water from the surface drains down through crevasses in the glacier. It enlarges the crevasses and melts away the ice at the base of the glacier.
These caves can be carved out of glaciers or snowfields by water and/or wind or in a rock cavity containing ice formations. As moisture in a cave is frozen it clings to the walls and continues to build up. When slight melting occurs or water enters the cave, it runs along the walls creating formations similar to calcite speleothems.
These type of caves are all created from flowing lava and the effects of volcanic gases. Categories of volcanic caves include lava tubes, pressure-ridge caves, spatter cone chambers and blister caves.
A massive movement of bedrock separates rocks along joints or fractures. The cave created in this fashion is usually a small, high, narrow fissure consisting of a single passage. The ceiling is often a flat section of rock that did not move, or moved in a different direction. Massive, brittle rocks such as sandstone and granite are the best rocks for tectonic caves; however, they can also occur in basalt and limestone.
This is the category of caves that is classified as caverns. They are formed by the dissolution of soluble rocks such as limestone (calcium carbonate), dolomite (calcium magnesium carbonate), gypsum (calcium sulfate dihydrate) and salt (halite).
Many people think that caverns are created by limestone bedrock being dissolved slowly by water. This is not true, limestone is not much more soluble than glass in water withc means that water alone could not form caverns but, however, when ground-water contains an acid in solution it can and does make caverns.
Older traditional theories identified dissolved carbon dioxide as the probable cavern forming acid. When carbon dioxide dissolves into water it forms a weak acid known as carbonic acid. It is the same acid that gives carbonated beverages the bubbles and the pleasant "bite" on the tongue. It is however a very weak acid and would not normally be able to explain most of the worlds caverns.
Unlike carbonic acid, the hydrogen sulfide is much better suited for the probable primary source of the cavern development acid because of its available in many natural sources. This can be generated by the anaerobic (oxygen free) decomposition of organic materials such as leaves, oil, coal and other natural deposits, with the oxidation of sulfide deposits such as iron pyrite when it is exposed to weathering or even produced in large quantities during volcanic actions and from geo-thermal springs and features.
When an underground source of hydrogen sulfide wells up and mixes with down-percolating oxygenated water from the surface the mixture of water, hydrogen sulfide and oxygen forms sulfuric acid which can perfectly dissolve limestones, (common limestone, dolomite and marble) for the caves creation.
Anyway carbonic acid is the most responsible factor for the caves look. It is mostly responsible for the "decorations," such as stalactites, stalagmites and flowstones that are formed which make caverns so delightful to look at. The solution of carbonic acid and limestone (calcium carbonate) forms calcium bicarbonate which when reaching the cavern interior can deposit calcite (calcium carbonate) that makes the formations within the cavern, and can even entirely fill the cavern.
St. Michael’s Cave
Gibraltar Rock is made of limestone so it’is not surprising that it contains a number of caves. St. Michael's Cave, located halfway up the western slope of the Rock, is a popular tourist attraction with two main caves: Old Michael's Cave and New St Michael's Cave discovered in 1942.
During 1942, a tunnel was being driven into the largest chamber of the Old St Michael's Cave so that it could be used as a store with reasonable air ventilation. This broke into the system now known as New St Michael's Cave.
New ST Michaels' not only contains stalagmite formations rivalling those in splendour of Old St Michael's, but also has a lake some 30 metres long and up to 11 metres wide and 6 metres deep. One of its remarkable features is the size of the calcite ledges formed at the lake margin by deposition from the surface of calcium bicarbonate-saturated water.
At some period during the history of this cave, part of a stalagmite became to heavy on one side and fell, possibly thousands of years ago. It now lies on its side at the far end of the main chamber, cemented through the years by nature to the floor of the cave.
In 1792 a slice 18” thick (45cm) was cut off from the top end. What remained was a cross-section which revealed the interior structure of the stalagmite in a most dramatic way. Within a diameter of approximately 4’6” (1.35m) can be seen the history of its growth. During periods of excessive rain its growth is clearly indicated by light-brown rings and patches. the darker areas were formed during periods of less rain.
But perhaps the two thin lines of crumbly white substance are the most interesting part of its structure. It is believed that these represent glacial periods. Besides the cross-section the stalagmite is also translucent in certain parts. This stalagmite which is centuries old enables visitors to see the unique beauty of crystallised nature.
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· What, and why, is the type of this cave?
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