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How does a phreatic cave passage form?

Updated: 4/27/2024
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Dlocklear01

Lvl 1
11y ago

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Caves, and other karst features, are produced because limestone is soluble in water containing dissolved carbon dioxide and organic acids. Rain picks up atmospheric carbon dioxide as it falls. On passing through the soil more carbon dioxide, from plant roots and decaying vegetable matter becomes dissolved in the water, along with complex organic acids called humic acids. This ground water is easily able to dissolve limestone.

phreatic conditions result in three dimensional solution either forming large dome-shaped chambers or complex three dimensional mazes. The rock may become like a Swiss cheese producing what is called spongework.

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With respect, you've summarised cave formation but forgotten to define "phreatic"!

A phreati passage is one formed by dissolution as above but withinthe saturation zone of the rock mass' i.e. entirely filled withwater underhydrostatic pressure.Because the passage is totally filled the rock is attacked on all its exposed surface,so creating characteristically circular or elliptical passage sections.

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11y ago
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AnswerBot

1w ago

A phreatic cave passage forms when groundwater containing carbonic acid dissolves limestone bedrock, creating a network of interconnected channels and chambers. As the water table drops, the passage becomes exposed, creating a dry cave system.

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"Virgin cave" or "virgin cave passage" refers to unexplored or untouched portions of a cave system that have never been visited or surveyed by humans. These areas may hold scientific importance for understanding cave geology, ecology, or for discovering new species.


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An interesting question: I don't know that anyone has studied the statistics! Sticking to caves in limestone - the most numerous by far - it's more accurate to consider the entire cross-section of the passage. Karst cave passages develop along joint and bedding-planes - also faults but leave them for now. Initially the water dissolving the limestone occupies the whole passage volume, from its start as a minute micro-conduit. In these "Phreatic" conditions erosion takes place place along roof, floor and walls at the same time. If the guiding conduit is along a joint, the passage becomes a tube of circular cross-section. If the guide is a bedding-plane, or the intersection of joint and bedding-plane, the passage becomes elliptical as the water spreads itself into the bedding-plane on each side of the conduit. In some circumstances under phreatic conditions a layer of sediment can protect the floor from dissolution, and the passage tends to develop upwards into the joint, giving a tapering or "pointed" roof. If the roof is an insoluble "aquiclude", phreatic passages are flat-roofed - the udnerside of the aquiclude. The water will also tend to spread out below the insoluble bed to form a network of small tubes, with one eventually becomg dominant. An example familiar to me is Blacknor Hole, in Portland, (Southern England), where the aquiclude is a tabular chert band about 100mm thick within the Jurassic-age Portland Formation of limestone. On the other hand, a thin bed of resistant micrite called the Porcellaneous Band influenced many caves in the NW Pennines (Northern England), particularly Gaping Gill, to give a series of flat-floored, semi-circular phreatic passages on its upper surface. If the cave's formative stream abandons phreatic passages relatively quickly their tubular cross-sections remain intact. If though the cave's outlet level falls, typically by down-cutting of the landscape holding the springs, the cave's stream level falls to leave air above it. This is called the Vadose Phase. Now the water can attack only the walls and floor, principally the floor, and a vadose Canyon of key-hole cross-section may for. The "stem" of the keyhole is the original cylindrical phreatic tube, called the passage's 'precursor'. The section may resemble a letter T, with an arched roof, if the precursor was a bedding-plane. Sometimes bedding-plane passages can be very wide. The largest in Porth yr Ogof (South Wales) is nearly 30m wide at its widest, but not much more than 2m high, with an unusually flat roof. In very old caves, the beds in the roofs of large passages and chambers may collapse in stages to develop a cantilevered arch. To go back to passages on faults (major tectonic fractures extending throughout the rocks, often from crust upwards). The way in which a fault forms shatters the rock on each side of the fault plane, giving ready access to the water. Such passages are marked by roofs of breccia and often obstructed by fallen angular boulders. So to summarise... there is a variety of passage forms, with little or no prevalence overall of one type over the other; and the actual shape and size of a cave passage is highly individual.