Early Cave Development
Created | Updated May 1, 2002
Caves
A cave is defined as a cavity large enough to admit a human being.
Caves are cavities typically formed in limestone, a sedimentary rock formed from carbonate minerals, most commonly calcium carbonate, but limestones with varying levels of magnesium carbonate (dolomite) are also found. The caves are formed principally by the chemical action of water dissolving the limestone, which is very slightly soluble.
Some caves can be formed in other types of rock by non-chemical action - some sea caves may be formed by the action of waves on insoluble rocks, particularly exploiting zones of mechanical weakness, though such caves tend to be of small length. Lava caves (lava tubes) are formed by the roofing-over of a flowing lava stream with a crust of air-cooled lava. The subsequent draining of molten rock from the now-covered channel, can leave a tube-like cavity behind. Such caves can be quite long (up to several kilometres), but very linear, and close to the surface. Once formed, little change happens, beyond the odd roof collapse.
Sediment Deposition
The biologically or physically concentrated calcium carbonate which forms the bulk of limestone rock builds up over geological timescales as a sediment under marine or fresh water, usually shallow and clear.
When significant carbonate deposition is occurring, other material washed in to the area of sedimentation can become incorporated in the future rock, leading to limestone of a different colour or texture.
During phases of minimal or zero carbonate deposition, or when the influx of material is rapid, other materials may be washed in and deposited on the accumulating sediment to eventually form distinct layers of non-carbonate material (such as shale or clay) in the future rock.
Distinguishable layers of rock that were deposited succesively over time are referred to as beds, and the junction between two beds is known as a bedding-plane.
Rock formation
Eventually, if the sediment becomes buried sufficiently deeply, the resulting pressure and temperature results in the formation of a solid limestone. The thickness of the individual beds within the limestone can vary from millimetres or centimetres up to several metres, and the resulting limestones are referred to as finely or massively bedded, respectively.
Subsequent geological processes can lift the rock to heights of hundreds or thousands of metres above sea-level. In the process, the rock may become stretched, thickened, folded, faulted, or tilted compared to the original horizontal orientation of the beds. In addition to the natural bed-structure, as a result of the upheaval procedure limestone often contains naturally occuring fractures, known as joints which are broadly planar, and roughly at right angles to the beds. (A fault is essentially a crack in the rock where the rock on one side of the crack has observably moved relative to the other side, whereas a joint is a crack where there is minimal or no movement.)
Carbon dioxide and limestone solubility
Acidity
The capacity of water to dissolve limestone depends on the acidity of the water, in which dissolved carbon dioxide plays a crucial part - the acidity increasing with the CO2 concentration. Rainwater containing low levels of CO2 dissolved from the atmosphere is a very weak solvent for carbonate rocks, hence cave development is generally a slow process. Due to biological action, the level of CO2 in soil is very much higher than in the atmosphere, and therefore water which has drained through soil may contain much more dissolved CO2 than rainwater, and thus dissolve limestone more effectively. In addition, other processes in soil may add different acids to the water draining through it, so the nature of the landscape above caves can have great influence on the speed of solution.
Stalactites
The dependence of limestone solubility on CO2 levels also helps to explain the formation of the well known cave deposits called stalactites (which hang from the ceiling) and stalagmites (which grow on the ground), and other carbonate deposits inside caves. These deposits are commonly formed from water seeping through small fissures, which has moved sufficiently slowly to allow it to become saturated with as much dissolved limestone as it can carry, given its level of dissolved CO2. When this saturated solution enters the cave passage, CO2 can leave solution, and (if the humidity is below 100%) water can evaporate, leaving the solution supersaturated with limestone which is then deposited. Since little or no CO2 loss would occur if the concentration in the seepage water was in equilibrium with that in cave air, it is generally water which has collected an increased CO2 load by passing through soil that will result in the growth of cave formations. This helps explain why many cave systems formed in mountain areas with limited and very thin soil cover often contain few formations compared to lowland caves.
Cave passage development
Initial channel formation
In the descriptions below, for the sake of simplicity, it is assumed that the beds of limestone are broadly level, and the joints are therefore relatively vertical.
The initial development process of a cave is extremely slow, up to millions, or tens of millions of years, and involves the very gradual dissolution of limestone by water either existing within the body of the rock from the time of its formation, or percolating very slowly through the rock. This dissolution often occurs preferentially along joints and bedding planes, and faults if present. If sulphur-containing minerals such as pyrite are present within the rock, as may be the case in bands of shale incorporated betwen limestone beds, any sulphuric acid formed by their oxidation can greatly assist this initial dissolution process.
Over time, a connected network of small channels may be formed in the rock. If, as a result of gradual erosion or glacial action, the surface relief changes such that a point of this network becomes exposed, water from places in the network above that point can drain out. If the network extends upwards to one or more points where streamwater or groundwater can enter the network at a higher altitude, a significant flow of water can start, and much more rapid solution can begin. The point where the water drains out onto the surface is known as a spring or resurgence.
Water table
The underground boundary dividing the saturated zone, where all pores and channels in the rock are filled with water, and the drier zone above is known as the water table. Initially all channels are formed below this boundary, are therefore flooded, and hence rock is dissolved from the roof, sides and bottom of the channel, which therefore naturally tend to develop a rounded cross-section.
In the case of joint-formed channels, it is common for the passage to become stretched vertically to some extent into a more oval form, either as a result of the original channel being itself flattened in the plane of the joint, or as other parallel small channels formed earlier in the same joint become incorporated into the main channel as it expands. In some cases the 'ends' of the oval are simply rounded, but in others, there is a noticable 'peaking' at the site of the joint itself.
The same effects can occur in the growth of bedding-plane channels, with the additional possibility that a band of physically weak material, such as a clay or shale layer, may exist between beds of limestone, and can easily be removed by the mechanical action of flowing water. This can cause a great sideways extension of the channel cross-section, leaving a conduit that could be centimetres high, but metres wide.
Another possibility in bedding-controlled channels is that the bed of rock on one side of the bedding plane may be significantly less soluble than the bed on the other side so the developing circular or oval pasaageway may be relatively flattened above or below. In the extreme case where a passage develops along the junction between a bed of limestone and a bed of effectively insoluble rock, the developing passage can be completely uneroded on the non-limestone side.
Subsequent passage development
Once a flow through the channel network has been established, the drainage of water from the rock can lower the water table such that channels above the level of the resurgence may become partially air filled. However, depending on the layout of channels, the level of the water table may be different at different parts of the network.
As a cave system develops, the water table can change further. Usually, this involves a lowering of water levels, as outside valleys deepen and uncover lower resurgences, or internal development of lower-level drainage networks.
However, it is possible, albeit much less likely, that passages may be blocked by material washed in to the cave system, or that resurgences may be blocked by movements of surface material, so local or global rises in the internal water tables can happen.
Vadose and phreatic passages
The zones of rock above and below the water table are known as the vadose and phreatic zones respectively, and the passages formed there are named similarly. As mentioned above, all passages are initially phreatic, but changes in the water table over time may leave some of them partially air-filled channels, and vadose development can begin.
Since there is no flowing water in contact with the upper parts of vadose passages, solution only affects the lower portion, the passage develops by downcutting, and commonly develops a canyon profile. Traces of the phreatic origins may remain visible at the top of the passage as a roughly semicircular cap (a phreatic half-tube) above the canyon if downcutting took place over most of the width of the initial phreatic passage, or the bulk of the initial rounded passage may remain above a narrower canyon, giving a classic keyhole profile. In the case where the initial passage was a relatively flat bedding-plane, the resulting passage may either be a flat-roofed canyon, or a passage with a T-shaped cross-section.
Roof collapse
In wide passageways, solution caused by water seeping through bedding planes and joints in the roof can cause weaknesses to develop to the point where collapse of boulders can occur, sometimes leaving the roof rather irregular, but other times giving a new flat roof in place of an old rounded or irregular one. This latter case can be explained by the post-collapse roof surface being part of a relatively thick, unfractured or otherwise mechanically competent bed of rock, which is separated from the underlying (fallen) rock bed(s) by a layer of particularly weak or soluble material.
In some cases, the fallen debris still remains as a boulder layer, often with distinctly flat-sided boulders.
However, if the last collapse was a long time ago, and the passage carries a stream, it is possible for the boulders to be eradicated by slow solution or to be physically moved downstream in the case of flooding.
