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Research Topic: How long do caves take to form?

By Ian Mander, 27 July 2009, updated 12 June 2011, updated 19 January 2014.

Question: How quickly does limestone dissolve, and thus how long do caves take to form?

Answer: Waitomo limestone dissolves at about 70 cubic metres per square kilometre of karst per year. Nelson caves dissolve at 100 m³/km²/annum. The Nelson rate is the equivalent of a 10 cm depth being removed over the entire area during one thousand years. These are moderately high compared to current observed rates in other places. Central Kentucky has a rate of 59 m³/km²/annum.

Carbonic acid is formed when carbon dioxide dissolves in water. There is actually not much CO₂ in the air (about 0.04% by volume, or 400 parts per million) and most of it does not dissolve. Even so, enough will dissolve to give water a sour taste. This will drop pH from a neutral 7 to 5.65.

This principle is used in disappearing ink, which uses an indicator (normally thymolphthalein) which changes colour from blue to colourless when an alkaline solution becomes a little more acidic because of the dissolving CO₂.

More CO₂ is found in the soil (up to 10% by volume), and water filtering down to cave systems through the soil becomes much more acidic than rain water normally is. This would reduce pH to as little as 4.42.

Even at this rate it would take many thousands of years for some caves to form, but that wouldn't be directly answering the question, which is in the present tense. So: How long did caves take to form?

The simplest answer is we don't know, because we weren't around to watch. It's possible that they took no longer than a few thousand years, but there is a huge amount more work that needs to be done on studying how caves form. It is believed the carbonic acid mechanism is the main way in which caves are presently made, but most carbonic acid from ground water only dissolves limestone in the top ten metres of limestone. Some large caves are dry enough to also show evidence that stronger acids like sulfuric acid were involved in their formation. (Large wet caves have of course had any such evidence washed away.) This makes me wonder if Waitomo and Rotorua were in the same place rather than on opposite sides of the island, how big our caves would be then.

Reference

Source: www.icr.org/index.php?module=articles&action=view&ID=161

The amount of carbon dioxide (CO₂ ) in solution is probably the single most important factor affecting solution because carbon dioxide combines with water to produce carbonic acid (H₂CO₃). The air, which normally has a pressure of 1 atmosphere, has a partial pressure of only 0.0003 atmosphere of CO₂. Rain water in equilibrium with air can dissolve very little calcite. Water containing oxygen and decaying organic material, however, can possess 0.1 atmosphere of CO₂ (over 300 times more CO₂ than normal rain water) and is able to dissolve a lot of calcite. ... A swiftly moving, turbulent flow promotes washing of the limestone walls of its conduit, and is, also, more effective at dissolving calcite.

...

Chemical analyses of the area's groundwater by Thrailkill12 indicate that mean calcium ion concentration is 49.0 milligram per liter and the mean magnesium ion is 9.7 milligram per liter. Because rain water has only trace amounts of calcium and magnesium, essentially all of the dissolved calcium and magnesium in the groundwater must come from solution of calcite and dolomite. By simple chemical calculation it can be shown that these concentrations represent 0.16 gram of dissolved calcite and dolomite per liter of groundwater.

...

... if the dissolving power of the acid in one square kilometer of central Kentucky is carried in one conduit, a cave 1 meter square and 59 meters long could form in a year!

Source: www.teara.govt.nz/en/limestone-country/page-3 (formerly www.teara.govt.nz/TheBush/Landscapes/LimestoneCountry/3/en)

Dissolving rock

The rate at which limestone dissolves depends on the amount of rainfall and the concentration of carbon dioxide in the water.

Carbon dioxide concentration

In the open atmosphere, carbon dioxide has a concentration of about 0.03% by volume. However, in gaps in the soil, concentrations are often 2%, and can even reach 10%. As rainwater runs through soil, its carbon dioxide content increases. A hundredfold increase in carbon dioxide concentration means that limestone will dissolve about five times as fast.

Rainfall

In terms of rates of weathering, the amount of rainfall is even more significant than carbon dioxide concentration. The wettest places in the world have the fastest rate of karstification (formation of limestone caves and other features). As New Zealand is relatively wet, there is plenty of water to dissolve karst rocks.

• At Waitomo (average annual rainfall of 2,350 millimetres), the limestone dissolves at an annual rate of about 70 cubic metres per square kilometre of outcrop.

• On Takaka Hill (average annual rainfall about 2,160 millimetres), the marble dissolves at an annual rate approaching 100 cubic metres per square kilometre of outcrop.

These rates are moderately high by world standards.

The top 10 metres

Most dissolving of limestone happens just beneath the soil. This is where carbon dioxide is generated by soil microbes, so percolating water has its highest level of carbon dioxide. Some 90% of dissolving can occur in the top 10 metres or so of the limestone outcrop. The heavily corroded rock layer beneath the soil is known as the epikarst.

Fissures are widest near the surface, and taper with depth, usually reaching about 10 metres. Rainwater drains in much more easily than it drains out, and after heavy rain, water accumulates at the base of the epikarst. This water can remain there for months, eroding the rock.

Source: en.wikipedia.org/wiki/Carbonic_acid#pH_and_composition_of_a_carbonic_acid_solution

At a given temperature, the composition of a pure carbonic acid solution (or of a pure CO₂ solution) is completely determined by the partial pressure PCO₂ of carbon dioxide above the solution. ...

PCO2 (atm)	pH	[CO2] (mol/L)	[H2CO3] (mol/L)	[HCO3-] (mol/L)	[CO32-] (mol/L)
10-8	7.00	3.36 x 10-10	5.71 x 10-13	1.42 x 10-9	7.90 x 10-13
10-6	6.81	3.36 x 10-8	5.71 x 10-11	9.16 x 10-8	3.30 x 10-11
10-4	5.92	3.36 x 10-6	5.71 x 10-9	1.19 x 10-6	5.57 x 10-11
3.5 x 10-4	5.65	1.18 x 10-5	2.00 x 10-8	2.23 x 10-6	5.60 x 10-11
10-3	5.42	3.36 x 10-5	5.71 x 10-8	3.78 x 10-6	5.61 x 10-11
10-2	4.92	3.36 x 10-4	5.71 x 10-7	1.19 x 10-5	5.61 x 10-11
10-1	4.42	3.36 x 10-3	5.71 x 10-6	3.78 x 10-5	5.61 x 10-11
1	3.92	3.36 x 10-2	5.71 x 10-5	1.20 x 10-4	5.61 x 10-11
2.5	3.72	8.40 x 10-2	1.43 x 10-4	1.89 x 10-4	5.61 x 10-11
10	3.42	0.336	5.71 x 10-4	3.78 x 10-4	5.61 x 10-11

• ...
• For normal atmospheric conditions (PCO₂ = 3.5 x 10^-4 atm), we get a slightly acid solution (pH = 5.7) and the dissolved carbon is now essentially in the CO₂ form. From this pressure on, [OH^-] becomes also negligible so that the ionized part of the solution is now an equimolar mixture of H⁺ and HCO3^-.
• For a CO2 pressure typical of the one in soda drink bottles (PCO₂ ~ 2.5 atm), we get a relatively acid medium (pH = 3.7) with a high concentration of dissolved CO₂. These features contribute to the sour and sparkling taste of these drinks.
• ...

Source: www.answersingenesis.org/tj/v12/i3/cave.asp

It appears that sulfuric acid has been primarily responsible for the excavation of at least 10% of the caves in the Guadaloupe Mountains of southeastern New Mexico and west Texas. This is especially the case for the larger caves, such as Carlsbad Cavern and Lechuguilla Cave.

...

... cave formation, in at least some cases, was much more rapid, since sulfuric acid is much stronger than carbonic acid. Sulfuric acid dissolution is not only postulated for the caves in the Guadaloupe Mountains, but it is thought that 10 % of known major caves worldwide were carved out by sulfuric acid.

...

It is possible that many more than the postulated 10 % of caves worldwide were formed by sulfuric acid dissolution, because these types of caves are recognised in dry areas where some of the dissolution products remain in the cave. However, in humid climates, the reactants may have been washed out of the cave. So, it is difficult to know whether a cave in a humid climate was excavated by sulfuric acid.

Source: creation.com/caves-for-all-seasons

We must first emphasize that despite much effort, secular science (karstology in this case) still does not have an acceptable explanation of how water can manage to form large caves hundreds or thousands of feet underground. To eat away the limestone, the water must be acidic. How does it get deep inside the rock without losing its acidity? Thousands of measurements show that by dissolving limestone, the water loses its acidity within some 10 metres of the surface. It is only possible for water to flow deep underground if it follows pre-existing conduits.

...

However, where growth rate has been measured in show caves (which are not necessarily the fastest growing), it varies from 0.1 to 3 mm (four thousandths to 1/8 of an inch) each year. Thus, to grow 2 m (7 ft), a stalagmite could take anywhere from 700 to 20,000 years.

...

Returning to our generic example: if a 2 m stalagmite were 200,000 years old, its annual growth rate must have averaged 0.01 mm per year. This is ten times slower than the slowest measured today! Long-agers try to explain this by saying that the growth occasionally stopped completely, perhaps for 10,000 years at a time. And after 10,000 years, they assume that nothing changed — the water drops start arriving again at exactly the same point, with millimetre precision, to fall on the tip of the stalagmite!

Such explanations require common sense to take a nap. Thorough investigation shows that the path followed by water from the surface to the dripping point of a stalagmite is long, winding and extremely sensitive to the slightest change (remember, chemistry is involved, too). Moreover, huge amounts of field data reveal that karstland surfaces change dramatically and quickly, in a matter of centuries.

Source: creation.com/the-riddle-of-paleokarst-solved

Naturally, corrosion by H₂SO₄ is much faster than classical CO₂-driven corrosion. The general chemical reactions are:

H₂S + 2O₂ → H₂SO₄ (1)
CaCO₃ + H₂SO₄ + H₂O → CaSO₄•H₂O (gypsum) + H⁺ + HCO₃^– (2)

It has been calculated that the amount of H₂S required to generate Carlsbad Cavern’s Big Room (in excess of 10⁶ m³) is less than 10% of one year’s commercial production from the nearby gas fields in New Mexico. No one has calculated the actual increase in the rate of limestone dissolution by H₂SO₄, but it is generally believed to be much higher than for CO₂. Furthermore, Bakalowicz pointed out that the aggressiveness of sulfuric acid solutions can be further increased by CO₂ generated in the limestone as follows:

CaCO₃ + 2H⁺ → Ca²⁺ + CO₂ + H₂O (3)

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