No. 79 Origin of Limestone Caves

By Steven A. Austin, Ph.D.*

INTRODUCTION

A cave is a natural opening or cavity within the earth, generally

extending from the earth's surface to beyond the zone of light. Three

genetic classes of caves can be recognized according to the major

sculpturing process: (1) caves formed by pressure or flow, (2) caves

carved by erosion, and (3) caves dissolved by solu-don. Those structures

formed by mechanical pressure or flow include lava tun-nels associated

with volcanoes (e.g., Catacombs Cave in Lava Beds National Monument,

California) and "badland caves" excavated from poorly consolidated

rock by hydraulic pressure (e.g., small caves of the and Badlands of

South Dakota). The caves carved by erosion include shoreline grottos

created by the mechanical action of waves (e.g., La Jolla sea caves

near San Diego, California) and rock shelters cut by river meanders

(e.g., the massive sandstone alcoves of the famed cliff - dwelling

Pueblo Indians). The caves dissolved by solution include ice caves

associated with glaciers and the familiar limestone caverns or caves.

Limestone caves are, by far, the most common type of caves.

The great size and beauty of limestone caves have made them features of public

amazement and wonder. More than 130 caves in the United States are open

commercially, and at least 13 national parks and monuments contain caves.

The world's longest cave appears to be Kentucky's Mammoth Cave which has

more than 240 kilometers (150 miles) of accessible passages. The largest

subterranean chamber yet discovered is the Big Room of New Mexico's

Carlsbad Caverns. The Big Room is about 400 meters (1,312 feet) long, 200

meters (656 feet) wide, 90 meters (295 feet) high, and contains the Great

Dome, a stalagmite 19 meters (62 feet)tall. Gouffre BergerCave neargrenoble,

France, descends at least 1, I 00 meters (3,680 feet) below the surface and is the

deepest cave yet explored by man. Records of the National Speleological Soci-

ety of America indicate more than 1 1,000 caves in the United States, and it ap-

pears likely that I 00,000 caves exist in the whole earth.

Caves are of interest to the student of the Bible because the Bible lands

are rich in limestone caves. In Old Testament times caves often served

as refuge or emergency shelter (Genesis 19:30; 1 Samuel 22:], 24:1-8;

Hebrews 1 1:37-40). Caves were also used as places of burial

(Genesis 23:17; John 1 1:38). After the great confrontation between Elijah

and the prophets of Baal, Elijah lodged in a cave and received the Word

of the Lord there (I Kings 19:9 -1 1). Psalms 57 and 142 were

composed in a cave by David after he fled from Saul. The famed

"Dead Sea Scrolls" were discovered in caves.

A great deal of scientific interest has been generated by caves.

Speleology is a multi-disciplinary science which deals with the

cave environment: cave discovery, exploration, surveying, archaeology,

zoology, botany, paleontology, meterology, and geology. Mineralogists

and gem collecters know that caves contain many large and perfect

crystals. Paleontologists have found fossils in caves which shed

light on the history of man (e.g., Neanderthal man). Geologists

have attempted to answer several theoretical and practical questions

posed by caves. One of the most difficult problems has been to

interpret the history of limestone caves in relation to the

Biblical framework for earth history.

SOLUTION OF LIMESTONE

Solution cave chemistry can be simply stated: limestone and dolostone,

the host rocks for most caves, are dissolved by natural acids (carbonic,

sulfuric, and various organic acids) which occur in groundwater. Calcite

(CaCO3), the principal mineral comprising limestone, is dissolved in

the presence of acid to produce calcium ion (Ca '-* ) and bicarbonate

ion (HCO3 ). Dolomite [ CaMg (CO3)2 1, the most important mineral

in dolostone, is dissolved by acid to produce calcium ion (Ca

magnesium ion (Mg **), and bicarbonate ions (HCO3 )-If the acid

is able to flow through the rock, ions will be removed and a

cavity or solution conduit will form.

That many limestone caves formed by the solution process is indicated by four

types of geologic evidence.

1. Modem limestone cases often show evidence of ongoing solution-the

groundwater leaving a cave often has a higher concentration of calcium and

bicarbonate ions than the water entering the cave. I Dripstone deposits

on the interior of caves prove that solution occurs above the cave.

2. The shapes of bedrock structures in limestone caves oflen resemble those

produced in solution experiments. For example, the shapes produced at inter-

-sections of joints in cave bedrock can be predicted based on the theory of solu-

tion kinetics.2

3. The passages in limestone caves usually followjoints, fractures, and the

level of the land surface in such a way as to suggest that the permeability of the

bedrock has influenced the position of cave passages. Maps and cross - sections

of caves often show the regular spacing and orientation of passageways caused

by joints.3

4. Caves resemibling those found in limestone and dolostone do not

occur in insoluble, nori-carbonate rocks. The apparent causal relationship

implies that some characteristic of the rock (i.e., solubility) has affected

the occurrence of the caves.

SOLUTION THEORIES

Thdt solution is a major factor in the formation of limestone caves

appears to be well substantiated. The hydrologic conditions and sequence

of events leading to cave formation, however, are poorly understood by

geologists. The Encyclopedia Americana begins its discussion of the

origin of solution caves with the following admission:

The origin of solution caves in limestone and related rocks is com-

plex, and scientists are not in full accord as to the exact sequence

of events that lead to the formation of such caves.4

The problem is that we are attempting to understand the origin of d cavity for

which the evidence of the events forming it has been largely dissolved.

Two basic types of theories concern the wilter conditions when the cave formed.

These are the vadose and phredlic theorieS.5 The vadose theory suggests that

solution of the cavity occurred while the limestone was above the level of

groundwater (water table) and that the cavity was largely filled with air. The

phreatic theory claims that the cavity formed when it was below the level of

groundwater when it was completely filled with water.

NON-SOLUTION PROCESSES

Althougli solution was a major process in the formation of limestone caves,

some major problems are encountered if these caves are considered to have

formed only by solution. The first problem is the origin of the original

fracture porosity along which circulation of acidic groundwater could

be initiated. The original hairline fractures in the limestone would not

transmit water, and, therefore, solution conduits would not he expected

to form. Davis6 suggested that groundwater flow could be initiated by

tectonic stresses on the rock which opened fractures and created the

driving force for fluid flow. The process of "piping" (the production of

underground conduits by removal of fine particles by water driven by

pressure through poorly consolidated material) may also be important

in producing fracture porosity. "Badlands caves" in shaly rock.,; form by

this process.

A second problem for solution theory is the evidence of erosion and abrasion in

limestone caves. Many caves contain large amounts of clay, gravel, cobbles, and

boulders which could not have been dissolved from the limestone. Instead, the

cave -filling material appears to have been transported by moving water from a

sediment source outside the cave. These cave deposits show that some caves at

one time were essentially "underground rivers," and, as such, could have ex-

perienced abrasion and erosion such as occurs in modern channels. The

amount of material removed from caves by this process, however, appears to be

small compared to that removed by solution.

FACTORS AFFECTING SOLUTION

The solubility of calcite and dolomite, and the rate at which solution

occurs, are dependent on at least eight factors: amount of carbon dioxide

in solution, pH, oxidation of organic matter, temperature, pressure,

concentration of added salts, rate of solution flow, and degree of

solution mixing. Calcite is more soluble if carbon dioxide is increased,

acidity is increased, oxygendnd orgdnic matter are increased, temperature

is decreased, pressure is increased, concentration of salts is increased,

rate of flow is increased, and degree of mixing is increased. 7

The amount of carbon dioxide (CO2) in solution is probably the single most im-

portant factor affecting solution because carbon dioxide combines with water to

produce carbonic acid (H2C03). The air, which normally has a pressure of I at-

mosphere, has a partial pressure of only 0.0003 atmosphere of Co 2- Rain water

in equilibrium with air can dissolve very little calcite. Water containing

oxygen and decaying organic material, however, can possess 0. I atmosphere

Of C02 (over 300 times more CO 2 than normal rain water) and is able to

dissolve a lot of calcite.8 it is possible to make undersaturated solutions

simply by mixing two types of water having different pressures of Co 2,

different salinities, or different temperatures. Undersaturation occurs

in the case Of CO 2 because a non - linear relationship exists between

the partial pressure of C02 and the solubility of calcite. A swiftly

moving, turbulent flow promotes washing of the limestone walls of its

conduit, and is, also, more effective at dissolving calcite. 9

RATES OF LIMESTONE SOLUTION

Because at least eight complex variables determine the rate of

solution of limestone, an estimate of solution rates based on the

theory of chemical thermodynamics and kinetics would be a monumental

task! A better way to estimate solution rates would be to go to

the cave environment, measure the various physical and chemical

parameters, and relate them to observed solution rates. Unfortunately,

the cave environment where solution may be occurring exists

deep in the earth, in total darkness, in passages which are completely flooded

with water. This environment is very inhospitable to man, and no data are

available.

Another way of attacking the problem is to study a large cave -containing area

where water chemistry and flow rates are known in order to estimate overall

rates. An excellent area for this type of study is the large limestone

and dolostone Sinkhole Plain -Mammoth Cave Upland region of central Kentucky.

The area is between Green River, Barren River, and Beaver Creek, and

comprises several hundred square kilometers. Although it receives

1 22 centimeters (48 inches) mean annual rainfall 10 and would naturally

have about 51 centimeters (20 inches) of average annual runoff,

the area has virtually no surface streams! The runoff is channeled

into sinkholes which distribute the water into a widespread

limestone and dolostone formation which is about I 00 meters (330 feet)

thick. Caves and solution conduits in the aquifer transport most

of the water northward where it discharges at springs into the Green River.

Chemical analyses of the area's groundwater by Thrailkill 12 indicate

that mean calcium ion concentration is 49.0 miiligram 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. 1 6 gram of

dissolved calcite ancl dolomite per liter of groundwater.

It is reasonable to assume that about 1.0 meter of the 1.22 meters

of mean annual rainfall go into the aquifer. Therefore, each square

kilometer (I million square meters) of central Kentucky receives about

I miilion cubic meters of infiltration each year (1,000,000 M2 X I M =

1,000,000 M3). Because a cubic meterof water contains I thousand liters,

I billion liters of water enter the ground through each square kilometer

of land surface each year.

The above data can be used to calculate the amount of calcite and dolomite

dissolved each year. This is done by multiplying the mass of minerals per liter

times the water infiltration rate (0. 1 6 g /1 x 1,000,000,000 I /yr =

160,000,000 g /yr). The answer is 160 million grams (I 76 tons) of dissolved

calcite and dolomite per year over each square kilometer of land surface. If the

mass of calcite and dolomite dissolved is divided by the density of the minerals,

thevolumeisobtained(160,000,000g/yr - 2,700,00og/M3 = 59 M3/yr).

Thus, if the dissolving power of the acid in one square kilometer of central

Kentucky is carried in one conduit, a cave I meter square and 59 meters long

could form in a year! 13

The high rate of solution oflimestone dnd dolostone should be a matter of alarm

to uniformitarian geologists. In 2 million years (the assumed duration of the

Pleistocene Epoch and the inferred age of many caves), a layer of limestone well

over I 00 meters thick could be completely dissolved off of Kentucky (assuming

present rates and conditions). Any reasonable estimate of the volume of

limestone actually removed by solution of Kentucky caves and karst would be

insignificant compared to that predicted by an evolutionary model.

The solution data are not at odds with a catastrophest interpretation of earth

history. The data of Thrailkill 14 show that the groundwater in central Kentucky is

actually undersaturated with respect to calcite and dolomite, and that the full

dissolving power of the acidic water is not being utilized in attacking the

limestone. Calcite is dissolved only to about 55 % of saturation and dolomite

only to about 14% of saturation. 15 Furthermore, climatic and geomorphic

evidence in Kentucky suggests that rates of groundwater flow and rates of so-

lution have not remained unchanged.The more humid, cooler climate of the

Pleistocene would have increasedgroundwater flow and increased rates of solu-

tion. It is also probable that the atmosphere had more C02- In the final

analysis there appears to be no major obstacle to a short time period

for the solution of limestone caves.

STALACTITES, STALAGMITES, AND FLOWSTONE

The formations which hang trom the ceiling of a cave are stalactites; those

built up above the floor of a cave are stalagmites; whereas those sheet

-like, layered deposits on the walls or floors are flowstone. A column

forms by the joining of a stalactite and a stalagmite. Together these cave

formations are known as speteothems.

The origin and age of speleothems is a controversial subject. A popular

theory for the origin of caves involves two stages. The first stage

was when the cavity was filled with water, and solution of limestone

occurred. The second stage was when the cavity or cavern was filled

with air, and deposition of speleothems began from solutions depositing

calcium carbonate. A less popular theory is that there was only one

stage in cave formation with solution occurring in the water - filled

part of the chamber concurrently with speleothem deposition in the air-

filled spaces.

Radiocarbon (C - 14) dating of speleothems has been used by some scientists to

support the great age of cave formations. However, dtteriipls to dote the car-

bonate minerals directly give deceptively old ages because carbon from

limestone with infinite radiocarbon age (cdrbori out of equilibrium with atnios-

pheric carbon) has been incorporated in minerals with dtinospheric carbon.

Most of the stalactites and stalagmites in modern caves are not growing,

dn(J it appears impossible to estimdte their former rate of growth.

The ones that are growing mdy be subject to extreme variation in growth

rate. ") Because the Pleistocene Epoch was a time of higher humidity

and rainfall than today, it is probable that more speleothems were

growing and that they were growing at faster rates than today. It

must be remembered that the rates of deposition of calcite are subject

to the same complex environmental factors which affect the rates of

solution of calcite (see ahove discussion). Therefore, some of the

great dges for speleothems claimed hy cave guides and "spelunkers" may be

significantly in error.

A large number of reports concern the rapid growth of stalactites and

stalagmites. 17 MoSt of these observations have been made in tunnels, bridges,

dams, mines, or other dated man-made structures which approximate cave

conditions. Fisher"' summarized some of the early literature where stalactite

growth averages about 1.25 centimeters (0.5 inch) yearly with some observed

to grow over 7.6 centimeters (3 inches) yearly. Stalagmites observed by Fisher

grew 0.6 centimeter (0.25 inch) in height and 0.9 centimeter (0.36 inch) in

diameter at the base each year. At this rate of height increase the 1,900 cen-

timeter tall stalagmite called "Great Dome" in Carlsbad Caverns might grow in

less than 4,000 years.

A ldrge stalagmite like Great Dome may contain I 00 million cubic centimeters

of calcite, which, if accumulated in 4,000 years, would require a deposition rate

of 25,000 cubic centimeters (67,000 grams) of calcite yearly. If the dripping

Wilter is assumed to deposit 0.5 gram of calcite per liter, 133,000 liters of water

would have to drip over the stalagmite each year. Because about 6,000 drops

comprise I liter, it would take about 800 million drops of water per year to form

the stalagmite. This works out to 25 drops of water per second, which is a con-

siderable flow. Whether a stalagmite would be deposited in the above hypo -

thetical situation is not known. One would want to carefully examine the

assumptions and the complex environmental factors which might affect stalag-

mite growth.

In addition to the observations of speleothem growth in cave or cave-like

natural environments, some interesting experiments have been performed

to simulate stalactite and stalagmite growth in controlled laboratory

situations. Williams and Herdkiotz 19 are studying the effects of

acidity, salinity, temperature, humidity, and other factors on rates

of stalactite growth in the laboratory. Their work applies to natural

cave environments, and indicates that stalactites can form very rapidly.

A MODEL FOR THE ORIGIN OF CAVES

Having examined the processes which can form limestone caves, we are now

ready to formulate a model which is consistent with the geologic data and in har-

mony with a Biblical framework for earth history. 10

Step 1 -Deposition and burial of limestone. The first step for the formation

of a cave is obviously to deposit the limestone. Most major limestone

strata appear to have accumulated during the Flood. After a lime

sediment layer (which later contained a cave) was deposited, it would have

been buried rapidly under perhaps several thousand feet of sediments. The

weight of overburden would compact the lime sediment and tend to expel

interstitial water. Although the fluid pressure would have been great

within the sediment, the lack of a direct escape route for the pore

water would impede water loss and prevent complete lithification. The

major means of water loss was probably through joints which formed during

the early stage of compaction while the sediment was only partially

consolidated,

Step 2-Deformation and erosion of limestone and overburden. As the

Flood waters receded, tectonic activity would deform the sediments and bevel

the upper layers down to a new level. The lime sediment layer would again be

near the surface. The tectonic forces would induce movement on joints and

build up fluid pressure, and the removal of overburden would make compaction

in and flow from partially consolidated sediments proceed at faster rates. The

pressure gradient would have been highest near the surface, causing sediment

to he removed by piping. As thejoints were opened, a conduitsystem for vertical

and horizontal flow would have been established.

Step 3-Horizontal groundwater drainage and solution of limestone. After

the Flood waters hdd completely receded, the regional groundwater level would

be in disequilibrium and horizontal flow would be significant. Acids from organic

decomposition at the surface and at depth would tend to move to just below the

water table where the highest horizontal velocities of flow would exist.

Solution of newly consolidated limestone would occur chiefly in

horizontal conduits at a level just below the water table. The mixing

of vadose water (CO2 rich, oxygen rich, organic poor, and low salinity) with

phreatic water (CO2 poor, oxygen poor, organic rich, and high salinity)

would also produce conditions ideal for solution of limestone near the

wilter table. As a result, a cave system would be developed at a certain

level.

Step 4-Deposition of speleothems. After the groundwater drainage had

been largely accomplished and the caves dissolved out, the water table would be

at a lower level and caves would be filled with air, not water. Thus, the final step

would be the rapid deposition of stalactites, stalagmites, and flowstone.

CONCLUSION

Caves are among the most fascinating structures in the earth's crust. The proc-

esses which removed material from caves in principle are rather simple, but they

were manifest geologically in response to many environmental factors. Deposi-

tion in caves was also complex. Although there is much in caves to challenge

further study, it appears that they can be interpreted within the basic framework

of earth history presented in Scripture.

REFERENCES

I . Thrailkill, J.V., "CdFboriate Chemistry of Aquifer and Stream Water inkentucky," Journal

of Hydrology, V. 16, 1972, pp. 93 104.

2. Lange, A.L., "Caves and Cave Systems," in Encyclopaedia Britannica, 15th ed., Chicago:

Encyclopaedia Britannica, Inc., V. 3, 1977, p. 1026.

3. Moore, G.W., "Limestone Caves," in R.W. Fairbridge (ed.) The Encyclopedia of

Geomorphology, New York: Reinhold Book Co., 1968, pp. 652 - 653; Davies, W.E.,

"Caverns in West Virginia," West Virginia Geol. Survey, Bull. I 9A, 1958, p. 330;

Thornbury, W.D., Principles of Geomorphology, New York: John Wiley, 2nd ed., 1959,

pp.324-331.

4. Davies, W.E., "Cave," in Encyclopedia Americana, New York: Americana Corp., V. 6,

1969,p.101.

5. Further details of these theories can be found in Thornbury, Principles of Geomorpholqqy,

pp. 324- 331 and in Ford, D.C., "The Origin of Limestone Caverns: A Model from the

Central Mendip Hills, England," Nat. Speleological Soc. Bull., V. 27, 1965, pp. 109 -132.

6. Davis, S.N., "Initiation of Ground Water Flow in Jointed Limestone," Nat. Speleolo_qical

Soc. Bull., V. 28, 1966, pp. II I - 1 18.

7. ThFailkill, J.V., "Chemical and Hydrologic Factors in the Excavation of Limestone Caves,"

Geological Soc. Amer. Bull., V. 79, 1968, pp. 19-46; Sipple, R.F. and Glover, E.D.,

"Solution Alternation of Carbonate Rocks: The Effects of Temperature and Pressure,"

Geochimica Cosmochimica Acta, V. 28, 1964, pp. 1401 - 1417.

8.

9. Moore, Encyclopedia of Geomorphology, pp. 652 - 653.

Kaye, C.A., "The Effect of Solvent Motion on Limestone Solution," Jour. Geology, V. 65,

1957,pp.35-46.

10. U.S. Geological Survey, The National Atlas of the United States of America, Washington:

Dept. of Interior, 1 970, p. 97.

11.

12. Ibid., p. I 1 9.

Thrailkill, Journal of Hydrology, p. 98. Analyses represent the mean of seven water

samples from Mill Hole.

13. For an estimate that a cave 3 by 6 by 1 20 feet could be dissolved in a single year per

square mile, see Swinnerton, A.C., "Origin of Limestone Caverns," Geological Soc. Amer.

Bull., V. 43, 1932, pp. 678 - 679.

14.

15.

16.

17. Thrailkill, Journal of Hydrology, p. 98.

Ibid., p. 98. Saturation data represent the mean of seven water samples from Mill Hole.

Lange, Encyclopaedia Britannica, p. 1028.

For introduction to various works see several articles in Volumes 14 and 1 5 of the

Creation Research Society Quarterly, 1977 and 1978, especially Helmick, L.S., et a].,

"Rapid Growth of Dripstone Observed," V. 14, 1977, pp. 13-17.

18.

19. Fisher, L.W., "Growth of Stalactites," American Mineralogist, V. 19, 1934, pp. 429 - 43 1.

Williams, E.L., and Herdkiotz, R.J., "Solution and Deposition of Calcium Carbonate in a

Laboratory Situation," Creation Research Society Quarterly, V. 13, 1977, pp. 192 - 199,

V. 15, 1978, pp. 88 - 9 1.

20. Williams and Herdklotz (ibid. pp. 197-198.) have suggested a similar model. Their model

cdn supplement ideas presented here.