Limestone Caves: Formation and Ages
Limestone caves are places of wonder and mystery. When meeting these phenomena,
one naturally wonders about their formation processes and the times involved. Limestone caves are also the most common caves. [1] They
exist in various places globally including under oceans.
How did ancient limestones form?
Limestone accounts for roughly 10% of all
sedimentary rocks by volume and “forms either by inorganic means or as a result of biochemical processes.” [2][3] Two theories on
the formation of sedimentary limestone are:
Old Age View
“[B]eginning about 300 million years ago, marine animal living in shallow salt
seas died and sank to the sea floor. Over tens of millions of years, their hard-shelled bodies built up hundreds of feet of limestone.[4]
Lime sediments in this view were generally native (autochthonous) to the location they were found.
Young Age View
A global flood
lasting one year occurred about 4,500 years ago. This catastrophic flood “tore apart the earth and stacked miles of sediment across
continents” [5]
Lime sediments in this view were sorted and moved in from other locations (allochthonous).
The age
of limestone caves is inherently constrained by the age of the host rock formations in which they are found.
How were limestone caves
formed?
Limestone caves are formed through the action of acidic water traveling along fractures in limestone, dissolving calcite and
creating cavities that progressively increase in size over time. In the past, it was conventionally assumed that limestone caves were
formed only from acidic waters moving downward from surface sources. Then, a change in basic assumptions began based on research presented
by speleologists showing that certain caves formed from waters ascending from within the earth unconnected to surface sources. The
downward moving water is called epigenic and the upward moving water is called hypogene.
Epigenic water comes from precipitation like
rain, snow, and sleet, and forms weak carbonic acid by absorbing carbon dioxide from the air and decaying matter. [6] Regions characterized
by high rainfall typically experienced increased cave formation, as the abundance of moisture enhances the infiltration of acidic
water into limestone formations. [7]
Hypogene water comes from deep sedimentary basins and other deep sources and ascends through fault
zones within the earth. It commonly has sulfur (as sulfate or sulfide), carbon (as carbon dioxide and bicarbonate), nitrogen (as ammonium)
and other chemical components. [8] Hypogene water holds elements that can form acids like sulfuric, carbonic, nitric, and hydrochloric,
raising overall acidity when combined in solution.
“When sulfur compounds are present in water or in the minerals in the cave walls,
chemical-loving bacteria use the sulfate as an energy source, producing hydrogen sulfide as a by-product. Oxidation of this hydrogen
sulfide then forms sulfuric acid.” [9] Sulfuric acid may originate from hydrothermal springs or rock minerals. [10] It is highly acidic,
with a pH between 2.75 at 1 mM and 1.01 at 100 mM. [11]
‘Since methane is much more common than H2S in the subsurface, it may play
a much larger role than the former in corroding large subterraneous cavities. There is one problem with methane; its oxidation does
not leave a “smoking gun.”’ [12] Methane oxidation: CH4 + O2 à 4H+ + CO2 [13]
When sulfuric acid (H2SO4) reacts
with limestone (CaCO3), the chemical reaction can be represented as follows: CaCO3 + H2SO4 à CaSO4 + H2O +CO2. This CO2 gas “then
can dissolve in water again and increase its aggressiveness even more.” [14] The above (CaSO4 + H2O) + H2O will form replacement gypsum
CaSO4 ·2H2O.
Some factors affecting limestone cave formation rates
The rates at which limestone caves dissolve depend on factors
like acid types and concentrations, pH, temperature, pressure, salt types and concentrations, rate of solution flow, and degree of
mixing. [15][16]
· The types of acids in solution influence dissolution rates: sulfuric acid, being strong,
dissolves limestone rapidly, while weak carbonic acid acts much more slowly. Limestone solubility is influenced by the concentration
of hydrogen (H+) ions present in the solution. Sulfuric acid solutions with pH 2.75 and pH 1.01 have 178 and 9,772 times, respectively,
more H+ ions than a pH 5 carbonic acid solution for dissolving limestone. [17]
· “A study showed that
limestone dissolved 100 times faster in a pH 5 solution compared to one with a pH of 7.” [18]
· Acid concentration
strongly influences limestone dissolution rate: higher concentrations increase the rate.
· “[T]he general rule
of thumb in chemistry states that for many reactions, the rate approximately doubles with every 10o C rise in temperature - though
this can vary depending on the specific reaction and conditions.” [19]
· Salt type and concentration can significantly
influence solution pH. [20]
· The rate of acid recharge will significantly affect the rate of limestone dissolution.
· Water flow moves dissolved compounds away from limestone, enabling cave formation. “Fast-moving water removes
dissolved ions from the surface quickly, making room for fresh water to keep dissolution going.” [21]
· Circulation
is key factor in the formation of limestone caves. “If the water is stagnant, its ability to dissolve limestone is limited. Once saturation
is reached, dissolution basically stops, and caves will not form or grow.” [22]
Stream abrasion in caves, as on the surface, can play
a role in cave growth. Sand and gravel in the stream will further speed up the erosion of limestone. [23][a] The rate of mechanical
erosion in caves is “impossible to quantify but sometimes” has been “significant.” [24]
“Dissolution of carbonate rock” under certain
conditions “is extremely fast compared to normal epigenic caves and can cause the formation of sizeable cavities in probably only
a few thousand years.” [25]
Where are limestone caves formed?
“Whether this cave forming activity goes on above the water table, at
the water table, or at some distance beneath it is a question that is still being argued. Most investigators, however, believe that
caves are usually formed below the water table and exposed later on, when the water table in the area is lowered by downward-cutting
surface streams.” [26]
“Most caverns are created at or just below the water table in the zone of saturation. Here acidic groundwater
follows lines of weakness in the rock, such as joints and bedding planes. As time passes, the dissolving process slowly creates cavities
and gradually enlarges them into caverns. The material dissolved by groundwater is carried away and discharged into streams and transported
to the ocean.” [27]
“Ascending groundwater charged with carbon dioxide and, especially, hydrogen sulfide can readily dissolve carbonate
bedrock just below and above the water table.” [28] Hypogene caves do not require surface outlets to form.
What are some indications
of epigenic limestone cave formation?
The morphologies below often indicate epigenic limestone cave formation, although some caves
may originate from hypogenic processes and later be altered by epigenic ones.
· Formed by acidic meteoric water
close to the surface
· Well-organized drainage patterns [29]
· “[B]ranchwork patterns”
with “conduits converging as tributaries in downstream direction” [30]
· “[G]reat continuity in dissolution
along groundwater flow paths.” [31]
· Phreatic loop: a U-shaped underground conduit that extends some distance
from input to outlet sites [32]
· Part of regional drainage systems [33]
· “Smoothed
and polished walls” [34]
What are some indications of hypogenic limestone cave formation?
· “Large chambers interconnected (if at all) by narrow
passages, shafts, passages of rough cross-section” [35]
· Calcite popcorn [36]
· Cavernous
edging along hypogene conduits [37]
· Ceiling bell holes [38]
· Ceiling channels [36][37][38][39]
· Ceiling cupolas [35][36][37][38][39][40][41][42]
· Ceiling half tubes [37][41]
· Chimneys[36][38]
· Chimneys (blind) [36][43]
· “[C]ollapse shafts over large hypogenic
voids” [37]
· Complex mazes with “jagged lines and multiple directions point to acids under pressure, pushing
through all the weak points.” [39]
· Condensation domes [36]
· Convection cupolas [35][41]
· Current and bubble trails [38][40]
· Feeders [37][38][41][42]
· Gypsum deposits [36][40][42][43][44][45]
· Half wall tubes [38][41]
· Individual passages or basic networks of interconnected passages [37]
· Isolated chambers [36][37][46]
· Mammillaries [40]
· Maze 3D systems [36][46]
· “Maze-like water table caves” [36]
· Megascallops [36][43]
· Passage with discharging
slots along its path up to a blind end [36]
· Passages single and isolated or rudimentary networks of passages[37]
· Passage with abrupt variations in passage size and morphology [38][45]
· “[P]assage size diminishes rapidly in the down-flow direction” [45]
· Passages in the lateral directions that
end abruptly [38]
· “[P]assages tend to form in well-defined levels with roughly horizontal floors” [42]
· Replacement pockets [36][43]
· Rising wall channels [37][41]
· “[R]ising, steeply inclined
passages or shafts” [37]
· Solutional wall notch with flat roofs at old water table level [40][42][43]
· Solution pockets [38]
· Sulfuric karren [36][42]
· Sulfuric
rills [40][45]
· Trays [40]
· Wall niches [36][41][43]
Have
the original hypogenic sources of some limestone caves been hidden or removed?
According to two resources, the earlier hypogenic stage
of some caves have been hidden by later epigenic imprints. [47] [48] Some ways epigenic processes conceal earlier hypogenic limestone
cave features are by eroding or dissolving them with meteoric water, filling feeders with debris, and covering them with mineral coatings
or speleothems.
“In the Alps and Prealps (Lombardy), some ancient high mountain karst areas exhibit evidences of an early hypogene
origin, deeply modified and re-modeled by later epigenic processes.” [49]
When did the older limestone caves begin to form?
One young
earth hypothesis suggests that caves began forming during the yearlong biblical Flood.
“Thus the process of forming the world’s cave
systems would have commenced during the Flood catastrophe itself, reaching its climax at the close of the Flood and in the subsequent
immediate post-Flood period, as volcanic, magmatic, and tectonic activity waned, geologic conditions began to re-stabilize and the
catastrophic process rates of the Flood year waned.” [50]
The age of limestone caves is often estimated by examining current natural
phenomena including radioactivity and geological processes, although this approach depends on uniformitarian assumptions that cannot
be verified through past observation, except for recent events. Four of these methods are:
· Using radiometric
dating methods
· Dividing the current length of speleothems like stalactites by their current growth
rate.
· Dividing the current cave volumes by the current dissolution rates (volume dissolved per time).
· Dividing the total weight of materials removed to make the cave by the current unit weight removal rate (weight of calcite removed
per time).
The article “Can radioactivity be used as a clock?” addresses significant potential issues associated with radiometric dating.
To use radioactivity as a clock, all three of the following requirements must be met:
· The rate of decay must
be constant over the whole period under investigation.
· Initial ratio of daughter to its parent must be known.
· There must be no migration in or out of parent or daughter isotopes.
The RATE study [c] proved that sometime in the past the decay
rates for certain radioisotopes greatly accelerated. Without a constant rate of decay, no radioactive clock is possible.
For any of
the other three methods to be effective, all factors influencing the rate of cave formation must remain constant during the process.
Evidence exists that this is not the case such as significant climatic shifts in the past, including wet periods and ice ages, likely
had a major impact on epigenesis.
Stalactites are mineral formations resembling icicles that hang from cave ceilings, typically composed
of calcite deposited by dripping water. Assessing growth rates presents challenges, as stalactites show variations over time; their
ringed cross sections reveal distinct concentric layers like tree rings, highlighting intervals of interrupted growth. [51] Notably,
most stalactites in contemporary caves have ceased to grow. [52]
Conclusions
The formation of limestone caves may occur from epigenic
or hypogenic or combined epigenic and hypogenic acidic sources. Dissolution of limestone to form caves can potentially be tens of
thousands of times more rapid by sulfuric acid solutions from hot hypogenic sources than by carbonic acid solutions from cooler epigenic
sources. When any ancient cave formation began, it is impractical to decide with any certainty from natural phenomena.
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