virtual cave maps
TRANSCRIPT
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VIRTUAL CAVE MAPS
Showerhead
Cave showerheads are a fairly rare formation generally only found in tropical caves.
They've been noted in Brazil, Mexico, Puerto Rico, and especially from Borneo. They
sprout from the ceiling at a seep site. The ones below are from a cave in Gunung Buda,
Borneo. It is in the form of a hollow cone, narrow above and broad below, not unlike a
bamboo Chinese hat. It measures approximately half a meter long and one meter in
diameter.
All surfaces of the Buda showerhead appear to be covered with fine calcite
botryoids (popcorn-like nodules), most likely the products of seep water issuing
through the spongy matrix of the speleothem. Presumably, the conical shape is due to
1) the downward flow of seep water under the influence of gravity, and 2) the
preferential deposition of calcite on the speleothem's outer surface, furthest from the
seep, where carbon dioxide partial pressure and humidity are at their lowest.
In Deer Cave, wihtin Gunung Mulu National Park, a showerhead previously
observed on British expeditions is the source of a spectacular falls (lower
right photo). This specimen clearly has a much larger volume of water issuing from it
than that observed at Buda. This showerhead likewise appears to be more elongated
and cylindrical in shape and features a robust lower rim that is broader than the
remainder of the formation.
Cave showerheads should not be confused with simple seeps that often occur in
conjunction with flowstone or drapery deposits and which may issue high volumes of
water following heavy rains.
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Moonmilk
Moonmilk is a white deposit formed of aggregates of very fine crystals of varying
composition. It is gooey and pasty when wet, with a texture like cream cheese. It is
crumbly and powdery when dry. Usually moonmilk is made of carbonate materials
(e.g., calcite, hydromagnesite, gypsum...).
Moonmilk is a very common cave deposit that probably precipitates from dripwater
entering the cave, but forming very fine crytals rather than the larger ones typical of
calcite deposits like flowstone. As can be seen in the photo below, moonmilk may take
on shapes somewhat like those of flowstone. Micro-organisms such as bacteria, algae,
and fungus have been found in moonmilk and may play a role in its formation, though
not all moonmilk deposits contain such organisms.
Bathtubs
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"Bathtubs" are unusual speleothems thus far known from only a few caves. The ones
here are from Snail Shell Cave, in the Gunung Mulu region of Borneo. Though taller
specimens grade into the more typical form of a stalagmite, such as the "kitchen sink"
variety in the upper photo, the broad, squat grouping of the lower photo bears the
undeniable aspect of a genuine bathtub.
Both of the specimens shown below are fed by seeps, as you might infer by the
falling water droplets in the upper photo and rippled pool surface in the lower. On any
particular day, one may find these seeps either rapidly dripping or perhaps gushing
with steady streams. The seep discharge seems to depend on whether it's merely
raining at the moment, or if there's a serious tropical downpour.
The pH of the seep water likewise fluctuates widely. On a drier day the seeps run
somewhat acidic, but on a wetter day, they are unusually alkaline. Such schizophrenic
water chemistry likely has something to do with the form of bathtubs, which seem as if
they can't make up their mind whether they're being built up or eaten away.
The temporal variations in the seep chemistry at Snail Shell Cave may be
analogous to similar spatial variations recognized in many caves. It is not uncommon
to find deep shafts carved by corrosive waters occurring side-by-side with banks of
speleothems deposited by supersaturated waters. This striking variation in neighboring
waters is attributed to differences in overlying plumbing. If waters are conducted to the
cave along open, gas-filled fissures, (in an "open-system") soil-derived carbon dioxide
is continuously absorbed by the water en route. This water may thus carry some
unreacted carbonic acid as it enters the cave, and so dissolve limestone or speleothems.
If, however, water completely fills its infeeder conduit (in a "closed system"), carbon
dioxide is not available to the solution beyond the soil zone. The initial carbon dioxide
will react completely with the conduit bedrock and most certainly arrive at the cave
atmosphere supersaturated, and ready to deposit speleothems.
It is clear that the conduits feeding "bathtubs" are generously proportioned, as they
consistently allow rapid through-flow of surface water. During "dryspells" the conduits
are not filled to capacity, and soil carbon dioxide is most likely drawn downward into
their depths, allowing open system dissolution within the conduit, and discharge of an
acidic solution at the cave seep. During heavy storms, however, the conduits may be
fully inundated by rainwater. The alkaline waters reaching the cave at these times are
consistent with closed system dissolution, in which the overlying plumbing is filled to
capacity.
The seeps overlying the bathtubs in the lower photo are, themselves, depositing
unusal ceiling features that are found in many caves in the Mulu region. An example of
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these "showerheads" may be seen in another alcove of the Virtual Cave.
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Deflected stalactites
Deflected stalactites are unusual stalactites that seem to defy gravity by forming in a
curve. Their formation is not completely understood, but is most likely due to strong
cave air flow. In some places this is quite evident, with groups of stals all curving in the
same direction, as in the bottom photo. But sometimes stals in one area will curve in a
variety of directions, and another contributing mechanism might be disruption of the
water flow down the outside of the stal by crusts or popcorn formed on the outside, as
in the top photo.
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Columns
Columns are formed by the unions of stalagmites and stalactites. As compound cave
formations, they include among their ranks the tallest free-standing speleothems in the
world. (Certain flowstone falls--sheets of calcite lining vertical shafts--are
undoubtedly taller, but rarely measured). The towering specimens of the upper left
photo, from Ogle Cave in Carlsbad Cavern National Park, New Mexico, USA, are
indeed impressive. These, however, are only about half as high as the 61-meter tall
column in Tham Sao Hin, a cave in Thailand. An image of this can be seen on the
Virtual Cave's page devoted to the world's largest speleothems.
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Deflected stalactites Deflected stalactites are unusual stalactites that seem to defy gravity by forming in a
curve. Their formation is not completely understood, but is most likely due to strong cave
air flow. In some places this is quite evident, with groups of stals all curving in the same
direction, as in the bottom photo. But sometimes stals in one area will curve in a variety
of directions, and another contributing mechanism might be disruption of the water flow
down the outside of the stal by crusts or popcorn formed on the outside, as in the top
photo.
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Trays
Trays are an unusual cave formation formed from clusters of popcorn which ends in a
flat-bottomed surface. They often have aragonite trees growing beneath them, as in
the photo below, taken in Lechuguilla Cave. Trays are generally calcite, but can also
be formed in gypsum.
The origin of trays is complex. They usually form on ceiling pendants, with aragonite
frostwork first forming along the edges where undersaturated water flows down the
pendant and evaporates. Water rising in the frostwork by capillary action causes the
frostwork to grown upwards and lateral, creating the flat-bottom surface. Over time,
the frostwork dissolves and is redeposited as calcite popcorn, due to the relative
differences in the solubility of calcite and aragonite.
Helictites
Helictites are contorted depositional speleothems which grow in any direction,
seemingly defying gravity. They occur in many forms from tiny filaments (as in the top
photo) to thick, antler-like forms (bottom photo). Most helictites are formed from
calcite.
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Helictites are formed by calcite-laden waters seeping through tiny pores in the rock.
Hydrostatic pressure forces a small amount of the solution out, carbon dioxide is lost,
and calcite is deposited. Growth continues through a tiny, central capillary chanel,
which the solution flows through via hydrostatic and capillary pressure to emerge and
deposit calcite at the tip.
Helictites are a very diverse group of speleothems, likely because different factors
influence them. There is a very rare category that forms underwater, best known from
Lechuguilla Cave, New Mexico. One show cave in California, Black Chasm Cavern,
was designated a National Natural Landmark because of this. Visit our special tribute
page to this cave's diverse helictites.
The twisted shapes are due to many factors, including:
(1) impurities in the deposited calcite
(2) wedge-shaped crystals causing uneven deposition
(3) plugging of the central channel may occur in dry periods, and when flow resumes,
the pressure may force a new channel out the side of the original one
(4) air currents may favor growth in a particular direction. Sometimes helictites are
found facing the same direction down a passage (see upper left photo in thumbnail table
below).
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beaded helictites
directional helictites
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subaqueous helictites
Balloons
A rare type of cave formation, balloons are a small, gas-filled pouch usually made of
hydromagnesite. Their origin is not completely known, but is likely related to
moonmilk, a material of high plasticity. Balloons probably occur when solutions under
pressure seep into a cave through cracks or out of porous walls of limestone. If they
meet moonmilk on their way out, the material may expand much like a rubber balloon.
The thin moonmilk coating which forms the cast may crack or dry out, exposing the
underlying hydromagnesite balloon.
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Halite Flowers
These curlicue extrusions (known as "flowers") are made up of a mineral more common
to your kitchen than to caves. Arid caves of Australia's Nullarbor Plain have supported
growth of these delicate fibers of halite, better known to surface-dwellers as table salt
(NaCl, or sodium chloride).
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Gypsum Flowers
Flowers are speleothems with crystal petals radiating from a central point. The petals are fibrous or
prismatic crystals growing in a parallel orientation. They are usually made of sulfate minerals such
as gypsum or epsomite, but can form from halite, or rarer (for caves) minerals. Flowers grow from
the base, not the tip as does a stalctite. Often they will carry a portion of the wall away as they
grow, forming a crust on the end of the flower. Flower growth in crevices may contribute to
portions of the wall breaking off and becoming breakdown. This growth mechanism is similar to
that for more fibrous forms of these sulfate minerals.
Flowers form in relatively dry, not dripping conditions. They result from local feeding of solutions
through pores in the rock, under capillary pressure. In the case of gypsum, the solution is calcium
sulfate. Due to changes in flow rate, the flower petals tend to curve, much like helictites.
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Aragonite
Aragonite, like calcite, is made up of calcium carbonate (CaCO3). It differs from
calcite, however, in its internal crystalline structure. Dense masses of tiny aragonite
crystals can be difficult to distinguish from calcite, but when the crystals are large,
they reveal a distinctive external form, or crystal habit. Aragonite crystals are long
and needle-like (acicular), whereas those of calcite tend to be stubby or dog-tooth-
like (often rhombohedral, but calcite is a master of disguises--easily the most
capricious of minerals when it comes to outward appearances). Bushes of acicular
aragonite crystals are also known as frostwork, as in the top photo.
If you simply can't resist the urge to ire some student of mineralogy, remind them
that aragonite and calcite are happy bedfellows in many caves. They'll stammer, stab
at phase diagrams like they were holy scripture, and insist that aragonite just plain
isn't stable at such low pressures and temperatures. But even as they speak, cave
aragonite grows on, seemingly in open defiance of the laws of chemistry. Its secret: a
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loophole clause known as "magnesium-poisoning".
"Magnesium-poisoning" is the name given by carbonate geologist Robert Folk to the
phenomenon by which certain solutions supersaturated with respect to calcite are
inhibited from actually precipitating calcite due to high concentrations of
magnesium. It seems that the magnesium ion somehow interferes with the lateral
growth of calcite crystals. As calcite deposition is suppressed throughout evaporation
or degassing of carbon dioxide, the higher supersaturation concentration of aragonite
is eventually reached. The lengthwise growth of aragonite crystals is not inhibited by
magnesium; they grow freely in an environment that would otherwise be foreign to
them.
Due to its dependence on the magnesium ion, aragonite is typically the product of
evaporation. Evaporation drives the concentrations of all ionic species up. As
calcium is lost to calcite precipitation, the magnesium-to-calcium ratio increases to
the point at which calcite growth is inhibited (about 2.9:1), and aragonite is
thenceforth deposited. A common progression from calcite to aragonite due to
evaporative enrichment is seen the bottom photo, where a nodular mass of calcite
popcorn is tipped by sprays of aragonite needles.
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Shields
A cave shield forms as calcite-rich seep water under hydrostatic pressure is forced
from tiny cracks in a cave wall, ceiling or occasionally, floor. As this seep water loses
carbon dioxide to the cave air, calcite is deposited as parallel extensions to the cracked
walls. The result: two thin, sandwiched disks separated only by a capillary, sheet-like
void that feeds their growth.
Often, seep water feeding a shield does not lose all of its carbon dioxide at the
shield rim.. Additional CO2 loss by water slowly dribbling from the shield rim gives
rise to draperies, as seen in the left photo. If a shield becomes clogged, perhaps due to
sealing of its rim during dry spells, backed-up seep water may find escape through
perforations in the shield disks. This may form stactites (as in the lower photo, or
masses of tangled helictites (left 3 photos in table).
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The third or center photo shows a series of miniature shields forming where water has
been squeezed through a crack in a flowstone wall.
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Blisters
Cave blisters may somewhat resemble balloons in appearance and genesis, but are
more common, and tend to be formed out of a larger variety of crystalline minerals.
They are rounded deposits often filled with sediments or a mineral different than that
forming its walls. They appear to result from solutions forced out of small cracks or
holes under capillary pressure.
Fibrous Speleothems
These formations are composed of aggregates of crystals which are fibrous or
filament-like in nature. Most typically they are gypsum, but can be composed of a
variety of other mineral salts like epsomite or halite. In form they are generally
classified into four subtypes: hair, cotton, rope, and snow. The latter type results from
a disintegration of any of the first three types.
The lefthand photo shows a classic gypsum rope formation, from a cave in the Grand
Canyon. The righthand photo shows a more cotton-like example from a cave in
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Tennessee. In the middel is an unusual coiled form of gypsum from a cave in New
Zealand known as the Spring.
All fibrous formations form from saturated solutions being squeezed out of pores in
the bedrock (usually limestone) and depositing as they hit air. They grow from the
base, with pressure forcing the deposited sections out as new sections are formed. The
size of the pores determines how thick the crystals are, finer pores forming cotton or
hair and larger ones forming rope. This is a similar mechanism to that which forms the
more rigid forms known as gypsum flowers.