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.

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

"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

these "showerheads" may be seen in another alcove of the Virtual Cave.

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.

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.

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.

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.

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).

beaded helictites

directional helictites

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.

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).

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.

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

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.

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).

The third or center photo shows a series of miniature shields forming where water has

been squeezed through a crack in a flowstone wall.

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

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.